The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

REPLACEMENT OF ACCELERATOR TUBES OF THE JAERI TANDEMACCELERATOR WITH TUBES CLEANED BY A HIGH-PRESSURE
WATER JET
S. Takeuchi, T. Nakanoya, H. Kabumoto, N. Ishizaki, M. Matsuda, Y. Tsukihashi, S. Kanda,
H. Tayama, S. Abe and T. Yoshida
Japan Atomic Energy Research Institute, Tokai Research Establishment
Tokai Naka Ibaraki 319-1195, JapanAbstract
In an effort of increasing the terminal voltage of theJAERI tandem accelerator by replacing the acceleration
tubes with compressed geometry tubes, we investigated ahigh-pressure water-jet rinsing of tube insides. There werea lot of microscopic particles loosely bound on insideceramic surfaces. The cleaning was found to be very
effective to reduce discharge activities as a result of a 3
MV test. And then, the replacement was carried out.

the tube was rotated on a turn table and the two jets weretilted by ±15 degrees, so that the tube inside withelectrodes were cleaned thoroughly. The air was filtered

INTRODUCTION
The JAERI tandem accelerator has been running over20 years. Its operating terminal voltage has been in arange of 16 MV these years, although it started at 18 MV
in 1982 or the design voltage was 20 MV. We intended toimprove the terminal voltage by replacing the 11
acceleration-gap tubes to 21 acceleration-gap compressedgeometry tubes.
An addition of more acceleration tubes in place ofuseless heater plates done at ORNL was certainly
effective[1]. But, we have to be careful to use new tubes
because one often needs very lengthy conditioning to get
rated voltages. We attribute the reason to pollution of tube
inner surfaces. We believed that cleaning the pollution
using a high-pressure water jet is effective to eliminate
lengthy conditioning as it is popularly used for
eliminating electron field emission in superconducting
cavities[2,3]. It is very important for us to resume themachine time as soon as possible after the tube
replacement.
We investigated how much the tube insides or ceramic
insulator surfaces were polluted with dust or microparticles and how much they were cleaned by the cleaning.
Old and new tubes and their ceramics were compared inthe investigation. We carried out a 3 MV test with newtubes for replacement at National Electrostatics
Corp(NEC).
After a cleaning over six months, the tube replacement
was carried out.

CLEANING METHOD
An apparatus of high-pressure water jet spray, used foracceleration tubes, is illustrated in Fig. 1. Air of 1 MPa
was fed to the nozzle to create two water jets. The jet
speed was 250-300 m/s. The nozzle moved up and down,Fig. 1. High-pressure water jet spray nozzle foracceleration tubes
and warmed to 40 °C. The water was 15 - 18 M·ohm·cm
de-ionised water passed through a 0.2 µm filter. Thespraying took about 25 minutes for a tube. After rinsing,
the tube was dried at 50 °C in a clean booth. Cleaned
tubes were baked and pumped at 200 °C for two weeks.
For more perfect cleaning, ultra-sonic wave cleaning
was added as a pre-cleaning process.

CLEANING EFFECTS
In order to know the cleaning effect of the highpressure water jet spray, the high-pressure water jet
sprayer was used without feeding water. Dry and filtered
air jet was sprayed to the inner surfaces of a tube andparticles in the air from the tube were counted by using aparticle counter. The measurements were done with an
old unused spare tube before and after a high-pressure
water jet spray and with another one after putting in an
ultrasonic wave bath for 30 minutes. As is presented inFig.2, millions of particles came out from the unused tube
and the numbers were reduced greatly by a high-pressure
water jet rinse as well as ultrasonic wave cleaning.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

COMPARISON OF CERAMICS BETWEENOLD AND NEW TUBES
Grain size
From observations through a microscope, ceramic
grains in the insulators were found to be very different
between old and new tube ceramic insulator samples. The
old ceramics contained a lot of grains as large as 40 µm
and the new ceramics had much fewer and covered withsmall grains. The mean values were 25 µm and 14 µm,respectively.

Fig.2 Particles which came out from the tubes sprayed byan air jet with increasing pressure before and after a highpressure water jet spray and ultrasonic wave cleaning.

Fig. 3. Particles which came out from the old(11 gap tube)
and new(21 gap compressed geometry tube) sprayed byan air jet with increasing pressure.

Loosely bound particles
By spraying air jets, particles from old and new tubes
were similarly measured as described in the previous
section. The results are shown in Fig. 3. The new tube hadmuch more particles which came off easily at a low airpressure of 0.5 MPa than the old tube had. These looselybound particles on surfaces must help discharge activities
develop under a high voltage gradient. Loosely boundparticles being hit by electrons are easily charged-up,
create secondary electrons or jump out with full ofcharges into the vacuum space between electrodes, andresult in a chain reaction of discharges. This result
indicates that the cleaning is much more important for the
new tubes than for the old ones.

Electron feed-and-collection experiment
In order to know the interaction between electrons andceramic surfaces under an electric field parallel to thesurfaces, we carried out an experiment of feeding
electrons(with an injection energy of 100 eV) to one edgeof a ceramic sample, which was put between parallel
electrodes, and collecting electrons, which flowed overthe sample surface, at the other side of the sample. As
shown in Fig. 4, nothing happened above a gap voltage of
1000V (the rated gap voltage in actual acceleration tubes
is 30 kV) and a transition-like abrupt increase wasobserved at several hundred volts for each sample duringa slow increase of the gap voltage. Such part wasirreversible for a decreasing path. The abrupt increase
seems to be a transition of electron multiplication factor
from <1 to >1. The transition voltages were high(380400V) for the old ceramic surfaces with large grains andlow(240-270V) for the new ones with small grains, andthose were greatly lowered by a sand blast surface
treatment and significantly recovered by high-pressure
water jet rinsing(as →200-220V → 260-280V, → 160180V → 220-240V, respectively). This phenomenon must
be due to loosely bound micro-particles on the surfaces.
The transition voltage can be regarded as a barometer ofsurface cleanliness.

Fig. 4 Electron current into the collector as a function ofgap voltage during 100eV electron feeding onto a ceramic
sample set in a pair of electrodes.

HIGH-VOLTAGE TEST RESULT
We carried out a 3 MV test with new tubes. Six of thenew tubes for the tube replacement were cleaned withultrasonic wave and high-pressure water jets and baked
and sent to NEC for testing. NEC carried out the test

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

according to a contract. The chart during the test is shownin Fig. 5. The rated voltage of 3 MV was achieved only in4 hours. There were only a few sparks in three days orover about 24 hours. The voltage reached at 3.2 MV
without increasing charging current and became very
stable after a small discharge.
For comparison, a chart of 3MV test for original newtubes before shipping from NEC is presented in Fig. 6.
Frequent small discharges and sparks happened to thetubes at 2.6-2.8 MV. The voltage would not haveincreased by continuing the conditioning.

TUBE REPLACEMENT
The tube replacement was executed after theinvestigations described above and 6 months longcleaning of 80 new tubes. The replacement work took
about 4 months as a result that it took several weeks morethan expected to clean the column, check and stop many
tiny vacuum leaks through gaskets and so on. The work
itself was successfully done, but we were allowed to have
only one week for conditioning. The conditioning wasdone in sectional bases. Stable voltages after ashort(several hours for each) conditioning were2.69MV(90%) for #1-#3 modules, 2.06MV(103%) for #4#5, 3.68MV (92%) for #6-#9 and 6.25MV(91%) for #14#20. The conditioning resulted in 15.8MV(80%) for thefull column. The fact that we could start the beam timeagain soon after the tube replacement was as successful as
we expected, although we are not satisfied with thevoltage. We will have to make further effort to attain avoltage higher than 18 MV.

SUMMARY

Fig. 5 Chart records during a 3 MV high voltage test with
6 new compressed geometry tubes after a high-pressure
water jet treatment.

Fig. 6 Chart record of the high voltage during a 3 MV testwith original new compressed geometry tubes.
There was a big reduction of 5-10 µA to 0.4-2.1 µA inthe lost charge between before and after the cleaning.
This reduction indicates that loose micro-particles wereplaying an important role of electron carriers in the tubes.

We planned an acceleration tube replacement program
for the JAERI tandem accelerator, expecting an increaseof the terminal voltages from 16 MV to 18-20MV byusing compressed geometry tubes and applying highpressure water jet rinsing to the tubes. For this program,
we investigated the tube inner surfaces or ceramic
surfaces and the effect of high-pressure water jet rinsing
on the high-voltage voltage performance before carryingout the tube replacement.
Millions of loosely bound micro-particles were foundin old and new tubes and cleaned well by high-pressure
water jet sprays. The new ceramics contained smaller
grains and more loosely bound micro-particles than oldones. This problem could be solved by cleaning.
The 3 MV high-voltage tests with six new tubes
indicated that the high-voltage performance of the tubes
were dramatically improved by the cleaning which
included ultrasonic wave cleaning and high-pressure
water jet spray rinsing. It should be noted that looselybound micro-particles are source of discharge activities inthe acceleration tubes.
The tube replacement was successfully done. We
obtained the former voltage of about 16 MV by only one
week conditioning. This program took a break for running
the accelerator for user’s experiments. We will need moreconditioning and further investigations about the real
accelerator system to get a terminal voltage of 18 - 20 MV.

REFERENCES[1] M.J. Meigs et al, Proc. Symp. N.E. Accelerator
Personnel(World Scientific, 1988) pp23-36.
[2] C.Z.Antoine et al, Proc. 5th Workshop on RFSuperconductivity, Hamburg(1991)pp456-462.
[3] K. Saito et al, Proc. 6th Workshop on RF
Superconductivity, Gif-sur-Yvette(1995)pp379-383.