-2

0

2

4

6

8

10

12

14

16

18

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