Periodic Klystron Maintenance Verification of vacuum integrity

Periodic Klystron Maintenance
If satcom Klystrons are to be stored for extended periods, it is recommended that they be
periodically checked for vacuum integrity. This can be achieved in a number of ways:
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Installation into a KPA system.
Use of the CPI Klystron Test Set.
Using a hi-pot high voltage (10kV), low current power supply.
It is recommended that this check be done after six months in storage, and then annually
after that.
Given the continual advances in klystron manufacturing technology and materials, CPI no
longer believes that it is necessary to perform six-monthly conditioning at full beam voltage.
Verification of vacuum integrity
Klystrons are manufactured using high quality materials and assembly techniques, and
while it is unlikely that vacuum integrity could be compromised under the normal use or
storage, CPI recommends verifying vacuum integrity periodically. A poor vacuum in a
klystron will cause breakdown between cathode and body. Depending on the internal
pressure, the breakdown voltage can be anywhere from 200V up to about 8.5kV. The higher
figure represents a C-band klystron that is at atmospheric pressure. Other effects of loss of
vacuum are inhibited beam current (as the cathode is chemically poisoned), and increased
heater current (due to changes in heat loss mechanisms, and changes of resistivity of
tungsten with temperature). The CPI Klystron Test Set operates at a maximum voltage of
1kV, and so proper use of this set depends on assessing heater current and beam current,
see the operating instructions.
Using the Klystron Test Set
The instruction manual for the Klystron Test Set describes a Performance Test to determine
the Perveance (the relationship between beam voltage and beam current) which confirms
that the klystron vacuum is intact by verifying cathode emission performance. In the case of
an MSDC klystron, a special connector is required to connect collector 1 to ground, and to
allow collectors 2 and 3 to float. The electrodes must be properly discharged after the test.
Consult CPI to obtain the required connector if you are working with an MSDC klystron.
Using a Hi-Pot tester
Using a Hi-Pot tester is similar in concept to using a KPA, described below. There are
additional safety considerations due to the nature of the equipment. In the case of an MSDC
klystron, a special connector is required to connect collector 1 to ground, and to allow
collectors 2 and 3 to float. The electrodes must be properly discharged after the test.
Consult CPI to obtain the required connector if you are working with an MSDC klystron.
1. Connect the heater and cathode leads of the klystron to each other. In the case of MSDC
devices, this is achieved in the same connector as is used to connect the collectors together.
2. Take all possible precautions to ensure that the Hi-Pot test unit is not energized, and is
fully discharged.
3. Connect the cathode leads to the negative terminal of the power supply.
4. Connect the body of the klystron to the groundside of the power supply.
5. Before switching on, take steps to ensure that personnel cannot touch the high voltage
terminals when voltage is applied.
6. To verify vacuum integrity, apply high voltage to the cathode and raise it gradually.
There may be bursts of current as voltage is raised. The final level depends on the klystron
type:
a. C-band (klystron models operating in the range 5.7Ghz- 7.2Ghz): 8.5kV.
b. Ku and DBS (12.75 GHz – 18.4GHz) : 6.5kV.
If current drawn at this voltage is negligible (less than 0.2mA) then vacuum integrity is
intact.
7. Remember to turn off and discharge the equipment (and floating MSDC collectors) safely
before touching connections.
Using a KPA
1. Connect the klystron into the KPA as per the operating instructions. The input and
output microwave connectors should be terminated with loads or attached to the
amplifier input and output assemblies.
2. Do not apply heater voltage, if possible. If your KPA does not allow this mode of
operation, see below.
3. To verify vacuum integrity, apply the lowest available beam voltage to the cathode
and raise it gradually to 10kV. If no current is drawn, vacuum integrity is intact.
Depending on the KPA model, it may be difficult to apply beam voltage with no heater
voltage. On other KPA’s, it may be difficult to control the beam voltage. In these cases, it
might be easier to run the klystron at full beam voltage, see the section on “Running Up
After a Period of Prolonged Storage” below.
Running Up After a Period of Prolonged
Storage
· Apply heater power in the usual manner. Allow the klystron to warm up for between five
and fifteen minutes. Do not leave it for longer than fifteen minutes before applying high
voltage.
· When applying high voltage, start from the lowest value possible.
· Set body current trip level to 20mA, as a precaution, while running up.
· There may be intermittent bursts of body current as gas is cleared. This should clear fairly
quickly, so that full operating voltage is reached in a few minutes.
· Once full beam voltage is attained, RF may be applied. The klystron should be run both
with and without RF input for a minimum of a half hour after the beam current has been
seen to stabilize.
· The time to stabilize will vary with the age of the klystron, storage time, storage
conditions, etc.
Operation
Once the klystron is in service, it is extremely important to operate the klystron correctly
with adequate maintenance. In achieving this, three items should be stressed:
i) Stand-by mode should be with beam switched on, preferably at the lowest beam voltage
necessary to achieve the required performance. Leaving the klystron on stand-by with
filament on but beam off, for extended periods (days rather than hours), can cause damage
to the cathode, resulting in premature failure of the klystron.
ii) It is most important that the collector cooler is kept clean. If the cooler becomes clogged,
the collector can become overheated. Over a sustained period, gas can evolve causing
damage to the cathode. Maintenance of cooling air filters is similarly important.
iii) The filament should always be operated at the nameplate voltage for a particular
klystron. Operating the filament too high or too low can cause premature klystron failure.
When the klystron is old, and the beam current drops due to cathode depletion, the filament
voltage can be raised 0.5V without causing any harm. This will temporarily extend the life of
the klystron for a short period of time until a replacement klystron can be obtained. There
is, however, no guarantee as to the period of extended klystron life, if any, as this does
vary from klystron to klystron dependent on the condition of the cathode.
Stand-By Klystron Operation
The following table details the recommended mode of operation for klystrons used in standby operation. This tends to occur when an amplifier is used in a 1:n configuration but can
also occur if an amplifier is being used to carry very occasional traffic and an operator may
want to save power during the "off air" period. The table starts with the most desirable
mode of operation and flows down to the least desirable. The most desirable mode will
provide best klystron life and the least desirable will result in worst klystron life.
Tube
Life
Best
Worst
Filament
Power
Off
On
On
On
On
Reduced³
Full
Beam
Power
Off
Reduced1
Reduced¹
Full
Full
Off
Off
RF
Power
Off
On
Off
On²
Off
-
Notes:
1. Assumes beam power reduced by at least 1 kW.
2. Assumes RF output power is at least 1 kW.
3. Most modern KPAs automatically reduce filament power when in this mode.
This document is not designed to replace safety, operational and maintenance instructions
given in the amplifier operations manual and should only be used to supplement information
given. For a complete guide of this information, please consult your operating instructions.