PRESENT STATUS OF UTTAC TANDEM ACCELERATOR
Seiji Seki
Tandem Accelerator Center, University of Tsukuba
Sakuramura, Ibaraki 305 Japan
ABSTRACTThe pelletron 12UD has been reliably operated up to11 MV on terminal" The highest terminal voltage available in experiments is limitted by deconditioning of a
few accelerator tube sections. These tube sections canbe conditioned up to above rated voltage gradient by applying voltage to 2 or 3 units alive.
Low beam transmission of the polarized ions is considered due to misalignment of the accelerator tube.
Alteration of the beam injection system is now in progress.
INTRODUCTIONSPUTTERION SOURCE

During the year from August 1, 1983 to July 31,1984,
the pelletron 12UD had been running quite reliably withmany experiments up to 11 MV on terminal. Charging
chains were running for 3612 hours with beam available
for 3260 hours. The tank was opened only twice for scheduled maintenance. All the research experiments wereproceded exactly on scheduled machine time. Major maintenance works at the tank opening were replacement ofstripper foils and cleaning or replacement of corona
points used as a means for voltage division along thetube and column posts as opposed to the use of resistors.

1400

1000

0~1--""-'
UJ

800

UJuu

c:(600:::;:c:(UJc::l400
20000pd4He
7L.
9 1
12Be
l6c
19°F
28s·32 s 1
35cl
37cl
63cu
93
127Nb
I
TOTAL

Beam Hours
58
3464
1628
270
35
99
69
362
128
55
17243
127
31
17233260

Use of the polarized beam, particularly polarized
deuteron beam showed gr·eat percentage. It is a prominent
feature of our laboratory that the beams of vector andtensor polarized deuterons with any spin direction areavailable with a combination of tandem and a Lamb-shiftpolarized ion source with an rf spin filter.
A heavy ion post accelerator of interdigital H structure linac has been installed in one of the beam linein the target room. Details of this booster linac is
described in a separate paperlo
A General Ionex Corporation (GIC) Model 860 sputter
ion source was purchased and is being tested.

As shown in Figo2, each of the low energy(LE) andthe high energy(HE) column of the pelletron 12UD is composed of 12 modular units, each of which contains threetube sections and nominally holds lMV. As mentioned above
the accelerator was running at the terminal voltage up
to llMV, slightly lower than the nominal 12 MV. The highest available voltage is limitted by deconditioning of afew tube sections in the LE column adjacent to the terminal. Operation up to 11 MV is very stable so that almost no conditioning operation is needed in routine operation. Terminal potential easily goes up to this value
within 30 min from starting. Even at the first operation
after the tank opening,this voltage can be realized bysix hours.
Vacuum discharge in the tube and dust in insulatinggas (SF6) are possible causes of accelerator sparks.
Since dust in the pelletron is much less than in an ordinary Van de Graaf which uses a belt charging system,
effect of dust is less important provided that carefulcleaning is made when the tank is opened. Dust induced
sparks were often observed at the first voltage elevation
after the tank opening. Several radial sparks occured at
the terminal voltage apparently lower than the tube conditioning threshold. However, these sparks caused no deconditioning. Sparking voltage went up gradually at eachsparko"":::>

c:(IonspVOL TI\.GE PERFORMANCE

12000zION SOURCE

1't

ACCELERATOR OPERATION:I:

Table 1. ACCELERATED IONSPOLARIZEDION SOURCE

The pelletron 12UD installed at the University ofTsukuba Tandem Accelerator Center (UTTAC) has been in
operation for more than eight years. As of July 31,1984,
the accelerator had logged a total of 27700 hours ofoperation with beam acceleration for 20260 hours. Thoughsubstantial alteration has not been made on the machine,reliability is considerably improved recently as aresult of careful handling and maintenance. In thispaper, operation experience is shown along with a discussion of tube conditioning and planned alteration of
the beam injection system"(/)

Beam acceleration hours at various terminal voltages
divided in 0,5 MV are shown in Fig.l" Most experiments
were carried out at the highest voltage, but wide energy
variability of the tandem satisfied the users' request
as low as 3.7 MV on terminal.
Tabe 1 summarises ·jon beams provided for experiments"_dl:

2 3 4 5 6 7 8 9 10 11 12
TERMINAL VOLTAGE(MV)

Fig. 1.. Distribution of beam acceleration hours
at different terminal voltages"(Aug. 1,1983--July 30, 1984)34

as a ratio of the currents read on FC-3 to FC-2. Average
beam transmission of the polarized protons at 22 MeV isabout 30%o Since beam emittance of the polarized ion
source is expected to be larger than acceptance of thetandem, it is suspected that the acceptance of the accelerator has decreased by distortion of the tube alignment.
A beam emittance measuring system will soon be installedin the vertical beam line between the inflection magnet
and the accelerator.

inflection magnet

FC-2ALTERATION OF BEAM INJECTION SYSTEM

LE column

Alteration of the beam injection system is underwayo
At present, two ion sources, a polarized source and a
sputter sou~ce, are alternatively operated. The GIC sputter source will soon be in operationo Switching of theinjection beam line from one ion source to another ismade by truning the inflection magnet about the verticalbeam lineo
Recently an experiment was proposed which aimed
searching for fractionally charged ions with the use oftandem acceletator as a mass analyzer4. Quarked nuclei,if exist, should have fractional charge but their massis unknown. Such ions are equally transported tf thebeam line was composed of only electrostatic elementso
A plan was proposed to replace the present inflection
magnet by an electrostatic deflector and electrostatic
quadrupole lenses.
In addition to the search for fractionally charged
ions. employment of the electrostatic deflector system
is prefered for switching the ion sources because thedeflector ins ta 11 ed in a vacuum chamber can be rotatedwithout breaking vacuum. Detailed desigh of the system
is completed.

FC-3

HE column

FC-5t~--- -- - 155,00c___ _REFERENCES

Figo 2 Layout of pelletron 12UD1) Ko Furuno et alo This Sympisium C-62) Ao Isoya et al, Proco 3rd Int. Conf. on Electrostatic
Accelerator Technology, Oak Ridge(IEEE Press, New York
1981) p983) G, Korschinek et al, Nucl. Instr. and Methods 220( 1984) 82
4) L Arai et al, UTTAC Annual Report 1983 p83

Sparks induced by vacuum discharge in the tube canonly be eliminated by tube conditioning. Experience sofar suggests that the tube conditioning is most efficiently made by applying voltage to 2 or 3 units at atime while the other units shortedo In this case, sparks
seldom caused deconditioning or damage of the tubeo
Unhealthy tube sections mentioned above could also
be conditioned up to lol MV per unit by this procedure.
However, these sections were liable to be deconditioned
gradually during normal operation or suddenly· by aspark along the column. Replacement of these unhealthytube sections will be done in the next year.
!soya et al. suggested that hydrogen glow discharge
in the tube was effective for improving voltage holding
capability as opposed to the high voltage conditioning
2. It was reported that the hydrogen discharge technique
was succesfully applied to the "NEC" accelerator tubes
in a small test equipment3. Still several problems lie
for application of this technique to existing largepelletron accelerators. We are preparing a test equipment to investigate the effect of this technique on o~r
accelerator tube configuration.
BEAM TRANSMISSION
At present, one of our greatest concern in accelerator operation is low beam transmission, in particularwhen the polarized ion source is used. Last year, wechecked accelerator tube alignment by using a laserbeam and found serious misalignment of the LE tube. Thelaser beam, about 1 em in diameter well aligned withcenters of entrance and exit apertures of the HE tube
was elipsed by nearly a quarter of its diameter throughthe LE tubeo Since the smallest aperture of each tube
section is 25.4 mm in diameter, distortion of the LE
tube is estimated to be about 18 mm. Alignment wasroughly corrected last year. In order to make fine correction, all the accelerator tube should be reconstructed from the begining. This work will be scheduled in thenext yearo
Beam currents are monitored by Faraday cup locatedat every slit sustem of the image of inflection magnet(FC-2), object and image of beam analyzing magnet (FC-3and FC-5, respectively). Beam transmission is defined35