This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Brown, R.

Derenchuk, V.

Ellison, T.

Friesel, D.

Hicks, J.

Jenner, D.

Pei, A.

Petri, H.

Schwandt, P.

Sowinski, J.

Wedekind, M.

English

IUScholarWorks

IUCF HIGH INTENSITY POLARIZED ION SOURCE 
M. Wedekind, R. Brown, V. Derenchuk, T. Ellison, D. Friesel, J. Hicks, 
D. Jenner, A. Pei, H. Petri, P. Schwandt and J. Sowinski 
Ind iana  Univers i ty  Cyclotron Facility, B loomington ,  I N  47408 
I. INTRODUCTION 
We have completed approximately nine person-years of work on the high intensity po- 
larized ion source (HIPIOS), or 45% of the total project. Due to manpower and operating 
budget limitations, we determined last fall that continuing with the ANAC ECR instal- 
lation was not compatible with finishing HIPIOS, the high voltage terminal (Terminal C) 
and the injection beam line (BLlC) on a reasonable schedule. Consequently, in October 
we redirected our project efforts toward HIPIOS. We are allowing the schedule for the 
Terminal C and BLlC projects to slip along with construction of the new building addi- 
tion in order to concentrate on completing HIPIOS testing. The NSF has awarded IUCF 
approximately $1M of the $1.3M HIPIOS/BLlC grant. The third and final disbursement 
is expected next month. 
11. ATOMIC BEAM SECTION ASSEMBLY AND TESTING 
We have started assembling the source (Fig. 1) in the south end of the Cooler building 
for off-line testing where it will remain until Terminal C is installed in the new building 
addition. Assembly of the dissociator vacuum chamber and vacuum system is complete. As 
of this writing, atomic beam velocity measurements are in progress. Over the summer we 
will perform a series of tests to look at the flux density of the cold atomic beam without 
the sextupoles, with the sextupoles in place but not energized and with the sextupoles 
energized. We will then run the same tests with the RF transition units in place. 
A. Dissociator 
Measurements of the atomic beam velocity without sextupoles have been made using 
an atomic beam chopper generously loaned to IUCF by Professor W. Gruebler of ETH. 
Results indicate that with an Hz flow rate of about 20 SCCM, and a nozzle temperature of 
25 OK, 30 OK and 40 OK, the atomic beam velocity was consistent with a beam temperature 
of 35 OK, 42 OK and 54 OK respectively (Fig. 2). 
There continues to be a build up of white powder in the cold nozzle and a milky film 
formed on the dissociator tube during operation. Replacing the original pyrex tube with 
a quartz one has not alleviated the problem. We are currently installing an LN2 zeolite 
trap on the hydrogen gas line to remove impurities. 
B. Sextupoles 
Both sextupoles have been mapped and are ready for installation. We increased 
the length of the first sextupole by 50% (from 10 cm to 15 cm) from previous designs 
to provide the increased integrated sextupole strength recommended by TUNL. The ray 
Hor./Vert. Steer- 
+10 kV Elnzel Len 
-40 kV Ehzel Lens 
Weak Fleld RF Transltlon 
Strong fleld RF Transltlon 
- 
I- 1 Ft. 4 
HIGH INTENSITY POLARIZED ION SOURCE 
Figure 1. High Intensity Polarized Ion Source 
Nozzle 
Nozzle 
Nozzle 
0 250 500 750 1000 1250 1500 
VELOCITY m/r 
Figure 2. Atomic beam velocity for Hydrogen measured with nozzle temperatures of 25 OK, 
30 O K ,  and 40 OK. Relative intensities between curves is arbitrary. 
trace code predicts an optimum integrated field of 105 kG-cm for the first sextupole; the 
actual integrated field is 112.7 kG-cm. We have measured a maximum pole tip field for 
the first sextupole of 7600 G at the design current of 250 Amps, and 5600 G for the second 
sextupole at the design current of 200 Amps. 
C. Rf Transitions 
The rf transitions, which convert the atomic polarization to nuclear polarization, uti- 
lize the conventional adiabatic fast passage principle causing transitions between particular 
magnetic substates. One strong field and weak field transition pair is located between the 
sextupoles and another pair between the second sextupole and the ionizer. 
Each strong field transition consists of a magnetic field transverse to the beam direc- 
tion (85 G for deuterons, 150 G for protons) and an rf magnetic field (330 MHz, 400 MHz, 
460 MHz for deuterons and 1485 MHz for protons) generated by a cavity. Each weak field 
transition consists of a magnetic field of 10 G transverse to the beam direction and an 
rf magnetic field of 15 MHz generated by a small coil with its axis along the beam. The 
strong field transition will be shielded from the fields of the sextupole magnet by a 1.6 mm 
thick iron field clamp. The weak field transitions are located inside a cylinder of magnetic 
shielding to prevent interference from other magnetic fields. 
The mechanical design of the cavities follows the design of Robinson e t  al.' For driv- 
ing the cavities, we opted to change from a medium power signal generator to a self- 
oscillating loop utilizing the cavities self-resonance and a high gain amplifier. Assembly 
of the 330 MHz and the 400 - 460 MHz dual unit has been completed and tested for 
resonance, quality factor and vibration characteristics. Teflon shims have been inserted 
between the cavity elements to reduce vibration. The driving ports of the 330 MHz and 
the 4001460 MHz dual frequency cavities have been matched to the 50 Ohm feeding cable 
impedance and power tests have been performed to determine thermal stability. After 
phase adjusting the output of the cavity with the input we were able to operate from 500 
milliwatts to over 5 watts. Data taken thus far show we have approximately 10 G per 
square root of rf power(W). 
Both strong field rf transition magnets have been built and mapped. Difficulties with 
non-uniformity of field in the strong field magnets of the TUNL design have been solved 
by adding Rose shims to trim the field while keeping the pole tips short. The weak field 
transitions are currently being fabricated. 
111. ECR IONIZER 
The HIPIOS ECR ionizer design closely follows the TUNL design2 which has the 
hexapole magnet mounted within a vacuum chamber on which the external mirror solenoids 
and return yoke are assembled. We have, however, made several modifications to the basic 
configuration to address special requirements for our application, accommodate competing 
technical requirements, and incorporate improvements resulting from source commissioning 
work at TUNL. 
A primary technical consideration involves the design and mounting of the permanent 
magnet (SmCo) hexapole, which can be damaged by plasma or beam heating above 300 "C 
or by exposure to the hydrogen gas in the ionizer vacuum chamber. An ECR chamber pres- 
sure less than 0.1 pTorr is required. A further complication is that the internal structure 
(hexapole magnet, pre-buncher, and extraction electrodes) of the ECR ionizer is isolated 
from and operated at 20 kV above platform ground to provide an extraction gradient. This 
allows the majority of the ion source apparatus to remain at ground potential, and also 
simplifies the pre-bunching of the low energy dc beam from the ionizer for later injection 
into the IUCF cyclotrons. 
To meet the competing requirements of vacuum, temperature, rf and high voltage con- 
straints on the mounting of the hexapole structure, it was decided to vacuum encapsulate 
the permanent magnets in thin walled (0.5 mm) stainless steel cans welded onto thicker 
ring brackets to form the hexapole field configuration. The magnets are inserted into the 
welded structure following fabrication, mapped and shimmed to minimize other multipole 
field components, and then epoxy impregnated. This structure is supported from one end 
of the ECR vacuum chamber by six 2.5 cm diameter ceramic insulators. A separate freon 
cooled copper heat shield is mounted inside the hexapole structure to conduct the heat 
generated by the plasma and rf (5  150 watts) away from the magnets. This overall design 
provides for an open geometry permitting good radial and axial pumping in the region of 
the ECR plasma. 
The rf generator for the ECR plasma will operate over a range of 2.6 to 4.7 GHz at 
150 watts. The system will consist of a voltage controlled oscillator based on an Avantec 
HTO-2600 oscillator and AGT-8235 variable gain preamplifier. Final amplification of the 
microwave system is provided by a Keltec TWTA 200 watt amplifier. Power will be fed 
into the ECR chamber via Heliax coaxial cable to an internally mounted horr, using a 
waveguide adapter at the chamber. We expect to achieve higher beam intensities and 
polarizations at the higher frequencies, as reported by the TUNL group. 
Optimization of the ECR beam extraction, focusing and acceleration geometry for 
minimum beam phase space and maximum transmission of protons has been performed, 
with the invaluable help of T. Antaya, using the ray trace code BEAM3D specifically 
designed for ECR extraction at Michigan State University. 
The ECR vacuum chamber and hexapole mounting structure have been fabricated, 
and we are currently assembling the hexapole for mapping. Fabrication of the remainder 
of the ECR assembly should be completed by the end of the month. 
IV. BUNCHERS 
A prototype of the wideband, ramp-waveform buncher to be incorporated in HIPIOS 
was built and tested at low power by Lars Hermansson, a visiting engineer from The 
Swedberg Laboratory in Uppsala, Sweden. In order to compress 90% of the DC beam 
from the source into the f 3" cyclotron phase acceptance, this buncher should have a 
linearity of f (30/n)% of the voltage amplitude over 90% of the fundamental rf period, 
where n is the pulse-selection ratio (n = 1,2,3) (i.e. faUnch,, = fcyclot,o,/n). A buncher 
with this linearity specification should ideally produce a beam phase width of less than 
f 30' at the entrance to the resonant buncher in BLlC, which always operates at fcy clotron, 
and the nonlinearity resulting from the sinusoidal waveform of this buncher should result 
in less than f lo beam phase width at the cyclotron (neglecting all other effects). The test 
results are compared with the design specifications in Table 1. One can see that, for the 
most part, the specifications have been met or exceeded. 
A mechanical design for this buncher has been completed. The length of buncher 
has been increased from 0.25 to 0.35 m so that the buncher power amplifier rating can be 
reduced from 1000 to 500 W. The electrical tests will be repeated after fabrication of the 
final unit. 
The charge to mass ratio, &/A,  of beams from the HIPIOS will vary by a factor 
of at least four. For the BLlC resonant buncher, the buncher length which optimizes the 
transit time factor varies by more than a factor of 2 112 as a function of &/A and cyclotron 
frequency. Consequently, a buncher optimized for beams with &/A = 1 would be totally 
ineffectual for beams with &/A = 4. A compromise buncher length would increase the 
amplifier power requirements by a factor of three. A second alternative would be to have 
two bunchers of different lengths. The third option, which we are currently exploring, is 
to have a single buncher with a variable length drift electrode. 
Table I 
Linearity Measurements as a function of frequency. 
V. TERMINAL C BEAM TRANSPORT 
h 
n 
1 
2 
3 
Design of the optics for transporting the beam from the source to the acceleration 
column has been completed (Fig. 3). The magnetic steerers have been assembled and are 
awaiting mapping. A prototype einzel lens has been installed in Terminal A for testing, 
and a prototype slit assembly is currently being fabricated. Designs for the electrostatic 
channel, vertical translator magnets and spin rotation solenoid will be completed over the 
summer. 
VI. TERMINAL 
Linearity 
(5  %) 
Detailed design work awaits the availability of the mechanical design staff, but a 
number of studies and tests are in progress. These include high voltage testing of a 
commercial ceramic support leg and a G-10 insulated drive shaft for AC power, research 
on a suitable drive motor for the insulated drive shaft, determination of filter needs for 
Specification 
5 
10 
15 
Fractions of Period 
(%) 
Achieved 
5 
5 
10 
Specification 
90 
90 
90 
Achi~ved 
91 
88 
81 
90' Spherical 
Electrostatic Channel 
1~~~1 / 
I 
Source exit 
Emittance Scanner . , . 
slits --! 
245: 20' Vertical Translator 
\ I  I 
H 8 V Electrostatic Steerers 
Spin Rotation Solenoid 
Einzel Lens --/ I I 
Terminal Wall 
A c c e l e r a t o r  Column 
Figure 9. Terminal C Beam Transport 
the AC power lines and design of new Voltage-graded water lines for supplying de-ionized 
water to the terminal through the 600 kV potential. 
VII. BEAM LINE 1C 
The BLlC design can be divided into two principal parts: the bunching and transfer 
of beam from the terminal to the cyclotron and matching the beam from the terminal to 
the cyclotron. 
The beam line is composed of 180" betatron phase advance sections between each 
45" dipole. This design automatically leads to alternating sections of high and vanishing 
dispersion. The high energy resonant buncher will go in one of the two zero dispersion 
sections. This symmetry also suggests a natural ordering of the steering dipoles and beam 
position measurement system: both steering magnets and position monitors are located 
modula 90" in betatron phase advance. This placement makes both local beam movement 
and steering simpler for the operator. Although we plan to have the computer tune the 
beam line using feedback from the beam diagnostic systems, we will also incorporate auto- 
matic display of the position monitor 90' downstream if the operator selects an individual 
st eerer. 
In addition to beam position monitors, there are wire scanners allowing measurement 
of the beam emittance at the beginning of the line so that the first four quadrupoles can 
be properly set, as well as another emittance measurement system downstream to check 
that the matching has been properly done. Radio frequency nonintercepting pickups are 
extensively employed. Besides being used to measure the beam position, various pickups 
will be used to provide automatic phase feedback to both the terminal buncher and high 
energy buncher. In addition, high bandwidth longitudinal pickups are being developed to 
allow realtime viewing of the beam bunching (i.e., viewing of the longitudinal focussing of 
the beam). 
Design of the beam line optics using TRANSPORT has progressed far enough to 
establish many of the basic features of the line, including the number of quadrupole magnets 
(21) and the number of magnetic steerers (22). The dipole magnet count remains at six. 
Magnetic designs of the BLlC dipole and quadrupole magnets are complete, including 
preliminary coil designs and a general description of the power supplies required. 
In order to complete the optics calculations, we mapped the six dimensional shape of 
the cyclotron system acceptance at various points. This will allow us to better match the 
beams from the terminals to the injector. Attention is now focussed on beam bunching and 
the diagnostics described above, as well as developing a potential pencil beam scanning 
system for optimizing beam transmission to the cyclotron. 
VIII. CONTROL SYSTEM 
We decided not to use the existing cyclotron control systems for HIPIOS since their 
limited functionality and numerous obsolete components placed limits on the proposed 
expansion. Instead, we are implementing a new system that will operate in parallel with the 
present controls and provide a platform for future facility expansion. A detailed description 
is included elsewhere in this report. 
1. H. G. Robinson, e t  al., Nucl. Instrurn. Methods, A278, 655 (1989). 
2. T. B. Clegg, U. Konig, P. A. Schmelzbach and W. Griiebler, Nucl. Instrurn. Methods, 
A238, 195 (1985). 


IUCF High Intensity Polarized Ion Source

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IUCF High Intensity Polarized Ion Source

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