Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

DETECTION OF X-RAY DUE TO GUN ARCING OF HIGH POWERKLYSTRON
Vladimir Vogel, Shuji Matsumoto, KEK, 1-1 Oho, Tsukuba, Ibaraki, 305-0801
Abstract
X-ray due to a klystron gun arcing was monitored by adetector consists of a plastic scintillation fiber and aphoto-multiplier. Observation of the X-ray was done
during the processing run of an X-band klystron. A clear
signal of X-ray burst is observed when the gun arcing
occurs. Possibility of the fast protection system for apulse modulator from the gun arcing is discussed.

in operation. Thanks to the fast time response of the fiber
and the PMT, we can identify the individual hit of X-rayon the scope. We confirmed that the sensitivity and thetime resolution of the detector are good enough for ourpurpose.

INTRODUCTION
For a future linear collider machine, GLC, some ofnovel machine components are proposed. One of these isa new kind of pulse modulator, which uses solid stateswitching devices, IGBT (Insulated Gate BipolarTransistor), instead of conventional thyratron tube. Oneof the main issues on R&D of this type of modulator isthe way to protect these devices from the arcing inklystron gun [1]. The protection should be fast enoughafter detecting a signal of the arcing. One possible
protection system is a combination of an X-ray detector
and a fast logic control in gate driver of IGBT.
It is naturally understood that a burst of X-ray appears
when the klystron gun arcing takes place. We study thiskind of X-ray production in a point of view of the future
protection system. In this report, we will give theobservation result of such X-ray during the processing ofa PPM X-band klystron and discuss the possibility of thefast protection system.

EXPERIMENTAL SETUP
The detector system consists of a plastic fiber and aphoto-multiplier (PMT). The system is as same as theones currently used as beam-loss monitors in KEK-PS(Proton Synchrotron) [2]. The plastic fiber is doped with
scintillation material. We use Kurarey SCSF-81M, 2mmφ.
The decay time of the scintillation material is a few nanoseconds (ns). Once an X-ray hits the fiber, it is converted
into a light pulse in the fiber and this light runs along thefiber. At the end of the fiber, a PMT is set. By observing
the output signal of the PMT, we can know the arrival ofX-ray. We use PMT Hamamatsu R-1635. Figure 1 is a
picture of the detector.
The klystron is a PPM-focussed X-band pulse klystron,
which is a prototype for GLC klystron. Its operation
parameters at AR-South Test Stand in KEK are in Table
1. In order to check the basic performance of the X-ray
detector, we did a pilot observation, a survey of X-ray
against whole of the klystron, at the test stand where weset the fiber along the klystron and monitored the PMT
signal on a scope. Clear signals of X-ray were seen
within every high voltage pulse when the klystron being

Figure 1: Picture of the X-ray detector. The plastic fiber(2mmφ) is covered by rubber tube for shielding of light.
The PMT is covered by lead (for radiation shield) and laidin an iron cylinder (shield from magnetic field). The
length of the fiber may be changed from one
measurement to another.
The observation of the X-rays from the gun is done
during the processing of the klystron. The fiber is set inthe gun coil housing. Figure 2 shows its location. The
fiber is set along the coil which surrounds the gun
structure. The fiber is turned three times around the gunand its both ends are extracted outside and they are
guided into a single PMT. The X-rays from the anode cancome to the fiber after penetrating through the gun
housing wall (stainless steel) and the return yoke of thegun coil (iron). Lead shields are properly set to preventthe background X-rays for better observation.
Table1. Operation of GLC prototype PPM klystron at
AR-South test stand.
Cathode Voltage
Cathode Current
Pulse width (RF)
Repetition
Operation Frequency
Output Power269

500kV nominal; 530kV max
283Amps (@500kV)
1.6 micro sec
50pps
11.424GHz
75MW

Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

the cathode voltage is much lower than our case [4]. At
any rate, we can utilize those X-rays as the precursor of
the arcing.
If we observe those waveforms over longer duration,
they look like as those in Figure 4. An additional minor
burst of X-ray is seen. The pulsed power from themodulator is moving back and forth in an oscillatory way
between the klystron and the modulator.

Figure 2: A cut view of the klystron gun region. The left
end of this figure is the central axis of the klystron. The
location of the fiber is indicated.

RESULTS
Examples of the wave forms appeared on the scope arecompiled in Figure 3, those in normal operation and in acase of arcing. The waveforms of cathode current, thecollector current [3] and the PMT signal are shown.
Figure 3C shows that the collector current drops to zero
within about 100ns while the cathode current goes up andfalls off in a rather slow way. This means “the current
from the cathode is missing” during almost latter half ofthe pulse duration. This missing current should run
through another path, possibly, directly to the anode in thegun and we may call this the arcing. The sudden changeof these currents (decay of the collector current or rise of
the cathode current) indicates the start of the gun arcing.
The cathode voltage starts to drop at the same moment
although it is not explicitly shown here.
Once the arcing starts, the electrons emitted fromsomewhere (probably other than the cathode) appear andhit the anode wall eventually. This should cause theproduction of X-rays much more than normal. As shownin Figure 3D, we have an X-ray burst. One can see theburst of X-ray starts somewhat earlier than the currents
start to change (compare Figure 3C with 3D).
Time response of the collector current pickup (CurrentTransformer) or the collector current pickup (resistors) is
supposed to be fast enough. The response of the fiber or
the PMT is also fast (a few ns). Even though taking into
account of the time delay due to the cables between those
pickups and the scope, the PMT signals still comes rise
earlier than actual change in the signals of currents.
The PMT signal starts increasing 200-300ns before theincrease of the cathode current starts. One can conclude
that some X-rays emitted in advance of the arcing. Thereis a discussion on these X-rays in a vacuum tube although

Figure 3: The observed waveforms in case of theoperation being normal (A and B) and the gun arcing (Cand D). The length of the pulse flat-top is about 1.6
micro seconds. The cathode voltage is 480kV. Figure Ashows the cathode current (dotted line) and the collector
current (solid line) whereas Figure B shows the X-ray
signals in normal operation. As for Figure B the data
accumulated over many normal pulses. Figure C showsthe current signals too in a case of the arcing. Figure D
shows a burst of X-ray due to the arcing. Single pulse.

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Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

Figure 4: The cathode current and the X-ray (PMT) signal
in case of the gun arcing. Waveforms for longer duration
than that of Figure 3.

DISCUSSION
The protection of the components in the RF systemfrom the damage due to the klystron gun arcing is an
important issue to make the system stable and reliable [5].
The X-ray detection of klystron gun arcing may be
applicable for the fast protection. It is possible toconsider a variety of protection schemes with a proper
control trigger signal prepared from the X-ray detector
signal.
In case of a PFN modulator, for example, one may usean additional thyratron tube together with dummy load forthe protection of the klystron. The second thyratronworks as a switch of the bipass line to the PFN, which isterminated with the load. Once extra X-rays from the gun
are detected, the second thyratron is fired. An appropriate
trigger for the thyratron is made from the detector signal.
The dummy load absorbs the energy stored in PFN, which
would be consumed at breakdown point in the klystrongun if this kind of protection system being absent. We
should consider the protection of klystron more seriously
when we operate the machine in high repetition rate.
In PFN modulators, the maximum peak current due tothe arcing is naturally limited to factor 2. However this is
not true for the modulator which is now developing atKEK. It utilizes IGBT each of which handles the peak
power of 3MW (3kV and 1kA). There are 40 circuits inseries to feed power to the klystron. One of the circuits is
schematically represented in Figure 4. The power fromthe each circuit is combined together through themagnetic cores and the circuits produce the peak power of120MW(3kV 1kA 40) as a total.

In our spark gap experiment, we found that the current
through IGBT CM600-90 type increased as 12-15kA/1µs
when the breakdown takes place. From this experiment,
we can conclude that we need to switch off the IGBT
immediately after the gun arcing, otherwise we would
have huge current through IGBT and it is surely harmful
or danger for IGBT.
The protection from the arcing can be accomplished byswitching off the IGBT immediately after the arcing isdetected. Such a fast protection system is possible to be
designed based on the X-ray detection system. If we havethe precursor X-rays and we can use its signal to make a
trigger to control the switching gate, the protection can beefficient. It is necessary to study more in detail the time
structure of the X-ray burst due to the gun arcing in orderto accomplish this work.
Note that we have already utilized the gun X-ray
detector signal as the one of the interlock signals at thetest stand. The X-ray interlock is not the fast one
considered above but it use (meter) relays. Once the extra
X-ray is detected, it makes the relay signal and stops thenext coming pulse. This works as a new additional
protection from the gun arcing at our test stand. We have
had the protection by monitoring the peak of the cathodecurrent.

REFERENCES[1] V.Vogel, talk at ISG-9 meeting, KEK, Dec. 2002.
http://lcdev.kek.jp/ISG/ISG9.Vogel.pdf.
[2] T.Kawakubo, T.Ishida, H.Yamaguchi, “High Costperformance Fast-response Beam-loss Monitor usinga Photo-multiplier”, in Proc. of 2nd Asian Particle
Accelerator Conf. (APAC01), Beijing, China, Sep.
17-21, 2001.
[3] The collector of the klystron is not electricallygrounded directly but so through some resistors. Thus
we can monitor the collector current by measuring
the induced voltage on the resistors.
[4] G.A. Mesyac, D.I. Proskurovskiy, “Pulser Electrical
Discharge in Vacuum”, Nauka, Novosibirsk , 1984
(in Russian).
[5] S.L. Gold, “Thyratron-PFN, IGBT Hybrid and Direct
Switched Modulator R&D As it Effects KlystronProtection”, SLAC-PUB-8475 (2000).

Figure 5: Schematic representation of a single IGBT
circuit.

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