The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Present Status of High Power CW Klystrons
Shigeru ISAGAWA
Accelerator Laboratory, KEK, High Energy Accelerator Research Organization,
1-1 Oho, Tsukuba-shi, Ibaraki-ken, 305 JAPAN

Abstract
In the KEK B-Factory (KEKB), high power CW
klystrons of lMW and 1.2MW that have been developed for
TRISTAN are to be used at a little bit higher frequency,
508.887MHz instead of 508.58MHz, without drasticchanges in other specifications. Various improvements andmodifications have been given so far, however, to bothtubes to get characteristics upgrading and higher stability.
Difficulties, like positive and negative spikes of anodecurrent, Side Band Oscillation (SBO) and Fast Self-recovery
Breakdown (FSB) in particular, that are caused by improper
arrangement of gun structure and/or back streaming electrons
have been extensively studied and solved by finding causes
and remedies. In this paper the present status of R & D isreviewed including new findings on these tubes and the most
updated summary of operation in TRISTAN.

1 Introduction
TRISTAN MR was terminated in December 1995, andwas disassembled by the end of March 1996. It is now
being rebuilt as KEKB composing of two rings installedside by side, the 3.5-GeV low energy ring (LER) for
positrons and the 8-GeV high-energy ring (HER) for
electrons. In TRISTAN two types of high power CW
klystrons have been developed: one is 0.8MW/l.OMW tube
called YK1302/YK1303 (Philips) and the other is 1.2MW
tube called E3786/E3732 (Toshiba).

They are to be successively used in KEKB in almost thesame frequency region (508.887MHz). Until now theoperational time of transmitters has totaled 1,413,000 h(161 years) in filament hours: 55 years for 0.8MW/l.OMW
tubes and 106 years for 1.2MW ones. Five tubes of thelatter have been continuously operated over 40,000 h.
Heater-on time of the top runner exceeded above 51,267 h.
Figure 1 summarizes the klystron age and life distribution
in 1,000-hour increments just at the time of shutdown ofTRISTAN MR.
Not all klystrons delivered to KEK worked completelywell. Typical difficulties met with are modulation-anodeinvolved instabilities: increased background level andpositive spikes of anode current and Side Band Oscillations(SBO) in 0.8MW/l.OMW tubes [1]-[4], and negative spikesof anode current and Fast Self-recovery Breakdown (FSB) in1.2MW tubes [5]-[8]. Both are related to excess barium
evaporation from cathode directly or indirectly [9]. FromFig. I one can notice that the main cause of tube's death ismodulation anode involved instabilities shown as Anode(the dotted boxes). The marked tubes, including Leak,categorized as alive are repaired in-house and aged in the testfield both by the author, and still in active service.
Shown in Fig.2 are the numbers of times of klystron
replacement versus year, excluding replacement due tostudies, rotations, acceptance tests and rearrangements for
power balance, etc. As the number of stations, for 1.2MW
tubes in particular, has increased, the smallness of thereplacement frequencies in last several years is remarkable.

YK1302
YK13030.8/1.0MW
TUBE

E3786
E3732

1.2MW
TUBE

f-------+4+-------13DIED

3DIEDALIVEor1111 LeakllEJ Anode111111 CoilDYING
221111 LeakttillJ Anode1111 Multi~ALIVEorDYING

111111 Mul!lEmission-10-8-6-4-200+++=+=++=+=I
6420Fig. 1 The number of klystron in each LV age category. Multi means difficulties of multipactoring at relatively lowerpower levels. Marked tubes, including Leak, categorized as alive are in-house repaired and still usable ones. Boxes
surrounded by dotted lines represent unaccepted tubes due to initial troubles or poor performance.

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The descending hatch in the graph means that tubes
developed troubles of various Ia related instabilities inYK130211303 and negative Ia spikes and FSB in E3786,respectively. Main efforts for improvement have been,
therefore, concentrated on such troubles and difficulties.

which could be driven independently but normally in series
with a current If. The bucking coil is not used in this tube.
In order to test the tube performance the top pair (H) wasexcited with a separate power supply with a current Irn.
Figure 3 shows an example of the ltH dependence of theanode current, Ia, and the body temperature increase, .tlTB.
The anode current is usually determined by smallinterception of the main beam and/or the wehnelt emission.
In the case of Fig. 3, however, it changes with the local
field made by the top coil, H, being apart farthest from thegun area. This fact clearly shows that in the 1.2MW tube,
too, there exists a back streaming of electrons returningfrom collector and/or output rfregion.
The difference of this tube from the 0.8MW/l.OMW one
is that the background level of Ia is low and no astablechange of Ia is found, indicating most returning electrons areinterrupted by a drift body but a portion of them can reachtne modulation anode rather constantly.
1.4
Ia (mA),...-...;__ _ _ _ _ _ _ _ _ _ _......:::..:..,::~Te (K)

Fig. 2 The number of tube replacement versus year. In thecauses of replacement, Multi means multipactoring,
Water denotes the leakage of body or boiler water,
Emission is the emission slump and Coil is thetrouble in focusing coil.

2 Positive Anode Current Spikes andBack Streaming Electrons

1.2

6.0

1.0

5.0

0.8

4.0

0.6

0.4
0.2

High efficiency high power CW klystrons tend to show
0.000
instabilities near saturation due to electron feedback
7.8 8.0 8.2 8.4 8.6 8.8 9.0 9.2 9.4 .
mechanism through back streaming electrons. They canltH (A)originate from collector and/or output rf region and can be
influenced by rf power level, load mismatches, focusing Fig. 3 The dependence of the anode current and the increaseof body temperature of the tube T40B on the top
fields and secondary electron coefficient of drift gaps, etc.
coil current, Irn. The main focusing current,
The last factor well accounts for the time dependent effect of
cathode and anode voltages, the beam current, thethe instabilities. Irregular anode current (positive anodemicro beam perveance and the rf power output werecurrent spikes) and spurious rf output (S B 0) can be
8.0A, 90kV, 62.lkV, 18.8A, 0.702J.LBP andobserved often simultaneously. If this behavior happens, it
1060kW, respectively.
may lead to beam loss in the storage ring accelerator, andthus a severe problem for its rf system.
Phenomenologically another distinct difference can be
In case of YK1302/YK1303 instabilities of this kind
have been observed above 900kW below saturation under observed between 0.8MW/l.OMW tubes and 1.2MW tubes
some combinations of focusing and prefocusing fields. The in characteristic curves of rf output vs. drive powers, withstability range often decreases with time and tubes become the anode voltage as a parameter. In the former, Ia generally
unstable even under matched operation after about 3000 h. has a maximum at a shoulder of the curves, about 0.5dB
Remedies can be obtained and tube's performance has been below saturation. ·The increase of Ia closely relates to the Ia
irregularities. In one of the latter tubes (T23B), on theimproved by some measures as reported before [4].
contrary,
the place where Ia is the largest locates far below,
A strong interest concerning this instability is why sucha phenomenon can never be found in 1.2MW tubes that are e.g., 250-300kW, only weakly depending on the anode andstable over whole range of output power level despite a cathode voltages. Not absence but constancy of thehigher efficiency (~63%) and a higher power gain (~50dB). returning electrons may be the main reason for the complete
The probable absence of back streaming electrons due to freedom from instabilities of positive anode current spikes·
geometry has long been believed· as a reason, but the and SBO in 1.2MW tubes.
simulation with FCI code has always shown their presence
3 Negative Anode Current Spikes andin these tubes, too [10]. A recent experiment on a 1.2MW
Fast Self-recovery Breakdown
tube well accounted for this discrepancy as follows.
·The focusing coil for 1.2MW tubes is composed of 8
Another instability of negative Ia spikes and FSB is, onpairs of inner and outer concentric coil elements. From thebottom to the top, the pairs are named as A to H, each of the other hand, unwanted phenomena typical of 1.2MW-186-

The 11th Symposium on Accelerator Science and Technology, Harima $cience Garden City, 1997

tubes. As repeatedly reported, however, this kind of temperatures, it could be increased even 'more. Although theinstability can be practically avoided in the most advanced optimum conditions are not universal, 66% and more hasversion, E3732, by adopting Ir coated M-type cathode, been preliminarily obtained in an average level tube (T40B)
Cr+CrOX. coated Cu modulation anode and conical shape of for lMW output by good combination of these parameters,
anode ceramics [6]-[9]. Fundamental study on the e.g. If 8.0A, ltH 8.0A, window 21.7°C and body 18.3°C.
When used at lower VK, say, 55.-6kV, a good saturation
mechanism of this phenomenon is being continued byefficiency above 39% has been obtained-for 228kW power
making a special type of socket as well as a gun tube.
output at 508.887MHz. Owing to general reliableness andtheofoneisM-type cathode coated with Ir thin layerof handling, we will use 1.2MW tubes for driving
easiness
betweenbreaking
insulation
suppressessential techniques toelectrodes and FSB [8]. The disadvantage of Ir, that is superconducting crab cavities at such lower power levels insusceptibility to gas poisoning and necessity of long term KEKB.
initial aging, is, however, still open question, as the new
5 Conclusion
techniques, like ion milling andfine-grained tungsten topaging
initialtheshorten
tounsuccessfullayer, proved to be
Instabilities typical of 0.8MW/l.OMW (Philips) andtime [8][9].
Compared with other parts, 0.8MW/l.OMW tubes are 1.2MW (Toshiba) CW klystrons are irregular anode currents
designed to have larger margin for gun ceramics in size. directed positive and negative as compared with stable
This fact, together with the use of sandwich structure of Cu values, respectively. They are normally accompanied byand stainless steel for a modulation anode, has long been SBO and AM in the rf output, which eventually increase thebelieved to be the reason of absence of this insulation power reflection from the accelerating cavities. Joint efforts
have been made with each maker to improve the tube
breaking in 0.8MW/l.OMW tubes.
stability by modifying the original design. In the mostsimilara(v04)
tubeRecently, however, in one 0.8MW
Fig.
advanced versions, both tubes behave very nicely [4][8].
inillustrated
asobserved
beenhaskind of phenomena
New findings reported here are summarized as follows: 1)
4, showing this instability not unique to special tubes but
more general to high power klystrons. This Ia spike The 1.2MW klystrons (E3786/E3732) are uniquely stablenormally· accompanies increase of beam current, lb, and tubes without irregular increase of anode current and SBO
seems dependent on VK and to be avoidable by adjusting the notwithstanding their high gain and high effiCiency. This
prefocusing coil current lpf and aging at relatively higher does not mean, however, the absence of back streaming.
electrons. Constancy as well as smallness in number ofVK.
such electrons must be a key reason to account for this fact.
8= = F=2) Negative anode current spikes and FSB were found for theh 0.02J.1A
first time in a 0.8MW tube. Although it is unclear whether=- - --,-:
the mechanism is completely identical or not, the FSB may
be the more universal breakdown phenomena between anode::± ±:::_ == t=== lil:::lb 3.9A
and body electrodes. 3) New techniques, like ion millingand fine-grained tungsten top la'yer, were not so successful•
as expected [9] to shorten the initial aging period of the Ir'coated M-type cathode. 4) By optimizing the magnetic field
It 8.3A •profile and body' and output window temperatures of 1.2MW-·' -·
tubes, efficiency can be increased above 66% for 1MWoutput, although further studies for optimization are needed.
••=~E E Ia -0.30mA

.

.

---.

References

Fig. 4 Negative anode current spikes observed without rf
for the first time in a 0.8MW tube. _The operational
conditions are as follows:Vh 21.8V, lh 21A,
VK 90kV, lb 3.9A, Va 19.4kV, Ia-- 0.3mA,
If 8.28A, and lpf 5.38A. Time scale is 30min/div.

4 Efficiency of KEKB Tubes, E3732
When operating storage ring accelerators, stability is themost important requisite of klystrons. As the power cost isgenerally increased with the scale-up of the machine,
however, the power efficiency becomes relatively important.
To optimize the tube regarding efficiency we prepared a
special focusing coil whose 8 elements can be excited
separately, and closed water chiller systems that can control
body and window water temperatures independently.
Efficiency of the 1.2MW tubes is in average 3-5% larger
than that of the 0,8MW/l.OMW tubes. By adjusting themagnetic field profiles and body and output window[1] . H. Frischholz, Proc. 1989 Part. Accel. Conf.,
Chicago, (1989) 1131.
[2] E.-G.Schweppe et al., Proc. 3rd European Part. Accel.
Conf., Berlin, (1992) 1215.
[3] S. lsagawa et al., Proc. 9th Symp. Accel. Science andTechnology, Tsukuba, (1993) 128.
(4] S. Isagawa et al., Proc. 4th European Part. Accel.
Conf., London, (1994) 1915.
[5] S. lsagawaet al., Part. Accel. 29 (1990) 793.
[6} S. Isagawa et al., Proc. 3rd European Part. Accel.
Conf., Berlin, (1992) 1206.
[7] M. Yoshida et al., .Proc. 9th Symp. Accel. Science andTechnology, Tsukuba, (1993) 131.
[8] S. Isagawa et al., Proc. 4th European Part. Accel.
Conf., London, (1994) 1912.
[9] S. Isagawa et al., Proc. 3rd Workshop,on PulsedRF
Sources for Linear Colliders, Hayama, (1996) 252.
[10] M. Yoshida, Private Communication.

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