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Submitted on 1 Jan 1989
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AN ECR ION SOURCE FOR RADIOACTIVE BEAMS
AT TRIUMF
P. Mcneely, G. Roy, J. Soukup, J. d’Auria, L. Buchmann, M. Mcdonald, P.
Schmor, H. Sprenger
To cite this version:
JOURNAL DE PHYSIQUE
C o l l o q u e C1, s u p p l e m e n t au n O l , T o m e 50, janvier 1989
AN ECR ION SOURCE FOR RADIOACTIVE BEAMS AT TRIUMF
P. MCNEELY, G. ROY, J. SOUKUP, J.M. D'AURIA*, L. B U C H M A N N * " , M. MCDONALD*
,
P.
W. SCHMOR* * and H.
SPRENGER*University of Alberta, Edmonton, Alberta T6G 2N5, Canada
* ~ i m o n Fraser University, Burnaby, British Columbia V5A 1N6, Canada **TRIUMP, 4004 Westbrook Mall, Vancouver, BC, V6T 2A3, Canada
Abstract
The TRRTMF/University of Alberta Ion Source is cumently undergoing its final consauction phase in preparation for rigorous off-line testing. The design uses a standard samarium-cobalt/copper coil combination to produce the required radial and axial magnetic fields whose calculated values were confirmed in recent magnetic field surveys. The source can operate at RF frequencies between 6.6 and 10.5 GHz. AU major elements of the system have been assembled with extraction of a beam expected in early August. Using an analyzing magnet and a precision gas leak, accurate measurements of the ionization efficiency of the source will be canied out. It is planned to test the source under a varity of conditions. These test are expected to be completed by the end of the year. The results of these tests will be used as a basis for the fmal design work that must be done before the source can be put on-line to produce radioactive ion beams at TRIUMF
One of the goals of the TISOL (Test Ion Source On Line) facility at TRIUMF is to produce intense beams of radioactive ions from a varity of sources. Since the amount of radioactive material produced by the target can be quite small any source used must be highly efficient. The source must also be able to operate in a high radiation field since it will be within 0.5 m of the 500 MeV proton beam, and it should require minimal amounts of maintence. The source which fits these requirements best is an ECR ion source. For this application we designed a single stage, variable frequency source based on the Karlsruhe design 111. At the time of this paper the source has been assembled in an off line mode and beam has been extracted from it for some four days.
Design:
The conceptual design of the source has been previously presented at Julich 121, and our initial progress was reported at East Lansing, Michigan 131. An assembly drawing of the completed source is shown in fig. 1. The source can be run at a microwave frequency of either 6.6 or 10.5 GHz which gives for the 10.5 GHz case a resonance of 3.572 kG. The microwaves enter the RF cavity radially and are fed into the plasma volume via slots between the poles of the hexapole. At this time we are operating at a frequency of 10.5 Ghz and a maximiurn RF power at the amplifier of
200 Watts.
The plasma volume is contained within two quartz tubes which are mounted coaxially in the bore of the hexapole. The outer quartz tube forms the vacuum seal while the inner quartz tube contains the plasma. There are two reasons for this, the first is that should the inner tube become radioactively contaimenated it could be easily replaced. The second is to thermally insulate the hexapole from the hot plasma. The dimensions of the outer quartz tube are 48 mm dia. x 195 mm, while the inner tube is 35 mm dia. x 175 mrn.
The axial magnetic field is produced by two sets of four copper, hollow core, water cooled pancake solenoids surrounded by an iron field clamp. For a mapping of the magnetic field see fig.
2. The coils are operated at currents of 360-380 A to produce a peak field of approximatly 4 kG with a mirror ratio of 1.3. The radial field is produced by a Sm2CoI7 hexapole magnet. This type of samarium-cobalt was chosen due to its radiation hardness. The permanent magnet is cooled internally by blowing compressed air through cooling holes in the magnet housing. If necessary water cooling could be easily connected. A map of the measured axial magnetic field of the hexapole is shown in fig. 3, it was not possible to measure the radial component due to the small size of the magnet's bore.
C 1-808 JOURNAL DE PHYSIQUE
The beam was extracted through a 2 mrn dia. hole by a grounded puller electrode. The hole in the puller electrode is 9 mm dia. Next the beam enters an einzl lens. The whole puller electrode1 einzl lens assembly can be moved remotely to give a range of gap size from 6 to 50 mm. Unfortunatly the puller electrode was not optimised to the Pierce geometry a factor contributing to a highly divergent beam at extraction and hence a low beam transmition.
Results:
A schematic diagram of the layout of the experimental system assembled to test the source is shown in fig. 4. The canier gas flow rate is set by a digital flow controller while the test gas is provided by a standard calibrated leak. The extraction voltage can be varied to a maxirnium of 20 kV, and the einzl lens can be set to a maximium of 15 kV. Both the forward and reflected power of the RF amplifier can be measured The current falling on the various faradhy cups can be measwed either directly on a Kiethly electrometer
orfe
Shown bebw is the results from one run, along with the parameters of the source for that run.Table
Source parameters of off-line test during which the data in fig. 5 was collectedGas: hydrogen (support gas-flow of 6.67x104 atm-ccls)
+
nitrogen (leak-flow of 6 . 2 2 ~ 1 0 - ~ atm-ccls)RF power: 136 W EE Energy: 5 keV
Extraction Hole Diameter: 2.0 mm Extraction Electrode Separation: 11 mm Total Current Extracted: 1 mA
Shown below in fig.5 is a graph of the current measured at the faraday cup after the analysing magnet versus the bending magnet current: the probable species is also indicated. Where no species is indicated it was not possible to identify the species due to ambiguous mass to charge ratio.
0 200 400 6 0 0 800 1000
Bending Magnet Current (arbitrary units)
Current in the second faraday cup versus the current in the analysing magnet
Based on these results we can give some tenitive efficiencies for the source. These efficiencies have been partially corrected to compensate for the poor transmition which was caused by the highly divegent beam at extraction. Below is a table which shows our efficiencies for the gases studied so far.
Table 2: Present efficiencies of the ECR Ion Source under conditions such as those listed in table 1
Future Develo~ements;
Since at the time of the conference the source had only been producing beam for four days much more remains to be done. The efficiency of the source will be measured for argon, helium, neon, and again for hydrogen and nitrogen. In addition helium will be tried as a support gas. Extensive efforts will be made to optimize the source and to determine the effect of RF power, extraction gap, extraction hole size, extraction voltage, gas mix, etc. on the sources efficiency. In addition based on discussions at the conference /4/ an extensive redesign of the extraction system will be camed out, which will hopefully remove the problem of our low transmition. The source is intended to begin its on-line tests on TISOL early in 1989.
References;
/I/ Bechthold, V., H. Dohrmann, S.A. Sheikh, The 7th Workshop on ECR Ion Sorces, Ed. H. Beuscher, Julich (1986) 248
121 Buchmann, L., M. Dombsky, J.M. D'Auria, M. McDonald, E. DeVitta, H. Schwider, A. Otter, H. Sprenger, P.W. Schmor, The 7th Workshop on ECR Ion Sorces, Ed. H. Beuscher, Julich (1986) 177
f3/ Roy, G., International Conference on ECR Ion Sources and their Applications, Michigan State University (1987) 466
This graph shows a cornparision between the mapped magnetic field (back diamonds) and the theoretical curve predicted by the POISSON program at R=O.O cm. The
ECR
resonance condition is shown as a horizontal line at 3572 G. The center of the solenoid is at the axial position of 0.
-
4000-
-
% 3000-
=
0 0 )-
N mAxfal Position (in)
2000
-
Fig& A graph showing the mapped magnetic field of the solenoid versus the axial position at a radius of 0.6 from the center line.
4
1 0 0 0 ,
.
I.
,
.
1.
I.
,
.
-15 -10 -5 0 5 10 15
