The 9th Symposium on Accelerator Science and Technology, Tsukuba, Japan 1993

Suppression ofSBO and Stability Improvements of TRISTAN 1MW High Power Klystrons
Shigeru Isagawa, Masato Yoshida, Yasunao Takeuchi, Masaaki Ono and Shin-ichi Yoshimoto

National Laboratory for High Energy Physics, KEK, 1-1 Oho, Tsukuba-shi, Ibaraki-ken, 305 Japan
Ernst Gunter Schweppe, Enzio Demmel and Horst Seifert

Philips GmbH, RHW, Stresemannallee 101, D-2000 Hamburg 54, Germany

Abstract.Philips IMW CW Klystrons have been well operated inlRISTAN for almost 7 years. Problems like degradation ofgun insulation and abnormal anode emission, both caused byexcess barium evaporation from cathode and by emission ofelectrons from hot electrodes of electron gun, have beensuccessfully solved by the use of M-type cathode as well as ofredesigned gun with decreased electrode temperatures. The
electrode temperatures are, however, still yet to be optimized.
Modifications should be refined much more.
Positive spikes of anode current and Side Band Oscillations(SBO) which are caused by back streaming electrons fromcollector andior output gap have been studied intensively.
Much improvements have been made so far by taking a goodcombination of modified drift tubes, anode shape, number ofcavities and redesigned output transitions.

I. INTRODUCTION
Initial problems encountered in operation of IMW klystronYK1303 have been solved almost successfully owing to KEKPhilips joint efforts. Among these problems, intrinsic ones
were all related to the electron gun structure. The phenomena
were caused by unwanted electrons which emitted fromincorrect places and were never focused nor simulated properly.
These electrons are shown schematically in Fig.l, withnumbers CD,® and®. Electron emission from wehnelt(CD)as well as partial interception is usually inevitable. If it'slimited in safe region, say below 2-3 rnA, no troubles canhappen in tube behavior. After a prolonged operation,
however, excess barium deposited on top of the wehnelt
cylinder, which increases the anode current due to thermo-fieldemission. Insulation between anode and wehnelt turns worseand HV becomes inapplicable due to heavy electron loading.

~\_GNDBODY<3) loSpikesn.

MOD.
L'== ANODE+WEHNELTCATHODE-90kVHEATERFig. 1. Gun structure and potential level of each electrode.

Furthermore, if the anode tip is thermally isolated, it is heated
up by the abnormal wehnelt current to the temperature highenough for the anode emission (®) to be triggered, which willcause the decrease and even negative going of the anode current[1]. Back streaming electrons (®) which induce the anodecurrent spikes and/or instabilities of SBO are not directly
related to the Ba evaporation. In some klystrons, however, thestable range decreases with time. They become unstable evenunder matched conditions after about 3,000h of operation [2].
The time dependent effect can be well explained by increase of
secondary electrons caused by unwanted electron emission of
CD and ® as well as by deposit of Ba on drift tube surfaces.
In order to cope with these instabilities, several remedies
have been resorted to as follows: 1] Reinforced oil cooling ofeach electrode; 2] Decreasing of electrode temperature, mainfeature of which is backlini11g of anode and wehnelt withcopper (® in Fig. 2 ); 3] Special vacuum pretreatment ofcathode and gun subassembly before installation; 4] Lowering
the cathode temperature and reduction of Ba evaporation by theuse of Os-Ru coated M-type cathode (CD in Fig. 2 ).
Much improvements have been found in tubes that abovemeasures were taken for. The anode current has become small
and very stable. No instabilities like negative going of Ia
have happened any more. Only the phenomena of Ia spikes
and SBO have been left unsolved even in the newly designed
tubes having the gun with the lowered electrode temperatures.
On top of that, after prolonged period of operation, new typeof problems have become actualized of late, which need furtherimprovements and more refinements of modifications on ourtube.

II. HOT OR COLD GUN ELEClRODES?
During the last cycle of lRISTAN operation, one tube
(V21) was troubled. On flat top, it showed all of a sudden asevere degradation of insulation between electrodes. A crowbar
fired. High voltage could not be applied any more due to highanode current accompanying a gas burst. High potting seemed
ineffective as the cold emission increased sharply only above
22kV. The filament time accumulated was about 14,300h.
In reality two other tubes (Vl4 and Vl7) died so far very
similarly. Their life times were 8,916h and 5,319h,
respectively. The phenomena were well accounted for by peel
off and dropping of Ba from the inner anode surface onto thewehnelt top as well as the cathode dish. When such a tube
was opened, a lot of flakes and small particles of Ba werereally found on these electrodes. The Ba on the annular surfaceof the anode less than about 1OOmm in diameter wascompletely removed out and the copper substrate was exposed.
All these tubes have guns with measures to hold the electrodetemperatures as low as possible to suppress primary emission.
They are equipped with B-type cathodes (5:3:2 mole-ratio ofimpregnant).
Analyzing these effects and comparing them with results
coming from data of hot-standing gun electrodes, thefollowing hypothesis can explain the phenomena: Ba on cold
surfaces is not well bonded and tends to crack and fall off when-128-layers become too thick. Additional impact by electrons orions may speed up this process. The adhesion of Ba on Cu is
sensitive to temperature. At elevated temperatures Ba-Cuprite
is formed with good bonding to Cu. About 300-400°C is thecritical temperature range for this reaction to occur. From thispoint of view, anode temperatures in cold gun case shown in
Table I are obviously too low.
Table IYK1303 Gun TemperaturesOHeater Current
21.5A (M)23.0A (B)(Cathode tyj:!e)Gun DesignHotColdHotCold
Wehnelt>600°C 260°C >600°C 280°C
3oooc 130°C 340°C 170°C
Anode+a3l
Anode+100W2)

480°C 250°C

510°C+~3)

Ill. Os-Ru M-TYPE CATHODE
In order to improve stability over lifetime we haveintroduced Os-Ru coated M-type cathode together with a
special cathode pretreatment to 7 tubes in total. Decreaseabout 70K in cathode temperature could be allowed and reduced
Ba deposition has been expected[3]. All tubes except 1 haveshown good emission behavior after aging in factory for aboutlOOh. Very recently, however, the second new problem hasarisen. The sudden degrading of the emission as shown inFig. 3 has been found in the most preceding M-cathode tube.
18~~~~~~~~~~~

1716~ 15E.,

280°C

t:: 14::l--<>-91/12/26

LV 5192.0h.,E"' 13-e -92/07/02

LV 8414.4h- • - 93/03/31

Lv 11938.7h

12--•- 93/04/05

LV 12058.7h-+ - 93/06/04

Lv 13331.1 huSock;et8()"C80°C
1) Data extrapolated from the other tube's experiment.
2) 100W=50kV x 2mA.
3) About zero when a "semi-cold" wehnelt is used.

o:l11Above discussions are summarized as follows: a) Excess
Ba evaporation must be minimized. Os-Ru M-type cathode at
low temperature is preferable. Further reinforcement ofvacuum pretreatment before installation is necessary; b)
Suppression of thermal emission requires temperatures ofwehnelt and anode below 550°C (better <500°C); c) Too lowtemperatures, however, promote peel off of Ba deposit.
Suppression of Ba blistering probably requires temperature
above 300°C.
COLLECTOR

7
MOD. ANODE

10 L............J..~....L..~.L.............J..~....L..~.L.o............J.~..J
250 300 350 400 450 500 550 600 650

Heater Power [W]
Fig. 3. Underheating data vs. time of V23.
From the beginning this tube has been operated with adecreased heater current of 21.0A (466W) very sufficiently.
Until now underheating data have been taken regularly during
each shutdown period. Just when new data were being
collected on 93/03/31, the gas level in the tube increased andas a consequence the emission decreased. In general M-type
cathodes are more sensitive to poisoning by gas than normal
B-type ones. Especially the reactivation time after gas bursts
is longer if an especial thermal flashing procedure is left
undone as it is in our case. Anyway in order to ascertain
whether the emission can be recovered or not, this tube must
be aged in our test stand further and probably this flushingprocedure must be tested. As there is also ample room for
doubt that poisoning was caused by underheating processitself, for the next M-type tube, only change of gun perveancewith time will be monitored at a fixed underheating point.

'>1-.r-tt-+-WEHNE LTIV. ANODE SPIKES AND SBO

Fig. 2. Modifications against backstreaming electrons.
In order to find out the compromised solution, newcombinations are to be tested in next two tubes: i) Coldwehnelt, hot anode and M-type cathode for new one; ii) Semicold wehnelt, hot anode and M-cathode for modified one. Hot,semi-cold and cold can be finely adjusted by removing,thinning and leaving Cu linings (® in Fig. 2). These
modifications should put wehnelt and anode temperatures innarrow optimum regions of 380/400°C and 300/500°C,
depending on interception, respectively. Sandblasting of innersurface of anode is also to be applied for getting higher
bonding strength between Ba and Cu.

The last but most serious problem left unsolved is theinstability due to irregular increase in modulating anodecurrent and Side Band Oscillation[2]. These phenomena can be
attributed to back streaming electrons whose hit on anode ringcauses current spikes and whose implication in a feed backmechanism involving high Q buncher cavities induces beamintervened self oscillations[3]. Many factors are involved in
the instability: drive power, load mismatch, beam andgeometrical perveances, focusing field, cavity Q, drift tube
diameter, anode ring diameter, electrode temperatures, extent of
Ba deposit on electrodes and drift tubes, etc.
Measures and modifications taken for improvement so far
are as follows: 1) M-type cathode instead of B-type in order tosuppress Ba evaporation (<D in Fig. 2); 2) Cold gun to-129-

suppress unwanted emissions from wehnelt and mod. anode(®); 3) Decrease of the 1st drift tube diameter at the transition
gun-RF section, aiming to hide the mod. anode ring from backstreaming electrons (®); 4) Decrease of the collector entrance
diameter in order to minimize the contribution of secondaryand backscattered electrons (®). The diameter was chosencarefully not to intercept the blown up beam and was decreasedin two or three steps with the additional modification of polepiece diameter (@); 5) Increase in diameter and oblique shapeof anode hole for the same purpose as the item 3) (®); 6) 5
cavities instead of 6. Table II summarizes these modification
points taken for recent 8 tubes.
Table IT Modifications made on YK1303
Tube NoV .•..

M2difisc1tism~~~4~~~!i

11A~714A!!lAl)M-typc2)Cold gun
3)1st drift tube4)Step I
Step 2 or 3
5)Anode ring

00
X
0X
X

X
0X
0X
X

000X
0X

000
X
0X

000X
X
X

000
X
0X

000X
X
0fil~ Carin~

X

X

X

X

X

X

X

000X
X
X
Q

Although the instabilities have not been completely
repelled out, the extent of the region which ensures a stable
operation has been made clearly wide. Interestingly, the best
stability and even the highest efficiency have been obtained bya modified tube (VOlA) with reduced cavity number (6~5).
VOlACD0Effects of modifications and new findings foundphenomenologically are as follows: A) Ia spikes and SBO
almost always induce increase in li (ion pump current) andmodulations (AM) in Po (output power) and Tc (collectortemperature); B) They are observed slightly under saturation.
Triggers are often necessary for their starting. In some cases
10 or even 30 minutes were needed for them to be triggered;C) They are very sticky phenomena with respect to Pdchange, showing a kind of hysteresis as illustrated in Fig. 5;D) Simultaneous modification of items 3)&4) seems very
effective on Ia spikes. Extent ofla seems limited, say below
1.5mA. Anode overcurrent (O.C) is scarcely found in thesetubes (V25, V26 and V27); E) Anode O.C. seems different
from usual Ia spikes prior to which grassy fluctuations andbackground increase are found. O.C. happens often all of asudden without any small symptoms while the tube isoperated very calmly and stably. Conditions seem to be
fulfilled in very narrow and limited region on lpf- If stability
map as experienced typically by Vl7A, V14A and VOlA; F)
Modification item 5) on Vl4A decreased the efficiency as wellas perveance as expected. More trials may be necessary which
combine it with items 3)&4); G) Ia spikes are not always
equal to SBO. In some cases only SBO can be observed withneither Ia spikes nor Ia O.C. Shown in Fig. 6(a) is an
example of V26. While coincidence is found between Ia
spikes and SBO, the envelope of the spectrum shows an AM
pattern which is always symmetric with respect to the carrier
frequency (Fig. 6{b)). The pattern is not discrete, but partly
continuous and changes with operating conditions. Relations
between these signal peaks and cavity tuning frequencies arestill uncertain, and not so clear as observed in LEP tubes[4].

00.500000000® 0 0 800008~~

000®

X 0

00000

®

X 00080 0 ®

~00000<D®

0.00~

4-0<D<D<DCDd\o o o om om m w·w

CD~00_....-0\A little bit high 6 Ta

-0. 5 L.l.--'--"--L....L--'--"--L_L-::-'::::_._L_L_,_::-'-_j_::--'
7.5

Stable & high efficiency
Stable<D

Stable & low efficiency

ElNo SBO, but gas burst

X

SBO & ~. spikes®

SBO & I, OC(b)(a)
Fig. 6. SBO spectrum: (a) when no instabilities were found in la;(b) when instabilities were found in la coincidentally.
Span 10MHz, Horizontal 1MHz/div, Vertical lOdb/div,
Center frequency 508.63MHz.

9.0

8.5

8.0•0l1 !AJFig. 4. A stability map vs. focusing & prefocusing currents.

V26

IV. CONCLUSION

1.0

A stable operation without Ia spikes, Ia OC and SBO has
been ensured over a wide range of focusing and load conditions
for modified versions of YK1303. Unknown factor has beenfound about poisoning of the Os-Ru M-type cathode, though.
Further improvements will be continued, including the findingof the best choice of operating temperatures of gun electrodes.

Tc I'Cl
130
0.5

120
'------c"-----"'------.J'------.J___j___j___j___j___j_---'--,-J50100
Pd IWlII 0150

V. REFERENCES

Fig. 5. Hysteresis of Ia and collector temperature Tc vs. Pd.
As an example a stability map of this tube is shown in Fig 4.
Other tubes also give a similar extent of stable region. If nochanges were expected of the map and the stable region should
neither shift nor shrink with time, a sufficiently stable
operation without instabilities would be ensured.

[1) M. Yoshida et al., Proc. 6th Symp. on Accelerator Science andTechnology, Tokyo, pp126-128, October, 1987.
[2] S. Isagawa et al., Part. Accel. 29, pp783-800, April, 1990.
[3] E.-G. Schweppe et al., Proc. 3rd EPAC, Berlin, ppl215-1217,March, 1992.
[4] H. Frischholz, private communication.

-130-