BAKING PROCEDURE OF THE ACCELERATOR TUBE BY MEANS
OF LOW VOLTAGE ARC DISCHARGE
A. !soya, Y. Miyake, K. Kobayashi, N. Kato, T. Nakashima, T. Maeda
T. Sugimitsu, and Y. Nakajima.
Tandem Laboratory, Department of Physics, Faculty of Science
Kyushu University
Experiences on the high-voltage electrostatic accelerator indicate thatthe baking treatme~t is very important to obtain a $table operation of theaccelerator tube at the highest working voltage.

For this reason, forexample, NEC tubes of the pelletron include a heating element in every tubeunits, which can be used during the high-voltage generation.

This provision

is an indispensable part.of this special type of tube for its remarkableperformance.
Recently in the tandem accelerator laboratory of this university a. neweffective method of initial baking is developed by means of gaseous discharge.
The accelerator tubes used here are ceramic ones fabricated by a metal bonding technique, eliminating organic material from the inside.

Originallybaking was performed by using electric mantle heaters, so the procedure wasvery tedious and troublesome. This procedure involved hard heating of thetube flanges, which often caused leakage trouble of the metal gaskets. In
the new method a dense plasma is produced along a long path of the accelerator tube with an oxide cathode at one end of the tube and an anode at theopposite end. The electrodes in the tube exposed to the plasma are easily
heated to high temperature of several hundreds degrees in centigrade. The
flanges are heated only weakly. Since the inside high temperature is wellinsulated from the surrounding by the ceramic insulator, this baking procedure can be carried out with the tube as completely equipped with such accessories as annular spark gaps and dividing resist'ors.
The effect of this discharge treatment is not only degassing by heat but
includes some degree of conditioning by ion- and electron-bomba rdment of thesurface of metal electrodes and ceramic insulators. One of anticipateddangerous events of this treatment is the contamination of the insulator
surface by sputtering. To avoid this it is necessary to use a discharge gasof the lightest elements,

like hydrogen or helium, and further, as a more
important thing, to produce a plasma of low electron temperature. Plasma of

the latter property can be produced by using an oxide cathode ("low voltagearc").

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We searched the way of producing the arc discharge most suitable forthe above requirements by using actual tube units of the tandem.

In thecase of H2 gas preferable pressure was found to be about 0.1 Torr, withwhich the average voltage gradient along the tube is about 40~50 volts perunit tube (25cm long), independently of the magnitude of the arc current.
Therefore, with the arc current of 10 to 20 A heating power of several
hundreds to one kilo watts can be easily supplied to each tube unit.

Thetemperature of demountable electrodes rises quickly (in a few minites) to an
equilibium temperature of more than one hundred degrees and then gradually up
to several hundreds degrees (in about one hour).

A small step of potential

is formed in diaphragm positions at the ends and oiddle of the·unit
tube, due to higher plasma iu;~pedance at these portions.
are easily red-heated by receiving more heat.
cathode requires special precausion.
of the tube.

These diaphragms

The location of the oxide

It should be set close to the entrance

Otherwise a large potential fall is generated at the entrance

portion and serious sputtering occurs from every points of the elect~ode
surface in this portion due to the effect similar to the hollow cathodephenomena, contaminating ceramic insulators.
The effect of the discharge baking was investigated by using lMVelectrostatic test generator.
remarkable.

It was found that the effect is apparent andMicrodischarges accompanying X-ray bursts due to the ion-exchange sputtering*, which usually started from the voltage of 300KV or lessper unit tube when it was not treated, were found to disappear with a moderated discharge treatment and did not appear up to the highest working voltage
of SOOKV (corresponding to lOMV of the terminal voltage).

Frequency of local

spark discharges, which are considered due to the flashover along the ceramic
insulator surface in the individual insulator sections decreased drasticallyand then SOOKV could be attained per unit tube in a short time with quite
iew breakdowns.

* The electrode arrangement in the present tandem accelerator tube of thislaboratory is designed so that the ion exhange sputtering process is confinedwithin the domain of one unit tube by placing diaphragms of concave shape atthe flange portions.

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