Nuclear Instruments and Methods in Physics Research A 635 (2011) 24–34

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

� Corr

E-m1 Pr

D-3811

journal homepage: www.elsevier.com/locate/nima

Gas jet studies towards an optimization of the IGISOL LIST method

M. Reponen a, I.D. Moore a,�, I. Pohjalainen a, T. Kessler a,1, P. Karvonen a, J. Kurpeta b, B. Marsh c,S. Piszczek b, V. Sonnenschein a, J. Aysto a

a Department of Physics, University of Jyvaskyla, P.O. Box 35 (YFL), FI-40014 Jyvaskyla, Finlandb Faculty of Physics, University of Warsaw, ul.Hoza 69, 00-681 Warsaw, Polandc CERN, CH-1211 Gen�eve 23, Switzerland

a r t i c l e i n f o

Article history:

Received 20 August 2010

Received in revised form

24 January 2011

Accepted 24 January 2011Available online 11 February 2011

Keywords:

Gas cell

Gas jet

Nozzle

LIST

02/$ - see front matter & 2011 Elsevier B.V. A

016/j.nima.2011.01.125

esponding author. Tel.: +358 14 2602430; fa

ail address: iain.d.moore@jyu.fi (I.D. Moore).

esent address: Physikalisch-technische Bun

6 Braunschweig, Germany.

a b s t r a c t

Gas jets emitted from an ion guide have been studied as a function of nozzle type and gas cell-to-

background pressure ratio in order to obtain a low divergent, uniform jet over a distance of several cm.

The jet has been probed by imaging the light emitted from excited argon or helium gas atoms. For a

simple exit hole or converging-diverging nozzle, the jet diameter was found to be insensitive to the

nozzle shape and inlet pressure. Sonic jets with a FWHM below 6 mm were achieved with a background

pressure larger than 1 mbar in the expansion chamber. The measurements are supported by the

detection of radioactive 219Rn recoils from an alpha recoil source mounted within the gas cell. A Laval

nozzle produced a well-collimated supersonic jet at low background pressures with a FWHM of

� 6 mm over a distance of 14 cm. Direct Pitot probe measurements, on-axis, revealed a non-uniform

pressure distribution in the gas jet of the Laval nozzle, supporting the visual observations. All

measurements are motivated by the requirement of a good geometrical overlap between atoms and

counter-propagating laser beams in the gas cell-based Laser Ion Source Trap (LIST) project. Computa-

tional fluid dynamics gas flow simulations were initiated to guide the future development of the gas jet

system.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

The helium-jet transport system has been used as a mechan-ism for the extraction of radioactive nuclides since the 1960s andhas developed over the years to become a standard technique atseveral facilities [1–3]. Later, the conventional ion source could beeliminated in the development of the helium-jet ion guidetechnique [4,5]. Of particular note was the discovery of thefavourable charge-state distribution brought about by the ther-malisation process within the gas, with a large fraction of nuclearreaction products settling in a 1+ charge state [6]. This character-istic formed the basis of the Ion Guide Isotope Separator On-Line(IGISOL) principle [7], allowing sub-millisecond conversion ofreaction products into a low energy radioactive ion beam.

In the IGISOL technique reaction recoils are stopped in a buffergas chamber filled with helium or argon at a pressure of typicallya few hundred mbar. The thermalized reaction products areguided by the gas flow towards an exit hole with a diameter ofapproximately 1 mm. The flow within the buffer gas cell has been

ll rights reserved.

x: +358 14 2602351.

desanstalt, Bundesallee 100,

studied both experimentally and by simulation [8–10] and is wellunderstood. Upon exit from the gas cell the pressure drops byorders of magnitude and the gas flow becomes either sonic orsupersonic and is highly expansive. Although the theory ofsupersonic flow required to describe the gas jet downstream fromthe exit hole is generally well-established [11,12], the computa-tional simulation of supersonic flow at boundaries is still an activefield of research [13,14]. Considerable effort has been madetowards creating mathematical models and computer codes forthe development of internal supersonic jet targets which utilizenovel nozzle designs and show considerable promise for avariety of experiments at storage rings [15,16]. Additionally, anexploratory study into the visualization of expanding gas flowfrom helium-jet and ion guide nozzles has been conducted byRasi et al. [17].

In the traditional IGISOL approach a skimmer system was usedto separate the ions of interest from the neutral buffer gasenvironment. The resultant ion trajectories are governed by acombination of the electrostatic skimmer potential and gas flow.More recently, multipole structures replacing the skimmer devicehave successfully improved the transmission efficiency and beamemittance of the gas cell-based mass separator system [18–20].The development of the radiofrequency (rf) sextupole ion beamguide (SPIG) at the Accelerator Laboratory of the University of

M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) 24–34 25

Jyvaskyla (JYFL) was motivated by the so-called Laser Ion SourceTrap (LIST) project [21]. This is a variant of the Ion GuideLaser Ion Source concept, IGLIS [22,23], where pulsed lasers areused to selectively ionize neutral atoms within the gas cellvolume [24,25]. Such a principle is motivated by the lack ofZ-selectivity inherent in the standard ion guide technique. TheLIST was originally proposed to improve the beam quality from ahot cavity laser ion source [26] by decoupling the hot productionand evaporation regions from the ionization volume. It has sincebeen adapted for coupling to a gas catcher system [27]. In thislatter approach, the reaction products are allowed to neutralizewithin the gas cell and upon exit they are selectively re-ionizedwith counter-propagating lasers within the supersonic gas jet.The photo-ions are captured by the RF-field of the SPIG locatedimmediately after the gas cell and are transported to the massseparator by a voltage gradient. A positive DC voltage applied tothe first electrode of the SPIG acts to repel any non-neutralfraction from entering the RF guide, ensuring the highest possiblebeam purity is maintained for subsequent experiments.

There are two parameters which determine the overlap effi-ciency between the atoms and the laser photons. The first isrelated to the duty cycle of the laser system. In order that eachatom has the chance to interact with the laser beam at least onceover a collimated interaction length of 10 cm, the needed repeti-tion rate is 10 kHz for a jet velocity of 1000 m/s. The secondfactor, the geometrical overlap, is directly affected by the jetcollimation and can be optimized using a special nozzle design orspecific background pressure outside the gas cell.

In the present work, we investigate the gas jet with differentbackground pressure conditions and exit nozzle shapes. In asimilar manner as Rasi et al. [17] we utilize an arc dischargesource to ionize and excite helium and argon buffer gas atoms. Inthe process of relaxation, afterglow light is emitted which can beseen by the naked eye downstream from the nozzle. In order tosupport the visual observations of the jet we have utilized thetransport and detection of recoils from a 223Ra source. In additionto these qualitative measurements we have used thin pressureprobes made from hypodermic needles in order to quantitativelyexplore the stagnation and static pressure distributions as afunction of distance, on-axis, from the exit hole of the gas cell.Finally, the measurements are supported by gas jet simulationsperformed at the University of Warsaw which are summarized inthis article.

1

0

1

2

3

4

5

6

Nozzle throat

Subsonic flow

Mac

hnum

ber M

Supersonic flow

A/An

10 100

Fig. 1. Velocity in the transition from a subsonic to a supersonic gas jet as a

function of the area ratio.

2. Theory of supersonic gas flow

The properties of the gas jet after the ion guide dependstrongly on the shape of the nozzle and the pressure boundaries.The theory associated with gas jet expansion is treated in detail inRefs. [11,30]; however, we provide a short introduction here forclarity.

The main features of the transition from subsonic to super-sonic flow in a converging-diverging nozzle can be described in aquasi-one-dimensional symmetry, assuming a constant velocityperpendicular to the symmetry axis of the boundaries. In the caseof an adiabatic flow, the change in velocity in terms of a Machnumber M is described by the so-called area-velocity relation [11],

dA

A¼ ðM2�1Þ

du

uð1Þ

where A is the cross-sectional area of the enclosing structure andu the gas velocity. Eq. (1) states that the velocity of the gas iscompletely determined by the boundary surface. A direct result ofthis relationship is that the speed of sound (M¼1) can only bereached in the throat of the nozzle where dA¼0. Inside the ion

guide, the gas accelerates as the area is decreased until a velocityof Mach 1 is reached at the minimal radius. After the throat, thegas accelerates during the expansion phase as dA40 and asupersonic flow is attained.

For a calorically perfect gas, Eq. (1) can be solved explicitly toobtain the area-Mach number relation:

A

An

� �2

¼1

M2

2

gþ11þ

g�1

2M2

� �� �ðgþ1Þ=ðg�1Þ

ð2Þ

where AnoA is the area of the nozzle throat and g the ratio ofspecific heats (5/3 for monoatomic gases such as helium andargon). For every value of A4An, two solutions of M fulfil Eq. (2),the subsonic and the supersonic solution. A visualization of theMach number as a function of area ratio can be seen in Fig. 1.

Once the variation in the Mach number through a nozzle isknown, parameters such as the temperature, density and pressurecan be calculated for a calorically perfect isentropic gas. Thepressure in the jet P is calculated using the relationship

P

Pin¼ 1þ

g�1

2M2

� ��g=ðg�1Þ

ð3Þ

where Pin is the static pressure for M¼0, in other words, the ionguide pressure. In a similar manner, the density and temperaturecan also be calculated [11]. Eqs. (2) and (3) can be combined toobtain an expression for the pressure ratio as a function of thearea ratio, illustrated in Fig. 2. The behaviour of the isentropicsolution for a supersonic jet is usually referred to as the design

condition. At the nozzle throat a value of M¼1 is reached andEq. (3) reduces to

P

Pin

����throat

¼2

gþ1

� �g=ðg�1Þ

: ð4Þ

For monoatomic gases such as He and Ar a value of � 0:49 isreached at the throat.

In reality, the gas jet does not follow the isentropic designsolution as it is forced to a fixed pressure boundary at the nozzleexit given by the pumping capacity of the vacuum system. As anexample of the dependence of the gas behaviour under differentboundary conditions one may consider a standard IGISOL ionguide with a simple converging-diverging nozzle. If no pressuredifference is applied between the gas cell and the vacuum

Fig. 3. Behaviour of the gas pressure ratio in a shaped nozzle for different

background pressures.

1.0

0.8

0.6

0.4

0.2

0.0

1 10 100A/An

P/P

in

Fig. 2. Pressure ratio as a function of area ratio in the transition from a subsonic to

a supersonic flow regime.

M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) 24–3426

chamber no flow occurs. Only when the background pressure Pbg

in the IGISOL chamber is decreased to a value of Pbg=Pino0:49 asupersonic flow condition is obtained.

A mismatch between the pressure boundary at the nozzle exitand the pressure of the jet is compensated by the formation ofshock structures in the jet. The impact of the background pressureon the behaviour of the gas jet exiting a converging-divergingnozzle is depicted in Fig. 3. The subsonic and supersonic isen-tropic design solutions are indicated as guidelines. Let us firstconsider the situation for a background pressure Pbg¼P1. AsP1o0:49, the nozzle is choked, however, the pressure at the exitis higher than the pressure Pdesign calculated from the isentropicsolution for the freely expanding jet. The only way to fulfil therequired higher pressure boundary is to form a normal shockwithin the nozzle as indicated by the vertical line in Fig. 3.Following the shock an immediate increase in pressure is seenin the nozzle. The flow becomes subsonic and the pressurecontinuously increases during the expansion phase to convergeto the given pressure boundary P1 at the exit.

By decreasing the background pressure Pbg further, the shockdisc moves downstream towards the exit of the nozzle. At apressure of Pbg¼P2, the shock is positioned at the nozzle exitand the jet is supersonic throughout the whole volume of thenozzle. If the background pressure is reduced further to a valuePdesignoPbg ¼ Pover oP2, an oblique shock occurs outside thenozzle area which increases the pressure towards the boundary

condition leading to a spatial compression of the jet. In this casethe nozzle is said to be overexpanded. Finally, if the pressure Pbg isreduced to below the isentropic value at the exit, Pbg ¼ Punder oPdesign, the jet can only reduce its pressure by forming an expan-sion wave. In this instance the nozzle is said to be underexpanded.In both overexpanded and underexpanded nozzle configurationscomplex shock patterns occur downstream from the nozzle whichare characteristic for the reflection of the jet on the free pressureboundary.

3. Experimental studies of gas jets

In the 1990s, an exploratory study into the nature of gas flowand expansion outside a gas cell was performed by Rasi et al. [17].This comprises visual observations made possible through excita-tion, by electrical discharge, of the gas within a chamber. The faintfluorescence visible from the gas following the excitation is aresult of relaxation, finally to the ground state. For metastablestates, this relaxation is of the order of several milliseconds andso, at typical gas cell–vacuum chamber pressure differentials,allows imaging of the gas jet far into the expansion region. Thegas-jet photographs obtained allowed qualitative informationabout the complete jet expansion and identification of the distinctshock-wave structures described by the fluid dynamics of a freelyexpanding gas. Additionally, observations were made with gasflow obstructions, in the form of circular discs with a centralaperture, placed downstream of the exit nozzle. This wasintended to simulate the interaction of the gas jet with a skimmerelectrode, usually placed a few cm from the ion guide. The studyconcluded that for large apertures, the gas flow remains largelyunperturbed. However, as the aperture diameter is reduced to lessthan the visible gas-jet width, a shock wave is observed and thegas flow after the aperture becomes more diffusive and expansive.

The work of Rasi et al. was conducted considering the IGISOLmethod to be used as an ‘‘ion guide’’ technique, whereby the iontrajectories after the gas cell are primarily governed by electricfields. The properties of the gas flow, including shock waves, werenot considered to be of importance. The present work, however, ismotivated by the development of the laser ion source and the useof the gas cell as an ‘‘atom guide’’. Furthermore, the local environ-ment within the jet is highly attractive for in-jet laser spectroscopy,with detrimental effects to the atomic linewidth due to tempera-ture and pressure broadening substantially reduced [27]. We haverevisited the method of Rasi et al. and have used the gas jetfluorescence as a means to probe the effect on the jet as a functionof nozzle design, gas cell pressure and variation in the backgroundextraction chamber pressure. It should be noted that in all experi-ments the ratio of Pbg=Pino0:49, and therefore, we are within theregime of choked flow conditions.

All studies discussed in this article have been performed in avacuum chamber designed for a new ion-guide quadrupole massspectrometer system. This provides more flexibility and ease ofuse than the IGISOL target chamber.

3.1. Gas cell exit nozzle shapes

Initially, two main types of nozzle design have been compared,a converging-diverging nozzle and a simple exit hole. The exithole is typical for all ion guides in use at IGISOL and cannot beshaped. However, the converging-diverging nozzle can be shapedto match the expected pressure conditions between the gas celland the extraction vacuum chamber such that it operates at thedesign condition. In reality we have been limited to practicaldesigns which can be produced by the technical workshop. In theIGISOL technique one critical parameter is the size of the exit hole

M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) 24–34 27

or in a shaped nozzle, the throat diameter, which determines theconductance of the gas cell. The typical exit hole diameter used is1.2 mm, however, when operating the gas cell under conditionssuitable for the laser ion source the nozzle diameter is a factor oftwo smaller.

We have used the solutions to Eqs. (2) and (3) as a means ofguiding our choice of nozzle sizes. When the quadrupole massspectrometer chamber was first being constructed, the pumpingcapacity was rather limited and a typical pressure ratio betweenthe ion guide inlet and extraction chamber was � 1%. Eq. (3) issolved at this pressure ratio to determine that a Mach number of4 is required to satisfy the design condition at the exit of thenozzle. With the design Mach number, Eq. (2) is then solvedindicating that the ratio of the nozzle exit area to the throat areashould be � 5:6, thus a diameter ratio of � 2:4. The converging-diverging nozzle tested in this work had a throat diameter of1.4 mm and an outlet diameter of 4 mm, reasonably close to thedesign condition requirements set by the earlier pumping capa-city (for which the nozzle had been designed). More recently,with a larger pumping capacity available for the studies describedin this work, the corresponding pressure ratio has decreased to� 0:2%. Fig. 2 illustrates that such a decrease in the backgroundpressure can be compensated by an increase in the diameter ofthe exit of the nozzle in order to maintain the design condition. Asmentioned previously, the design pressure condition for a stan-dard exit hole is 0.49 (under the assumption that the throatdiameter is equivalent to the exit hole diameter). With ourexperimental pressure conditions the jet following the exit holeis consequently highly underexpanded, and an expansion wave isformed. Although this condition is not preferred, as an exit hole isin typical use at the IGISOL facility we have studied this designwith two different diameters, 0.7 and 1.34 mm. Both nozzle typesare illustrated with appropriate dimensions in Fig. 4.

4,0 1,401.34 or 0.7

2,0

1,9

6,0

Fig. 4. Schematic drawings of the standard exit hole (A) and converging-diverging

nozzle (B) tested in this work. All dimensions are in mm. The gas cell would be

attached to the right-hand side of nozzle B in this figure.

3.2. Experimental set-up and gas jet photography

Fig. 5 illustrates, among other details, a schematic of the gascell used for the gas-jet observations. Helium or argon gas,regulated using a needle valve, was injected into the inlet of thegas cell. A Leybold capacitive pressure sensor (model DI2000)attached to the gas line was used to measure the inlet gaspressure. The extraction vacuum chamber was pumped withone Roots blower with a specified pumping speed of 4000 m3/h.A Balzers Pirani gauge (model TPR 010), positioned on one sideflange of the chamber approximately 20 cm from the gas jetregion, allowed monitoring of the background pressure. In prac-tice, the minimum pressure achieved within the extractionchamber (without gas injection) was of the order of 10-3 mbar.The background pressure could be adjusted in a crude manner bymanually closing the valve to the roots pumping line. The gas celland extraction region were visible from the side of the vacuumchamber through a glass window (see photograph in Fig. 6).

A single electrode inside the gas cell was connected to a 50 mAcurrent-limited DC power supply. To initiate the discharge, avoltage in excess of 300 V was required. With an increase to700 V, the gas emerging from the nozzle exhibited a glow easilyvisible to the naked eye. In off-line experiments at IGISOL, sparkdischarge source conditions are rather sensitive to the ion guidepressure. In this work a variety of gas cell pressures were useddepending on the stability of the discharging electrode. Theexcited states and the reactions in which they are populatedand subsequently quenched are difficult to determine withoutspectroscopic measurements of the afterglow, which is beyondthe scope of this work. For example, the population of metastablestates can occur via inelastic scattering of electrons, ion–atomimpact excitation, or in electron–ion recombination. The relaxa-tion of states can happen by a number of reactions, whose rates

Gasin

Spark electrode

60 mm

Sidetector

Fig. 5. Schematic set-up of the gas cell for use with either a spark electrode or a223Ra a-recoil source. The moveable silicon detector is used to count the alpha

recoil products.

Fig. 6. Photograph of a gas jet measurement set-up. The gas cell is situated on the

right and the perspex SPIG structure on the left. The distance between the gas cell

and the SPIG structure has been arbitrarily chosen in this picture. The nozzle of the

gas cell is interchangeable.

Rad

ial d

ista

nce

(mm

)

0.2 mbar

−30

−24

−18

−12

−6

0

6

12

18

24

5.6 mbar

48 42 36 30 24 18 12 6 0−30

−24

−18

−12

−6

0

6

12

18

24

M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) 24–3428

can be affected by impurities in the gas. In the analysis of the gasjet fluorescence detailed in the following section we, therefore,neglect possible quenching of excited states. For more detailsregarding the production and relaxation of metastable states werefer to Refs. [28,29].

The gas jet was photographed using a Casio Exilim Ex-F1 PROdigital camera in manual mode with the sensitivity set to ISO 100,an aperture of F7.5 and an exposure time of 1 s. The lowsensitivity and relatively long exposure time, combined with thecamera mounted on a fixed support (� 20 cm from the gas jet),resulted in low noise photographs. The photographs, of 6 Mega-pixel resolution, were saved in a RAW format which contains theRGB intensity data of each pixel allowing, for example, a morecomprehensive analysis afterwards.

When coupling the gas jet into a SPIG structure the dimensionsof the jet are particularly important [13,14]. The entrance to theJYFL SPIG is defined by the first electrode aperture of 6 mm indiameter [20]. The typical operating distance between the SPIGand the exit hole of the ion guide is � 5 mm. As discussed by Rasiet al., if the gas jet diameter is larger than the aperture the jetexperiences a normal shock and becomes subsonic after thenozzle leading to an almost spherical distribution of the gas. Inthis work we have utilized a perspex SPIG structure of similardimensions to the first sextupole used at IGISOL [20], shown inthe photograph of Fig. 6. In this manner, the interaction andcoupling of the gas jet to the structure can be visually studied.

Longitudinal distance from ion guide (mm)

Fig. 7. Contour map following the image analysis of a gas jet photograph taken at

two background pressures, 0.2 mbar (top) and 5.6 mbar (bottom). The SPIG is

visible on the left and the ion guide on the right. The dark regions visible above

and below the exit nozzle are the screws connecting the nozzle to the gas cell.

105

104

103

102

-60 -40 -20 0 20 40 60Radial position (mm)

Inte

nsity

(arb

itrar

y un

its)

Fig. 8. Gas jet profiles � 2 cm from a 1.34 mm diameter exit hole as a function of

background pressure. The gas cell was operated with helium at a pressure of

56 mbar. Colour online.

4. Analysis of the gas jet fluorescence

A number of photographs were taken using the nozzle typesdiscussed in Section 3.1, for a constant gas cell pressure of eitherhelium or argon. Greyscale images were then created by summingthe RGB colour channels of the unmanipulated raw images. Inorder to represent changes of intensity, reduced noise contourmaps were made from the greyscale images. Noise reduction wasperformed using the pixelwise adaptive Wiener filter of MATLAB’simage processing Toolbox [31]. An area of 20 � 20 pixelscorresponding to 1.42 �1:42 mm2 was used in the smoothingprocedure.

Fig. 7 is an example of a contour map which illustrates the gasjet fluorescence profile at a background pressure of 0.2 and5.6 mbar, respectively. The converging-diverging nozzle with athroat diameter of 1.4 mm was used and the gas cell was operatedwith helium at a pressure of 56 mbar. The contour lines representexponentially increasing intervals of intensity, such that each linecorresponds to about a 26% increase of brightness radiatingoutwards from the gas jet. Due to the expansion of the fluorescentjet, the ion guide head (on the right) and the perspex SPIG (on theleft) are visible in the figure. Given the ratio of the nozzle exit tothroat diameter (Fig. 4), an isentropic design pressure ratio of� 0:5% can be deduced. In order to achieve this pressure ratio, fora given inlet pressure of 56 mbar the background pressure shouldbe � 0:28 mbar. Consequently, the two jet structures in Fig. 7correspond to an underexpanded and overexpanded regime,respectively. In both situations the nozzle is choked. With abackground pressure of 0.2 mbar, less than the design pressureof the nozzle, the jet freely expands to supersonic velocities in theextraction chamber. A shock disc is not visible, however, whichmay be due to the increasingly diffusive nature of the gasdownstream from the nozzle caused by the interfering presenceof the SPIG structure. At a background pressure of 5.6 mbar,significantly larger than the design pressure, a shock is formedwithin the nozzle and the gas downstream is moving at asubsonic velocity. It is clear that the increase in the backgroundpressure improves the coupling to the SPIG structure.

In order to understand the effect of the background pressureon the gas jet diameter, Fig. 8 shows intensity plots of gas jetcross-sections at a distance of � 2 cm from the ion guide. In thisinstance the gas cell was again operated with helium at a pressureof 56 mbar; however, a standard exit hole of 1.34 mm diameterwas used. The thickness of the cross-sectional slice used to extractthe information was 40 pixels or 2.5 mm wide. The gas jetintensity and its corresponding error were calculated as a func-tion of radial distance from the average and standard deviation of

104

103

Inte

nsity

(arb

. uni

ts)

-60 -40 -20 0 20 40 60Radial position (mm)

Fig. 10. Gas jet profiles as a function of axial distance from the 1.4 mm

converging-diverging exit nozzle. Colour online.

Rad

ial d

ista

nce

(mm

)

Longitudinal distance from ion guide (mm)42 36 30 24 18 12 6 0

−12

−6

0

6

12

18

M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) 24–34 29

each 40 pixel row. In order to characterize the gas jet morequantitatively, the Full Width at Half Maximum (FWHM) of theprofiles was determined from either Lorentzian or Gaussianfitting. We note that the choice of distribution has not beenmotivated by any physics of the gas jet and simply provides anappropriate metric of the jets’ profiles. Due to the diversity in theoverall shape of the profiles, particularly at low backgroundpressures in which the presence of the SPIG may have aninfluence on the tails of the jet (as suggested by the contourdistribution in Fig. 7), each individual intensity plot was checkedfor overexposed areas or other artifacts not directly related to thejet (for example illuminated areas of the chamber seen in the rawimages).

Fig. 9 illustrates the change of the gas jet FWHM as a functionof background pressure for all the nozzle types and two differentgas cell pressures, as indicated in the figure legend. A number ofconclusions can immediately be drawn from the figure. Firstly,the FWHM of the gas jet shows a lack of sensitivity to the twohelium inlet pressures. Secondly, the gas jet diameter is insensi-tive to the type of nozzle used. The most striking dependence,however, is that all results show a rapid reduction in the FWHMfrom close to 40 mm at the lowest background pressures(� 0:1 mbar), followed by a slow levelling off at 0.5 mbar,wherein the FWHM has reduced to � 6 mm. We note thatalthough the gas jet can couple into the SPIG (aperture 6 mm indiameter) with minimal interference at background pressuresgreater than � 1 mbar, this is not an ideal scenario for IGISOLoperating conditions due to an increase in the probability ofdischarge between electrodes of the ion guide system.

In addition to the gas jet measurements as a function ofbackground pressure, we also studied the profile of the gas jet asa function of longitudinal distance from the exit nozzle, withoutthe perspex SPIG. An example of such a measurement is shownin Fig. 10 for the case of the 1.4 mm converging-diverging nozzleattached to the ion guide with an inlet pressure of 50 mbar ofargon. In this instance the maximum pumping capacity of theextraction vacuum chamber was utilized and the correspondingbackground pressure was 9� 10�2 mbar. The most interestingobservation occurs within a longitudinal slice taken 17 mm fromthe nozzle, in which a ‘‘rabbit-eared’’ structure is seen. Thecontour map in Fig. 11 illustrates the reason for such a structurein the gas jet profile, a rarefied region of expanding supersonic gas

100

10

10 1 2 3 4 5 6 7

Background pressure (mbar)

Jet F

WH

M (m

m)

Fig. 9. Gas jet FWHM as a function of background pressure for the different nozzle

types and two gas cell pressures.

Fig. 11. Contour map of an argon jet expanding from the converging-diverging

nozzle. The two dark curved bands extending from the ion guide represent the

oblique shocks. The normal shock, or Mach disc, is located at a distance of

� 22 mm from the nozzle. The white region below the nozzle screw was

overexposed in the original photograph.

surrounded by oblique and normal shock waves. In this instancethe pressure in the ion guide had been increased to 250 mbar andthe corresponding background pressure 4� 10�1 mbar.

Fig. 11 indicates that the converging-diverging nozzle behavesin a similar manner to the converging-only nozzle (exit hole),probably because the diverging duct is relatively short. Thepressure difference between the inlet chamber and extractionchamber is now considerably larger (� 600) and thus the type ofjet formed for the nozzle in use is said to be strongly under-expanded. Under such conditions the gas exits the nozzle andstarts to accelerate to high speeds (Mc1) while the density drops.Due to radial acceleration the jet cross-section increases (as seenfrom the two visible oblique shocks whose outer surfaces definethe jet boundary). The jet pressure reduces until it becomes lessthan the background pressure, at which point the jet moves intoan over-expansion region. Due to the radial gradient of pressure,the radial cross-section of the jet reduces as the gas is drivenback towards the nozzle axis. The normal shock, or Mach disc,

M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) 24–3430

surrounds the first region of fast, low density gas expansion and isimmediately upstream of a compression wave in which the gasdensity is considerably greater. The Mach disc appears to berather more diffuse than the oblique shocks (which are alsodefined by the term barrel shock), illustrated by the density ofthe contour lines. In our pressure regime, the shock wave is less ofa sudden discontinuity and can be thought of as a band in whichthe changes in pressure, mass density, temperature and flowvelocity are rather gradual.

For a monoatomic gas such as argon, the shape and size of theMach disc and barrel shock, as well as their location with respectto the nozzle, is simply determined by the ratio of the inlet tobackground pressure and the nozzle diameter d [32]. The positionof the disc xM can be calculated for the condition 15rPin=Pbg

r17 000 (which is satisfied for all pressure ratios in this work):

xM ¼ 0:67dðPin=PbgÞ1=2: ð5Þ

According to this relationship, as the ratio between the inletpressure of the gas cell and the background pressure becomeslarger, the Mach disc moves downstream from the nozzle.

We have estimated the Mach disc location from the jetphotographs as a function of inlet pressure. Using Eq. (5) andthe known throat diameter of the converging-diverging nozzle,we have extracted the background pressure from the Mach disclocation and compared it to the measured pressure from thePirani gauge, located 20 cm from the gas jet region. The results ofthe comparison are shown in Fig. 12. The error on the pressureinferred from the position of the shock disc is representative ofthe uncertainty of the location, whereas the Pirani gauge mea-surements have a 20% error according to the model specifications.We note that at the lowest inlet pressures the Mach disc is barely

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.00 50 100 150 200 250 300 350

Gas cell pressure (mbar)

Ext

ract

ion

cham

ber p

ress

ure

(mba

r)

Fig. 12. A comparison between the extraction chamber pressure measured using a

Pirani gauge and that determined via Eq. (5) and measurements of the Mach disc

location, vs the ion guide pressure.

Fig. 13. Schematic of the Laval nozzle tested in this work (right) followed by a contou

waves are visible until viscous effects finally dissipate the jet energy. The background

visible as the gas jet is no longer strongly underexpanded, thusthe location becomes more difficult to determine. The corre-sponding variation in the position of the Mach disc is � 7 mmover the range of inlet pressure. In general, the results are in goodagreement, supporting our understanding that the Mach discrepresents the location, where the jet pressure rapidly changesto match the background pressure.

The diameter of the Mach disc DM is related simply to thelocation of the disc xM [32]:

DM ¼ 0:42xM for Pin=Pbg ¼ 20

DM ¼ 0:48xM for Pin=Pbg ¼ 1000: ð6Þ

These relationships have been found for air, however, an accuratevalue depends on the ratio of specific heats, g, and the ratio ofinlet to background pressure. We have extracted values for thediameter from the jet photographs, and for a change of inletpressure from 300 to 100 mbar, DM varies from � 10 to � 4 mm.

We note that the gas jet profiles of Fig. 10 and the contour plotof Fig. 11 illustrate the rather wide radial distribution of the jet atthe lowest operated background pressures. Although a low back-ground pressure is desirable at IGISOL, the figures clearly demon-strate a detrimental behaviour towards coupling the jet into theSPIG, thus resulting in a poor laser-atom overlap efficiency. Thekey to providing a highly uniform, low divergent expansion at lowbackground pressure is to design the contour of the nozzle suchthat the flow at the exit has a constant Mach number, hence thedensity and temperature at all points within the subsequent post-nozzle flow are fixed. Such a nozzle design is called the Lavalnozzle. It is known that the design of the convergent and throatareas are less critical to the performance of the nozzle, althoughin general the convergent section should have a relatively largeradius of curvature near the throat region (five times the throatradius) [33]. However, the shape of the divergent section has to beprecisely calculated and manufactured in order to minimize thethickness of the boundary layer [34]. Such a layer describes theregion where the nozzle wall influences the flow (heat transferand viscous effects become important) and can be calculatedusing computational fluid dynamics methodology.

A Laval nozzle has some disadvantages when compared toother supersonic nozzle designs. Firstly, there are complicationsassociated with the fabrication of the nozzle and, furthermore, itis designed to work best only at one pressure, which means that ifthe ion guide pressure is tuned the jet shape suffers due to achange in background pressure. Although the effect is small itcould be avoided by using so-called altitude compensating nozzles

such as the aerospike nozzle [35]. Nevertheless, we recentlymanufactured a proof-of-principle Laval nozzle based on designswe found in the literature [34,36] with the practical criteria thatthe throat diameter be similar to the other nozzles tested in thiswork (� 1:4 mm) and the exit of the nozzle equivalent to thediameter of the entrance to the SPIG (6 mm). By solving Eq. (3),the design Mach number for this nozzle is � 6:2 and thus theisentropic design pressure ratio is � 0:14%.

r map of the resulting argon gas jet (left). A series of expansion and compression

pressure was 0.33 mbar.

M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) 24–34 31

In comparison to our previous converging-diverging nozzle(Fig. 4), the distance from the nozzle throat to the exit isconsiderably longer in the Laval nozzle as illustrated in Fig. 13.In addition to the nozzle schematic, Fig. 13 shows one example ofthe resultant jet structure visualized as a contour map. The ionguide was operated with argon at a pressure of 250 mbar. Atthis pressure the background pressure should be 0.35 mbar tomeet the design requirements and in reality it was very close,0.33 mbar. Unlike the single dominant shock structure in Fig. 11,the Laval nozzle exhibits a series of expansion and compressionwaves which, following reflection from the jet boundary, intersectto form a curved shock (seen at distances of 40 and 85 mm fromthe ion guide). The process repeats itself until viscous effectsfinally dissipate the jet energy. The initial results of Fig. 13 arevery encouraging, the gas jet maintains a reasonably collimatedstructure over a considerable distance of 14 cm while remainingfully supersonic. Following the first expansion, the radial profileof the jet has a maximum diameter of � 6 mm, a promisingdevelopment towards an improved overlap efficiency in a futureLIST experiment.

100

80

60

40

20

060 45 30 15 0 -15

Longitudinal distance from ion guide (mm)

Sta

gnat

ion

pres

sure

(mba

r)

Fig. 14. Stagnation pressure measurements as a function of distance from the exit

of the Laval nozzle for different ion guide pressures. The vertical dotted line

represents the nozzle exit. The nozzle throat begins at ��15 mm in this figure.

5. Direct pressure measurements

The previous section detailed our extensive studies of the gasjet fluorescence in order to gain insight into the behaviour of thejet as a function of background pressure and nozzle type. To gaina more quantitative understanding of the pressure of the jet it isvery useful to determine the impact pressure at a given point inthe flow. The pressure distribution of the gas jet was studied on-axis as a function of distance from the nozzle for a fixed gascell pressure, and also as a function of pressure at the fixedlocation of the exit of the nozzle. Here we only studied theLaval nozzle, detailed in Fig. 13, using argon gas. The measure-ments were performed using thin stagnation and static pressureprobes (Pitot probes) made from hypodermic needles of typically1 mm in outer diameter. It is well known that results obtainedwith this method are of modest accuracy, nevertheless, theyprovide direct and quantitative information, unlike the qualitativemeasurements based on the analysis of the gas jet fluorescence.We note that all pressures are absolute and not relative toatmosphere.

The needles were attached to 4 mm outer diameter aluminiumtubes located � 5 cm from the needle tip, which in turn weremounted to a skimmer plate drive, to be used in the future ionguide-based quadrupole mass spectrometer system. Such a set-upallowed the needles to be moved using computer control with submm precision. The pressure is measured via a low-cost siliconpressure sensor with an amplified voltage output (model PX138-015A5V from Omega Engineering), operated from a 8 Vdc regu-lated power source. The sensor was calibrated using a PfeifferCMR 261 capacitance gauge.

A simple Pitot probe with a hole facing the gas jet measuresthe so-called stagnation pressure, or total pressure. This is a sumof the static pressure and dynamic pressure (or impact pressure).The static pressure probe is generally measured via a hole on theside of the needle. Both measurements are required in order toderive the dynamic/impact pressure which is extremely sensitiveto the Mach number. The Pitot tube induces a standing shockwave in the flow and by assuming that the slowing down of theflow downstream of the shock wave is isentropic, the staticpressure ps and impact pressure pi are linked to the Mach numberthrough the Rayleigh–Pitot relationship [37]:

pi

ps¼

gþ1

2

� �ðgþ1Þ=ðg�1Þ

M2g=ðg�1Þ gþ1

2gM2�gþ1

� �1=ð1�gÞ: ð7Þ

If we assume once more an isentropic expansion within thenozzle we can extract the relationship between the impactpressure, the ion guide pressure pin and the Mach number:

pi

pin¼ðgþ1ÞM2

ðg�1ÞM2þ2

� �g=ðg�1Þ gþ1

2gM2�gþ1

� �1=ðg�1Þ

: ð8Þ

Fig. 14 shows the results of the stagnation pressure probe as afunction of distance from the nozzle. This measurement wasrepeated for several ion guide pressures, listed in the figurelegend. Measurements were not restricted to regions of the gasjet downstream from the nozzle but also included the divergingregion of the nozzle. A striking first maximum can be seen with atypical FWHM of between 6 and 8 mm, which moves downstreamas the ratio of the ion guide to background pressure increases.This behaviour is qualitatively similar to that of the Mach discresulting from the converging-diverging nozzle discussed in theprevious section. The break in the data points at � 20 mmin Fig. 14 is where the needle had to be physically moved dueto the limited travel of the skimmer drive mechanism. Followingthis, a second, smaller amplitude and broader peak is visible. Notethat fewer ion guide pressures were used to study this secondpeak. Fig. 14 can be compared with the gas jet fluorescenceanalysis of Fig. 13. The overexposure of the photograph in the first10 mm from the nozzle limits the comparison; however, thestructure between 40 and 50 mm is clearly supported by thedirect pressure measurements.

Measurements of the static pressure were also performed.From the nozzle exit as a function of axial distance downstream,the pressure was rather constant at 1–1.5 mbar. This is in reason-able agreement with the earlier work of Iivonen et al. whodemonstrated that, after a sharp drop near the nozzle, the staticbackground pressure in the channel between the target chamberand skimmer electrode was fairly constant and typically of theorder of 1 mbar [38]. As the static pressure is a negligible fractionof the total pressure we can assume that the stagnation pressureis equivalent to the impact pressure in our conditions. Fig. 15illustrates the ratio of the ion guide pressure to impact pressureas a function of the ion guide pressure, at the nozzle exit.

At low pressures the ratio Pin=Pi sharply decreases as the gasflow approaches sonic conditions. As the shape of the nozzle hasnot been properly calculated in this first design, the supersonicflow is not smooth and the peak between � 15 and 100 mbar

Fig. 16. Normalized a-recoil counts as a function of pressure for different Si

detector positions. The detector was positioned either on-axis (A, C) or � 15 mm

off-axis (B, D) at two distances from the ion guide, 40 and 80 mm, as labelled in

the figure. The solid lines are used to guide the eye.

6

5

4

3

2

1

00 100 200 300 400

Pin (mbar)

P in

/ Pi

Fig. 15. Measured ratio of the ion guide and impact (stagnation) pressures as a

function of ion guide pressure. The inset shows the Pin=Pi dependence on the Mach

number according to the supersonic Rayleigh–Pitot formula (Eq. (7)).

M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) 24–3432

represents the first pressure maximum of Fig. 14 as it moves pastthe nozzle exit. The supersonic Rayleigh–Pitot relationship ofEq. (7) is used to estimate the jet Mach number from the Pin=Pi

ratio (see inset of Fig. 15). The deduced Mach number at thenozzle exit is 3.7–4.6 for 100oPino400 mbar. Using the isentropicrelations, similar to that of Eq. (3), we can calculate fundamentalparameters such as the jet temperature and density using thededuced Mach number. For an ion guide pressure of 300 mbar theMach number is � 4. The measured temperature within the gas cell(without discharge source) is 296 K, and therefore, the jet tempera-ture at the nozzle exit is 46 K. The corresponding jet density for anion guide pressure of 300 mbar is � 5� 1017 atoms cm�3. Forcompleteness, the static pressure in the jet has been calculatedusing Eq. (3) to be 3 mbar at the nozzle exit. This is in reasonableagreement with the static Pitot probe measurement.

One of the interests in the LIST project is to study thefeasibility of laser spectroscopy inside the gas jet for exotic nuclei.The LISOL group of the University of Leuven have performedstudies on the impact of the gas jet on the resonant linewidth of aspecific element and have compared this with pressure conditionsinside the gas cell and in a reference cell under vacuum [27]. Inaddition, using laser Doppler-shift velocimetry, the group wereable to determine the flow velocity of moving nickel atoms in thejet. For a longitudinal ionization geometry a supersonic velocity of560 m/s was quoted for an Ar gas jet. Similar studies are currentlyunderway at the IGISOL facility, JYFL.

In this work however, it is possible to obtain the gas jet velocityvia the deduced Mach number and the isentropic relationship [33]:

V2 ¼g�1

2M2 1þ

g�1

2M2

� ��1

� V2max: ð9Þ

Here, Vmax is the thermodynamic limiting velocity (the velocity theflow would reach at M¼1) and is a function of the gas celltemperature Tin, the ratio of specific heat capacities and the massm of the flowing gas. It is constant for a given expansion and canbe calculated using

Vmax ¼2gg�1�

kbTin

m

� �1=2

ð10Þ

where kb is Boltzmann’s constant. For argon gas, Vmax ¼ 555 m=s,and therefore, the jet velocity (at the nozzle exit and for a Mach

number of 4) is deduced to be 509 m/s. A direct comparisoncannot be made with the LISOL measurement as the nozzle in thiswork differs from the simple exit hole used in the work of Sonodaet al. [27]. However, a comparison will be made using the laservelocimetry technique in the near future.

6. Alpha decay recoil measurements

An important question to be answered is whether the visiblefluorescence is representative of not only the excited gas densitybut also neutral gas atoms. In order to provide a first insight anattempt was made to study the transport of particles within thegas jet. An a-recoil source of 223Ra was collected on to the tip ofan aluminium needle (� 5 mm long section) and mounted withinthe volume of the gas cell as shown in Fig. 5. The needle waspositioned approximately 10 mm from the exit of the gas cell,defined by a simple exit hole. 223Ra is a commonly used radio-active source for off-line IGISOL testing due to the suitablelifetimes of the daughter products 219Rn (3.96 s) and 215Po(1.8 ms). The source is originally collected as the granddaughterof the decay of a 200 kBq 227Ac (21.8 yrs) source [39].

The ions recoiling into the gas cell following the alpha decay ofthe source were transported out of the cell and carried into thevacuum chamber within the gas jet. A small silicon detector(15 mm diameter active area) was placed on a movable mountwithin the vacuum chamber and could be positioned at variouslocations within the previously observed gas flow region. Theprobability of particle detection at a given location within the gasjet is proportional to the gas-jet density at this point.

The normalized alpha recoil counts from four different detec-tor positions (A–D) are shown in Fig. 16 as a function of thebackground pressure. The carrier gas used in these measurementswas helium and the gas cell was operated at a fixed pressure of200 mbar. Close to and on the axis of the nozzle, an increase in thebackground pressure greatly improves the transport efficiency ofparticles as seen in detector position A. On the other hand,� 15 mm off-axis, where a wide gas jet is required to transportparticles to the detector (position B), the ion count rate is greatestfor the lowest background pressures. The situation is strikinglysimilar at a distance of 8 cm from the nozzle. Although only one

Fig. 17. Simulated contours of gas density in a plane parallel to the flow direction.

The colour scale covers the range 0�3� 10�3 kg/m3. Inlet pressure is 200 mbar

and the background pressure is set to 1.5 mbar. The nozzle on the right is the same

as in Fig. 4, nozzle B. (For interpretation of the references to color in this figure

legend, the reader is referred to the web version of this article.)

M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) 24–34 33

measurement was made at a background pressure greater than1 mbar, a consistent feature of apparent saturation of the a-recoilcount rate is seen with increasing pressure. A possible explana-tion is that for a pressure greater than 1 mbar, the gas jet width isless than the diameter of the detector surface. For detectorpositions B and D, the gas jet completely misses the detectorsurface at this pressure whilst with the detector in position A or C,the jet is entirely within the detector surface. A further test wasperformed with the detector on-axis at a distance of 12 cm fromthe gas cell, outside the observable range of the gas jet fluores-cence. Over a 2000 s integration time, the count rate for abackground pressure of 0.16 mbar was � 0:4 a=s whereas for abackground pressure of 1.1 mbar, the rate was � 2:2 a=s.

The results shown in Fig. 16 can be compared with the trend ofthe gas jet FWHM as a function of background pressure detailedin Fig. 9. For the Si detector on-axis, the alpha counts show a trendtowards saturation at background pressures greater than 1 mbar,from which we postulate that the gas jet diameter must be lessthan the 15 mm diameter active area of the detector. The gas jetfluorescence measurements show a direct correspondence withthe background pressure and above 1 mbar the FWHM is wellwithin the size of the silicon detector. We note, however, that thepresence of the detector will disturb the gas jet, and therefore, theinterpretation of the above results remains a question. Althoughthese measurements were not used in any quantitative analysis,they are useful in establishing some confidence in the ability torely on the fluorescence variations in the photographs as a truerepresentation of the gas jet density distribution.

7. Computational fluid dynamics simulations

Theoretical models, as presented in Section 2, must rely onmany simplifying assumptions to be solvable. Thus, such modelscan only provide an approximate guideline for the constructionand operation of a shaped nozzle required, for example, for theIGISOL LIST method. In the experimental conditions we deal witha three-dimensional flow of compressible gas including turbu-lence and temperature variations. A link between the theory ofsupersonic gas flow and experimental requirements for thestudies of gas jets can be established by the use of computationalfluid dynamics (CFD) simulations.

A key element in the application of CFD simulations to theoptimization of the IGISOL LIST method is finding the appropriateparameters of the gas flow model. Such a ‘‘calibration’’ of thecomputational model can be done by experimental study of gasflow as initiated for the IGISOL system by Rasi et al. [17]. Furtherdevelopment of those investigations has been presented in thepreceding sections. Comparison of experimental results to anoutcome of CFD simulations can serve as a test of the computa-tional model. Fig. 17 presents an example of the gas densitysimulated for helium flowing out from a converging-divergingnozzle for a pressure ratio of 0.75%. The figure can be qualitativelycompared to the experimental results as shown in Figs. 7 and 11.Moreover, Fig. 17 shows that with CFD methods it is possible toget information for pressure ratios which are beyond the currentpumping capacity of the gas jet measurement set-up. It is alsopossible to compute the pressure, velocity and temperature fieldsin a gas jet which are difficult to determine experimentally.

As described in Section 6, an a-recoil source can be used tostudy the transport of particles by gas flow. In particular, due tothe suitable lifetimes of the daughter products, one can estimatethe travel time of particles from the gas chamber to a Si detectorplaced after the nozzle. It is possible to simulate a mutualinteraction between a discrete phase (such as atoms of interest)and flowing gas, which is another way of comparing simulated

and experimental results. Such atom transport simulations werefeasible for the Warsaw IGISOL system [40] and provided evacua-tion times of the same order as experimental tests [41].

The computer model discussed here consisted of nozzle B asshown in Fig. 4 and an arbitrary cylindrical volume (diameter220 mm, length 350 mm) after the nozzle. The nozzle geometrywas imported from a CAD file used by the JYFL mechanicalworkshop to fabricate the actual nozzle for the gas jet measure-ment set-up shown in Fig. 6. The geometry of the whole modelwas created and meshed with the Gambit programme. Computermodelling of fluid flow in three dimensions was performed withthe Fluent software package [42].

A properly defined computer model can be used to investigateelements of the ion guide system which directly interact with theflowing gas. In this manner, their shapes and sizes can be optimizedwith the CFD simulations. In the IGISOL LIST method the simula-tions are ongoing and are being used to find an appropriate shape ofa nozzle which should result in a sufficiently compact jet such thatan optimum overlap with the laser beams and efficient transport ofions into the SPIG structure can be attained.

8. Conclusion and outlook

In summary we have studied the gas jet expanding from an ionguide as a function of different background pressure conditions andexit nozzle types. This work is motivated by the requirement for anoptimal overlap efficiency between atoms and counter-propagatinglaser beams for the gas-cell-based LIST technique. Qualitativeinformation has been obtained via visual observations madepossible through excitation, by electrical discharge, of both heliumand argon gas within a chamber. In order to represent changes ofgas jet intensity, reduced noise contour maps were made fromgreyscale photographs. Quantitative measurements of the gas jetdiameter were made at a fixed distance of � 2 cm from the ionguide by extracting intensity plots of the radial gas jet cross-section.

We have shown that the FWHM of the gas jet is ratherinsensitive to the use of a simple exit hole or converging-divergingnozzle, and also to the different operating gas cell pressures, 25 or56 mbar. On the other hand, the jet FWHM shows a strikingdependence on the background pressure, rapidly decreasing from� 40 mm at the lowest pressure (� 0:1 mbar) before leveling offafter 0.5 mbar, wherein the FWHM is � 6 mm in diameter. This isan important observation as the aperture of the repeller electrodeof the JYFL SPIG is currently 6 mm, and therefore, background

M. Reponen et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) 24–3434

pressures greater than � 1 mbar are required in order to effi-ciently couple the gas jet into the SPIG with minimal disturbance.

Increasing the background pressure as a means of reducing thegas jet diameter is far from ideal, resulting in possible dischargebetween electrodes of the ion guide system and reduction of gasflow intensity. In order to address this concern we have tested theuse of a Laval nozzle based on designs in the literature. The initialvisual observations are very encouraging. A gas jet � 6 mm indiameter has been photographed over a distance of 14 cm fromthe nozzle (approximately the length of the SPIG).

A more quantitative understanding of the Laval nozzle hasbeen achieved via a direct probe of the gas jet pressure distribu-tion on-axis as a function of distance from the nozzle. Despite thefact that such measurements are prone to misinterpretation, theresults clearly show the presence of non-uniform density waveswithin the jet which support the clear visual observations ofexpansion and compression waves. Observations of shock struc-ture are also strikingly evident within the expanding gas jet fromthe converging-diverging nozzle and are well understood. Thepressure measurements can provide important information on theMach number via the Rayleigh–Pitot relationship. We have usedour results in connection with isentropic solutions to show thatthe gas jet temperature at the exit of the nozzle is reduced by afactor of � 627 compared to room temperature gas cell, and thatthe gas jet density at the nozzle exit is reduced by a factor of � 16.

In the near future, studies with the Laval nozzle will continue byutilizing resonant laser ionization within the jet. Laser Doppler-shiftvelocimetry will be used to probe the velocity of heavy atoms alongthe jet. At the same time a careful study of the effect of the reducedtemperature and pressure on the resonant linewidth will be made,in a similar manner as has been performed in Leuven. Thepossibility of utilizing the gas jet environment for laser spectro-scopy of short-lived exotic nuclei is highly attractive and thecurrent studies of the geometrical properties of the jet are animportant development towards this goal. In addition, laser ioniza-tion provides a means of verifying our work utilizing the alpharecoil source, whereby the interpretation of the results are still anopen issue due to the disturbing influence of the Si detector.

In parallel, computational fluid dynamics simulations areongoing at the University of Warsaw. The current experimentalwork is driving the development and optimization of the computermodels, discussed in this article. Important properties difficult todetermine experimentally include the pressure, velocity and tem-perature fields. Such models are currently being used to determinethe conditions expected for the pumping capacity available in theIGISOL target chamber. Importantly, we expect that the CFDsimulations will guide the development of the shape of the Lavalnozzle such that a more uniform jet can be attained whilemaintaining the optimal geometry for future LIST applications.

Acknowledgements

This work has been supported by the Academy of Finlandunder the Finnish Center of Excellence Program 2006-2011(Nuclear and Accelerator Based Physics Program at JYFL). Numer-ical simulations have been done at the Interdisciplinary Centre forMathematical and Computational Modelling at the University ofWarsaw, Grant number G33-2.

References

[1] R.D. MacFarlane, R.A. Gough, N.S. Oakey, D.F. Torgerson, Nucl. Instr. and Meth.73 (1969) 285.

[2] A. Ghiorso, J.M. Nitschke, J.R. Alonso, C.T. Alonso, M. Nurmia, G. Seaborg,E.K. Hulet, R.W. Lougheed, Phys. Rev. Lett. 33 (1974) 1490.

[3] D.M. Moltz, J.M. Wouters, J. Aysto, M.D. Cable, R.D. von Dincklage, R.F. Parry,J. Cerny, Nucl. Instr. and Meth. 172 (1980) 519.

[4] J. Arje, K. Valli, Nucl. Instr. and Meth. 179 (1981) 533.[5] J. Aysto, J. Arje, V. Koponen, P. Taskinen, H. Hyvonen, A. Hautojarvi,

K. Vierinen, Phys. Lett. B 138 (1984) 369.[6] J. Arje, J. Aysto, H. Hyvonen, P. Taskinen, V. Koponen, H. Honkanen,

A. Hautojarvi, K. Vierinen, Phys. Rev. Lett. 54 (1985) 99.[7] J. Aysto, Nucl. Phys. A 693 (2001) 477.[8] K. Perajarvi, P. Dendooven, J. Huikari, A. Jokinen, V.S. Kolhinen, A. Nieminen,

J. Aysto, Nucl. Instr. and Meth. A 449 (2000) 427.[9] Yu. Kudryavtsev, B. Bruyneel, M. Huyse, J. Gentens, P. Van den Bergh, P. Van

Duppen, L. Vermeeren, Nucl. Instr. and Meth. B 179 (2001) 412.[10] T. Kessler, Development and application of laser technologies at radioactive

ion beam facilities, Ph.D. Thesis, University of Jyvaskyla, 2008.[11] J.D. Anderson, Modern Compressible Flow: With Historical Perspective, vol. 3,

McGraw-Hill Professional, New York, 2002.[12] Y.P. Golovachov, Numerical Simulation of Viscous Shock Layer Flows,

Springer-Verlag, 1995.[13] M. Jugroot, C.P.T. Groth, B.A. Thomson, V. Baranov, B.A. Collings, J. Phys. D 37

(2004) 1289.[14] M. Jugroot, C.P.T. Groth, B.A. Thomson, V. Baranov, B.A. Collings, J.B. French,

J. Phys. D 41 (2008) 025205.[15] V. Varentsov, Nucl. Instr. and Meth. A 317 (1992) 1.[16] V.L. Varentsov, A.A. Ignatiev, Nucl. Instr. and Meth. A 413 (1998) 447.[17] M. Rasi, R. Saintola, K. Valli, Nucl. Instr. and Meth. A 378 (1996) 251.[18] H.J. Xu, M. Wada, J. Tanaka, H. Kawakami, I. Katayama, S. Ohtani, Nucl. Instr.

and Meth. A 333 (1993) 274.[19] P. Van den Bergh, S. Franchoo, J. Gentens, M. Huyse, Y. Kudryavtsev,

A. Piechaczek, R. Raabe, I. Reusen, P. Van Duppen, L. Vermeeren, Nucl. Instr.and Meth. B 126 (1997) 194.

[20] P. Karvonen, I.D. Moore, T. Sonoda, T. Kessler, H. Penttila, K. Perajarvi,P. Ronkanen, J. Aysto, Nucl. Instr. and Meth. B 266 (2008) 4794.

[21] I.D. Moore, J. Billowes, P. Campbell, T. Eronen, C. Geppert, A. Jokinen,P. Karvonen, T. Kessler, B. Marsh, A. Nieminen, H. Penttila, S. Rinta-Antila,T. Sonoda, B. Tordoff, K. Wendt, J. Aysto, AIP Conf. Proc. AIP, Isle of Kos(Greece) 831 (2006) 511.

[22] P.V. Duppen, Nucl. Instr. and Meth. B 126 (1996) 66.[23] Yu. Kudryavtsev, B. Bruyneel, S. Franchoo, M. Huyse, J. Gentens, K. Kruglov,

W.F. Mueller, N.V.S.V. Prasad, R. Raabe, I. Reusen, P. Van den Bergh, P. VanDuppen, J. Van Roosbroek, L. Vermeeren, L. Weissman, Nucl. Phys. A 701(2002) 465c.

[24] T. Kessler, I.D. Moore, Y. Kudryavtsev, K. Perajarvi, A. Popov, P. Ronkanen,T. Sonoda, B. Tordoff, K. Wendt, J. Aysto, Nucl. Instr. and Meth. B 266 (2008)681.

[25] I.D. Moore, T. Kessler, T. Sonoda, Y. Kudryavtsev, K. Perajarvi, A. Popov,K.D.A. Wendt, J. Aysto, Nucl. Instr. and Meth. B 268 (2010) 657.

[26] K. Blaum, C. Geppert, H.J. Kluge, M. Mukherjee, S. Schwarz, K. Wendt, Nucl.Instr. and Meth. B 204 (2003) 331.

[27] T. Sonoda, T. Cocolios, J. Gentens, M. Huyse, O. Ivanov, Y. Kudryavtsev,D. Pauwels, P. Van den Bergh, P. Van Duppen, Nucl. Instr. and Meth. B 267(2009) 2918.

[28] R. Deloche, P. Monchicourt, M. Cheret, F. Lambert, Phys. Rev. A 13 (1976)1140.

[29] A. Bogaerts, R. Gijbels, J. Appl. Phys. 92 (2002) 6408.[30] R. Courant, K.O. Friedrichs, Supersonic Flow and Shock Waves, Applied

Mathematical Sciences, vol. 21, Springer, Berlin, New York, 1999.[31] The MathWorks, Inc., MatLab R2008b version 7.7.0, Natick, MA 01760-2098,

USA, 2007.[32] G. Sanna, G. Tomassetti, Introduction to Molecular Beams Gas Dynamics,

Imperial College Press, London, 2005.[33] D.B. Atkinson, M.A. Smith, Rev. Sci. Instrum. 66 (1995) 4434.[34] V.A. Souhanovskii, H.W. Kugel, R. Kaita, R. Majeski, A.L. Roquemore, Rev. Sci.

Instrum. 75 (2004) 4320.[35] R. O’leary, J.E. Beck, /http://www.pwrengineering.com/articles/nozzledesign.

htmS.[36] G. Dupeyrat, J.B. Marquette, B.R. Rowe, Phys. Fluids 28 (1985) 1273.[37] I.R. Sims, J.-L. Queffelec, A. Defrance, C. Rebrion-Rowe, D. Travers, P. Bocherel,

I.W.M. Smith, J. Chem. Phys. 100 (1994) 4229.[38] A. Iivonen, K. Riikonen, R. Saintola, K. Valli, K. Morita, Nucl. Instr. and Meth.

A 307 (1991) 69.[39] J. Huikari, P. Dendooven, A. Jokinen, A. Nieminen, H. Penttila, K. Perajarvi,

A. Popov, S. Rinta-Antila, J. Aysto, Nucl. Instr. and Meth. B 222 (2004) 632.[40] J. Kurpeta, M. Kasztelan, M. Gierlik, Z. Janas, W. Kurcewicz, A. P"ochocki,

A. Syntfeld, in: Poster Presented at the Fourth Joint Collaboration Meeting ofNIPNET, HITRAP and Ion Catcher, Munich, Germany, 26–28 May 2005.

[41] A. Wojtasiewicz, G. Canchel, A. Emsallem, M. Gierlik, Z. Janas, M. Kisielinski,W. Kurcewicz, J. Kurpeta, H. Lefort, S. Lewandowski, H. Penttila, A. P"ochocki,E. Roeckl, B. Roussiere, S. Sidor, A. Syntfeld, Nucl. Phys. A 746 (2004) 663c.

[42] /http://www.fluent.comS.