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0 5 10 15 20 25 30 35 40 45
Position [ mm ]
Wir
e C
urr
ent
[ u
A ]
Results
For the 1 mm solid wires, tungsten with the highest melting point
melted and broke when the total beam current exceeded 60 µA,
tantalum broke at 40 µA and stainless steel and copper at about 20
µA. Table 1 shows the melting and breaking points for various
water cooled tubes.
Methods
The experiments were carried out at the TPC CC18/9 cyclotron using
proton beams of 18 MeV. An U-shaped wire holder was used to
support a 1 mm diameter wire or 0.8 - 0.38 mm diameter tubes.
For wires, cooling was achieved by heat conduction from the
wire material to the wire holder for current measurement and by
thermal radiation.For the tubes a HPLC pump were used to force cooling water
through the tube and thereby make the cooling more effective.
Various materials were tested; tungsten, tantalum, stainless steel
(316L) and copper.
Wire scanner for beam profile of high current particle accelerator beams
Stefan Johansson 1, Per-Olof Eriksson 1, Johan Rajander 1, Jan-Olof Lill 1, Olof Solin 1,2
1 Accelerator Laboratory, Turku PET Centre, Åbo Akademi University2 Radiopharmaceutical Laboratory, Turku PET Centre, University of Turku
Introduction
Accelerator beam profile scanners are used to determine particle
beam dimensions and intensity distributions. The goal is in many
cases to achieve uniformity of the beam for radionuclide
production. The maximum proton beam current for a wire scanner is
typically low, a few tens of µA, limited by the beam-induced heating
destroying scanning wires. For some applications higher beam
currents are needed. Our goal was to develop a beam profile
scanner that can be used at currents beyond 100 µA..
Conclusions
Water cooling enhances the resistance of the tube against
breakage. High thermal conductivity is more important than a high
melting point. Obviously, the thinner the tube the better as less
thermal power is deposited in the tube. Thus, the thin-walled copper
tube was the best of the materials studied to date when
looking for high beam current resistance.
Figure 2. Thermal conductivity for various metals as function of melting points..
0
100
200
300
400
500
0 1000 2000 3000 4000
Melting point [ °C ]
Th
erm
al c
on
du
ctiv
ity
[
W/(
m·K
) ]
W
NbTi
SS
Ta
Ag
Cu
Au
Al
Figure 1. Light emission from a non-cooled Tantalum wire at different positions when it is
scanned through a proton beam. In this case the beam current is over the limit, but wire not
visually broken. The noise on top of the curve shows that material is close to meltingpoint
Material Outer diam. Wall Breaking Point
[ mm ] [ mm ] [ µA ]
Tantalum 0.80 0.15 85
Stainless Steel 0.80 0.31 55
Stainless Steel 0.80 0.28 40
Copper 0.75 0.26 90
Copper OFHC 0.38 0.08 >120 *
The wire or tube was moved through the beam at a speed of 2 mm/s
by a stepper motor driven mechanism, picture 2. Only vertical scans
were performed at this stage. Target-, collimator-, and wire currents,
water temperatures and pressure were monitored. The scanner unit
was mounted on the 40 mm diameter beam line at 15 cm from a
target. The beam was collimated to a diameter of 10 mm, and the
beam current target/collimator ratio was 70/30 %.
Figure 3. Setup for wire scanner. Two units are used in X-and Y directions
Stepper Motor
Cooling water vessel
Measuring tube with holder GearWater pressure
Flexible tubing
Linear feedtrough
Water feedtrough
Table 1. Melting and breaking points for water cooled tubes
Figure 4. Setup for wire scanner.
* Maximum current tested to date
Acknowledgment: Simo Vauhkala and Jimmy Dahlqvist are acknowledged for expert help at the workshop of Åbo Akademi University
Temperature