PRESENT STATUS OF THE JAERI 20MV TANDEM ACCELERATOR
Michie Maruyama
Japan Atomic Energy Research InstituteThe 20MV heavy ion tandem accelerator is being installed atTokai Research Establishment of Japan Atomic Energy ResearchInstitute. It is the Model 20UR folded tandem electrostatic
accelerator manufactured by National Electrostatics Corp.(NEC)in
Wisconsin, U.S.A. This project started in late 1975 and is now
in the final stage.
The accelerator system is designed to produce energetic ionswith masses from l to 240 amu at terminal voltages of 2.5 to 20MV
with four different negative ion sources; the direct extraction
duoplasmatron, the Heinicke-Pennin g source, the sputter-conesource, and the charge-exchange helium source. It is equippedwith a nanosecond klystron bunching system in the low energy beamline for high energy pulsed beams of light and heavy ions and ananosecond bunched positive ion source in the high voltage
terminal to produce intense pulsed proton or deuterons. It isalso equipped with dry high vacuum system of all metal andceramic construction bakable in place to over 200°C, including
the accelerating tubes. The vacuum system includes forty-one
sputter ion pumps, two getter pumps, and two turbo-molecularpumps.
The column structure of the high voltage generator is housed
in a accelerator tank which is 26m in height and 8.2m in innerdiameter and designed for pure SF 6 gas at a maximum pressure of
8.8Kg/cm2. The 4m-long high voltage terminal is on the verticalcolumn structure of modular design l3.7m in length and 2.7m indiameter, and it houses a double focusing 180° magnet with a massenergy product of 55 amu-MeV and a radius of curvature of 7lcm.
There are six 1.25" diameter Pelletron Chains in the column todeliver a maximum charging current of 600~A from grund to theterminal. The chains are charged from drive sheaves at ground
potential by the electrostatic induction.
The low energy and high energy accelerating tubes are
installed in parallel in the column and connected with the 180°
magnet to make the folded configuration. The foil and gas strippers are located before the magnet. There are two dead sections
at the 7MV and l3MV levels of the column. They house the ion
pumps, electron traps, and beam monitoring components. The lower
dead section also houses two electrostatic quadrupole triplet
lenses on the both accelerating tubes, and the upper dead section
the second foil stripper.
There are three separate corona needle potential grading
systems along the accelerating tubes and the column. Drain
current during normal operation is around 20~A in each system.
Two rotating shafts provide power to the compornents in the terminal and the dead sections and to the baking heater plates in
the accelerating tubes.
An Interdate 7/32 computer system takes care of digital-11-

communication between a control console and end controllers ormonitoring devices at accelerator site by using serial CAMAC highways at a maximum rate of 5MHz. Light links between the pressure
vessel wall to the terminal and the dead sections extend theCAMAC highways to the high potential region.
High Voltage Tests
The column voltage test without the accelerating tubes wasperformed in December, 1978. After column conditioning of 80
hours over 8 cays, the terminal potential reached 23.4MV at a SF 6
gas pressure of 6Kg/cm~ which was estimated from current drain
through the column corona needle system. During the conditioning
160 terminal-to-tank sparks and one charging chain spark wereexperienced, but none of column spark. A lot of tank spark marks
and continuous corona discharge marks were found on the terminal
shell and the column modules higher than 15MV level.
Accelerating tube conditioning was tried in the period from
December, 1979 to March, 1980. In the course of this conditioning
the corona needle systems were frequently damaged by sparks andthe highest terminal voltage reached was 17.4MV. The needles
seemed to be too week to be used in 20MV accelerators whosestored energy is three times as much as that of existing 14MVaccelerators.
In April, 1980 the three corona needle systems
were replaced by the new ones which have been improved by NEC
and were equipped with much thicker needles. Afterwards any
serious damage has not been observed with the improved corona
needle systems during the tube conditioning and the terminal
pot,ential reached 19MV late June.
Beam Tests
In parallel with the voltage conditioning, beam tests wereconducted since September, 1979. An initial ion beam was transmitted through the whole accelerator at a terminal voltage of
9.8MV on October 30, 1979, which was an o+ 6 beam of 2~A and68.6MeV.
In the following test at a terminal potential of 16MV,
almost same amount of o+6 beam was also observed. Concerning
protons, a beam current of 2.5~A was measured in a target room at13MV, and a beam of 4~A exceeding the guaranteed valve (3~A) wassuccessfully obtained at 2.5MV with a good transmission. This is
the lowest voltage in the guaranteed performances specified toconnect the existing 5.5MV Van de Graaff accelerator to the tandem
accelerator. To date an I+ll beam of 50 nA was accelerated up toa beam energy of 180MV. During the beam tests an instability ofion beams due to an inadequate operation of the terminal potential stabilizer was found and it is now under improvement.
In general electronic circuits including CAMAC crates in theaccelerator tank are Imlch stronger for high voltage discharges thanexpected. Double shielding and grounding design seem to be very
effective. Damages of integration circits due to the SF6 gaspressure were experienced in the early stage of the accelerator
tests. More primitive technical troubles have caused many tankopenings, for example troubles in the terminal water coolingsystem or in the rotating shaft mechanism.

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