ICIS2015,Aug. 23-28, 2015, New York, USA Further improvement of RIKEN 28GHz SC-ECRIS for production of highly charged U ion beam T. Nakagawa (RIKEN, Nishina

ICIS2015,Aug. 23-28 , 2015, New York, USA

Further improvement of RIKEN 28GHz SC-ECRIS for production of highly charged U ion beam

T. Nakagawa (RIKEN, Nishina center)

1. Introduction RIKEN RIBF

2. Effect of magnetic field distributionBmin effectEffect of magnetic mirror

3. Highly charged U ion beam with sputtering methodConsumption rateBeam intensity

4. Summary


M. Nishida et al, PASJ 2015,FSP003

28GHz SC-ECRIS

Liquid He free SC-ECRIS(18GHz)

New 18GHz ECRIS( under construction)

18GHz ECRIS

RILAC

RILAC II

AVF cyclotron

SRC

IRC

fRC

RRC

~345MeV/u


10.8MeV/u 50MeV/u 345MeV/u0.6MeV/u3.2keV/u

35+ 35+ 65+ 65+ 86+

He gas stripper C stripper

Efficiency ~15% Efficiency ~30% Efficiency ~4.5%

U beam

U,Xe beam

O, Ca, Kr beam

Due to low transmission efficiency ( ~4.5%), we surely need the intense beam production form the ECRIS

Construction of the New SC-ECRIS(2009~ )



28GHz SC-ECRIS　 (SS chamber)

18GHz ECRIS28GHz SC-ECRIS(Al chamber)

ICIS2013 ICIS2015RIKEN 28GHz SC-ECRIS

RILAC II

G. D. Alton and D. N. Smithe, Rev. Sci. Instrum. 65, 775 (1994).



ECRIS-2014, Aug 24-28, Nizhny Novgorod

1. Minimization of the consumption rate

we need long term continuous beam production (1~2 montns) for RIBF experiment. To meet this requirement, even if we can install 10gr of metal U, the consumption rate should be lower than ~7 mg/h.

2. Optimization of the magnetic field distribution

The optimization of the magnetic field distribution is important task to increase the beam intensity. For this reason, we intensively searched the optimum distribution to maximize the beam intensity.

2015~ Increase the beam intensity for RIBF


Magnetic field distribution Binj, Bext, Bmin, Br

Mirror ratio Binj/Bmin……

Microwave power absorptionBmin

Microwave power and frequency

Gas pressure

Geometrical effectchamber sizechamber shape

Becr

Key parameters of ECR ion source

Key parameter of ECRIS


Bmin effect

(Bmin)opt


Bmin (microwave absorption)　 gas pressure effect

Gas pressure was tuned to maximize the beam intensity

Gas pressure effect ?

Ar11+ ion beamLiquid He free Sc-ECRIS(18GHz)Binj~1.87T, Br~1.1T Bext~1.2T, RF~400W, Vext=10kV

When the gas pressure was tuned to maximize the beam intensity at each Bmin, (Bmin)opt was ~0.48 T, which is almost same as the value shown above figure


Bmin ( U35+ ion beam) 28GHz microwaves

RF power effect and X-ray heat load Charge state

The beam intensity of U35+ ions and X-ray heat load as a function of Bmin at two different RF power (~1.5kW and ~1kW), respectively. Binj, Bext and Br were ~3.1, ~1.75 and ~1.88T, respectively. (Bmin)opt was ~0.65T for both RF powers. The X-ray heat load increased with increasing the Bmin, which is mainly due to the magnetic field gradient at ECR zone. It was saturated at Bmin~0.7T, even though the magnetic field gradient become gentler.

RF~1.5kW

~1.0kW


Bmin ( U35+ ion beam)(summary)

Figures show the summary of the (Bmin)opt for several heavy ions with 18 and 28GHz. In case of 18GHz, (Bmin)opt was ~0.5T, which is ~0.8Becr. For 18 GHz microwave operation, Binj, Bext and Br were fixed to 2.3, 1.19 and 1.2T respectively. On the other hand, (Bmin)opt with 28GHz was ~0.65T, which is slightly increased with increasing the charge state. It is correspond to the ~0.65Becr, which is lower than that with lower frequency of 18GHz in this experiment.

18GHz 28GHz

RIKEN 28GHz SC-ECRIS


Magnetic mirror (Br/Bmin, Binj/Bmin, Bext/Bmin)

It is obvious that Binj, Bext and Br work as a part of the magnetic mirror to confine the plasma. In the mid 1990s, so-called “High B” mode, which basically gives high magnetic mirror ratio to confine the plasma, was proposed to increase the beam intensity of highly charged heavy ions Many laboratories adopted this empirical formula to design the ECR ion source.Using it, they successfully increased beam intensity of highly charged heavy ions.

28GHz U35+

Bext~1.75T ~1.45T


Binj/Bmin Bext/Bmin

𝑉⊥

V//

𝑉⊥

V//

Binj/Bext

Magnetic mirror

Binj/Bmin~Bext/Bmin

Binj/Bmin>Bext/Bmin

Particle flow


Br effect

The beam intensity increases with increasing the Br and saturated at certain Br. The saturation point increases with increasing Bext. Figure shows the bema intensity as a function of Br/Bext. Very roughly speaking, the beam intensity is saturated at Br/Bext~1.2

(Br)thresh ~1.2Bext

18GHz Bmin~0.5T

Xe20+, 18GHz


(Br)thresh ~1.2Bext

Br effect

To investigate this effect for 28GHz operation, we measured the U35+ ion beam as a function of Br. To measure it in a wide range, Bext was fixed to 1.42T. The beam intensity increased with increasing Br and saturated at Br~1.2Bext, which is same tendency for 18GHz operation.

28GHz

Bmin~0.5T


Loss cone ( )Br

BminLoss cone ( )Bext

Bmin

> Loss cone ( )Br

BminLoss cone ( )Bext

Bmin

<

Br effect

At lower Br/Bext (<1), the loss cone size at Br side is almost same as that at Bext side. Therefore, the plasma confinement and ion flow is comparable for both side. When increasing Br, the loss cone at Br side becomes smaller. At higher Br/Bext (>1.2), Bext may govern the plasma confinement, because the loss cone at Br is smaller than that at Bext. For these reasons, the beam intensity is saturated.

𝑉⊥

V//

𝑉⊥

V//


Br effect

The magnetic field gradient is strongly dependent on the Br. At higher mirror ratio, the electron energy may not be high enough to produce highly charged Kr ions. Therefore, the beam intensity decreases with increasing the Br.

(x10-4)

Kr18+


Binj effect

Bext~1.75TBext~1.45T

(Binj)thresh(Binj)thresh

The beam intensity increased with increasing Binj and became constant above Binj/Bext~1.6


Plasma chamber

Biased disc

U-rod

Support rod(water cooled)

To obtain the consumption rate, we measured the total amount of consumption and total sputtering current for long term at fixed sputtering voltage. We obtained that the consumption rate was ~1mg/h for 1mA of sputtering current at the sputtering voltage of -2kV.

Rconsumption~ f( Ion, Vsputter )Isputter

U production (sputtering method)


U production (sputtering method)

To obtain the consumption rate, we performed long-term measurements of the total amount of material consumption and total sputtering current, at fixed sputtering voltage. For example, we obtained the consumption rate of ~1 mg/h for 1 mA of sputtering current (ion current + current due to the secondary electron emission) at the sputtering voltage of -2 kV. Consumption rate strongly depended on the sputtering voltage3) and was proportional to the sputtering current at fixed sputtering voltage.

𝑓 (𝐸)∝𝐸1 −√(𝐸𝑏+𝐸) / Λ 𝐸1

(𝐸+𝐸𝑏)3

Eb: binding energyE1: incident energy of ions

Energy of sputtered particle

Energy of sputtered particles decrease with decreasing the incident energy of the ions

M. W. Thompson, Philos. Mag. 18, 377 (1968).


U35+ ion beam production

201emA(~2.6kW)

Binj ~3.11T,Bmin ~0.62T Bext ~1.78T Br ~1.87T RF 18+28GHzVext ~22kVWX-ray ~2.8W


Summary

1. 200euA of U35+ was produced at the RF power of ~2.6kW and the magnetic field strength lower than the ordinary High-B mode operation for 28GHz.

2. The consumption rate of metal U was dramatically reduced from ~8.6 to ~2.8mg/h with sputtering method to produce intense beam of U 35+ (~150euA). Based on these results, we successfully produced stable intense beam of U35+ ion in RIBF experiment for long term (more than one month) with low material consumption this year.

3. The beam intensity of highly charged heavy ions was saturated at Binj>1.6Bext and Br>1.2Bext for U35+ ions in a wide range of Bext.

4. The optimum value of Br to maximize the beam intensity is strongly dependent on Bmin in a certain condition

5. Optimum value of Bmin for maximizing the beam intensity was dependent on the microwave frequency and the gas pressure.

Documents

ICIS2015,Aug. 23-28, 2015, New York, USA Further improvement of RIKEN 28GHz SC-ECRIS for production of highly charged U ion beam T. Nakagawa (RIKEN, Nishina