E3730A OPERATING INSTRUCTIONS
－ 20 －
Check the ultimate pressure in the waveguide system is less than 1.3x10
-6
Pa (10
-8
torr),
and evacuating speed of the vacuum pump and pumping conductance of the vacuum channel is
large enough to keep the waveguide pressure at the klystron output window less than 1.3x10
-5
Pa
(10
-7
torr) even in the full power operation.
The waveguide outside of window must be conditioned with RF output power of the klystron.
The surface of the new waveguide may be contaminated with foreign material, or may absorb gas
molecules, and may have fine scratches or metallurgical defect. If high power RF with long pulse
duration, that means high RF energy, is applied to the waveguide, these irregular points can be
origins of electric break down, and may sputter metals onto the surface of the ceramic disk of
klystron output windows. The metal layer on the ceramic disk causes excessive heat onto the
output window and results in window failure such as puncture or crack due to thermal strain.
To avoid a serious damage in the early stage of conditioning, short pulse conditioning is
recommended. Make the RF pulse width shorter, less than 1 micro second preferable.
Apply high voltage pulse to the klystron. Less than 150kV is recommended in the first stage
of conditioning, especially after long period of storage. Apply drive power to the klystron. Be careful
the klystron ion pump current as well as the output waveguide pressure not to exceed the
prescribed limit. In many cases non-conditioned waveguide shows serious electric break down
even at the power level of a few MW or hundreds kW, accompanied with tremendous amount of
out-gassing. In case of a large break down, a part of or almost of the output power is reflected
backward to the klystron. Interlock system to detect the backward power must be provided and
must cut off the drive power or the HV by pulse-to-pulse basis.
If the waveguide material, finishing, cleaning, and evacuating system are properly designed
and successfully operated, it is not so difficult to overcome the outgassing. When the waveguide is
well conditioned, the klystron output power is rather easily increased up to tens of MW or more. If
the vacuum in the waveguide is poor, the conditioning will take much longer hours or even days.
When the waveguide system and the klystron is stably operated at its full rating for several hours,
then make the RF pulse width longer to the required pulse width. Start conditioning with longer
pulse width again from a reduced power level of hundreds kw. Raise the output power gradually by
means of raising the HV or1 raising drive power up to the rated power. Be careful the waveguide
and klystron pressure not to exceed prescribed limit.
Measurement of the klystron performance should be made with the well-conditioned
waveguide.
6.2.1.3. Conditioning of electron gun
The electron gun is carefully designed and fabricated with certified materials and special
processes, and already conditioned. However, there may still occur arcing (high voltage
discharge) in the electron gun assembly at some possibility. Start conditioning at the beam voltage
less than 150kV and gradually raise the beam voltage up to the value indicated in the individual
Test Report for each tube.
When arcing occurs almost all of the electric energy stored in the PFN capacitors is
concentrated between the anode and cathode and may give a damage to the electrodes. Once
arcing occurs, large amount of gas or ion is emitted and some are still in the vicinity even when the
next HV pulse is applied. That means the possibility of successive arcing is very high, and the