N-3AGING OF THE NEW ACCELERATING TUBE OF THE RCNST TANDEM
IsaoYamane

Research Center for Nuclear Science and Technology,Univ. of Tokyo
HiroshiYamashita

Faculty of Science, Univ. of Tokyo1.

New Accelerating Tube

Both negative and positive ion accelerating tube consist ofnine sections. Each section is 508 mm in length. Ceramic
insulators of Murite (3Al2032Si02) are 190 mm diameter and29 mm in thickness. Electrodes consist of two parts. Outer ones
are flat disks of covar and cemented to the insulators withAraldite. Dismountable stainless-steel inner electrodes are ofre-entrant type with 90 mm inner diameters. Sixteen permanent
magnets for electron suppression are installed between tube
sections.
2.

High Voltage Generation and Aging of the Accelerating Tubes

Fig. 1 shows the progress of aging of the acceleratingtubes. While high voltage was generated, pressure of insulatinggas (N2+ 10% C02) was maintained at least higher than 10 atm,the dewing point of gas at about -60°C, and vacuum degrees of
the tubes at 1~2xl0-6Torr. Vacuum degrees of the tubes, column
currents, and intensities and energy spectra of X-ray generated
in the tubes were watched to monitor extraordinary discharges
in the tubes.
The procedure of raising voltage was as follows: After theconditioning run of 30~ 60 minutes, the belt was graduallycharged. At first voltage was smoothly raised without discharge.
At some voltage, vacuum degrees and column currents began tooscillate and bursts of X-ray were generated in coincidence withoscillation. This indicated occurence of small discharges inthe tubes. Then, the generator was leaved as it was. Thesedischarges gradually calmed down and amplitude of the oscillation was reduced. After these discharges sufficientlycalmed down, the belt charging current was slightly increased
to raise voltage. Then small discharges c3me back again andthe same process was repeated. In this procedure rate of voltage
increase was about 100 kV per hour.
The voltage, at which small discharges occured, graduallybecame higher as aging of the tubes progressed. But after
evacuation of the tubes had been stopped for some period, orafter stripper gas had flowed into the tubes during accelerationof beam, the voltage lowered a little.
41Sometimes, but not frequently,
another type of discharge occured.
In this discharge, vacuum degreeof a tube and column currentof the same side swing to about
ten times of the amplitude ofoscillation in the small discharge,
and voltage was reduced by a few
hundred kV. This type of discharge
was payed with special attention.
When it seemed to occur frequently,
charging current was reduced
a little to lower voltage.
Energy spectra of X-ray had
a broad peak around the energy
correspondin g to two thirds ofthe voltage of one section ofthe tube. This showed that electrons generated in the tubes weretrapped by .the fields of installed
permanent magnets.

Time(hr )1 000 2000

Voltage( MV)

. ·.

Nov

1Dec
1'78
Jan
1FebFig.l

Aging of the new accelerating tube of the RCNST
Tandem'.__.~.

....
:z·· ·;. .:. :•-/it.. ..
... .
·~.
~

••4~

Sep
1Oct
1

54

3

2

1
1'77
Apr1 \\\\
III\May \I,.
\\1 \\\ \\\JunI\\\1\'\JulIIII1IIAugIIII1• •I.

:.I'\.,' \ '?". ...
."\·.~\\\z\·.·.

.. :>···.. ·'i'i.. ·S

J\ \ \~.::~~ ~.:·.

\\ -).~.
\.

\-.....!,

I •(i)spray time <2) belt time
@voltage,discharges start@voltage, maxi mum reached42