The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

OPERATION OF THE UHF CW KLYSTRON AT THE
WORLD’S HIGHEST POWER LEVEL
S. Isagawa, High Energy Accel Res Org, KEK, Tsukuba-shi, Ibaraki-ken, 305-0801 Japan
Abstract
One of the leaked KEKB CW klystrons was repaired
in-house and aged at the klystron test field to recover its
original rf function. At 508.887 MHz the world’s highest
power level of 1.3 MW was reached. The coaxial rf
window cooled by a hybrid system using air andcirculated water, its titanium nitride coating and theimproved version of slanted-tube water load proved towork normally and stably under such super-high power rf
conditions. This paper describes a performance and highpower characteristics of this tube. The tube shows atendency, however, to lose its aged memory after somestandby periods. Limitations of such in-house repairing
are also discussed.

dc- and rf-aged at the klystron test field [1]. Applicationof high voltage was, however, very difficult even with areduced beam current, because the beam voltage of ourklystron power supply (KPS) cannot be lowered below 47
kV. Initially, about 110 times’ switch-on trials wereneeded, which extended over 3 calendar days. Thenafterwards, the tube was firstly operated in diode modeby using another low voltage power supply (e.g. 1 kV, 50mA). After decreasing the background gas pressure
sufficiently, switching on the tube with KPS in triode
mode showed no difficulties. Owing to the beam
pumping effect vacuum in the tube was made better andbetter. Then the rf ageing smoothly progressed up to 1.3
MW with neither serious gas bursts nor abnormal arcs.

IN-HOUSE REPAIRINGIn KEKB 24 high power CW klystrons are now being
used, of which 4 are 1 MW ones from Philips and other20 are so-called 1.2 MW ones from Toshiba. Both tubes
have been developed and improved originally for
TRISTAN in close collaboration between KEK and eachindustry. The MTBF of the latter is for example 34,636 hand, even 46,917 h, if initial failures during the period
below 1,000 LVh are not included, considering InfantMortality. The heater-on time, LVh, of five tubes
exceeded 50,000 h and that of top runner (T27 installedin D7-B station) has almost reached 70,000 h. Stable,
reliable and long life performances have been assured,
which contributed much to KEKB’s success.
Until now many leaked klystrons, irrespective of 1
MW or 1.2 MW ones, have been repaired in-house bybrazing (hard soldering) and by the treatment withvacseal®. They have then been aged in test fields [1] andreturned to active service. The leak test was based only
on 8 l/s ion pumps. Freon, butane, ethanol and water
were used as leak hunting probes. Leakage of one
klystron (E3786, T41) occurred while stored on standby
and its mode was unique. Two of three stainless steel
sealing members, 0.5t / 0.5t and 5 mm in diameter,around the output coaxial line were located as leaksources. They play roles to seal the holes that are used toalign an inner conductor during the shrinkage fit process[2]. One leak was on the welding bead but another was
located about 1 mm below the bead, indicating leaks
were caused by sensitization although parts made of low
carbon stainless steel (SUS304L 0.5t / 0.5t) must havebeen used.
The leaks were sealed with vacseal® resin. Leakpoints were further localised by removing the sealantwith acetone and again sealed with it as small quantity as
possible. After curing in warm atmosphere the tube was

Figure 1: Klystron characteristics at KEKB frequency,
508.887 MHz. Constant collector loss curves (in kW) areshown up to 1000 kW. The highest rf output is 1.3 MW.

TUBE PERFORMANCE
CHARACTERISTICS AT KEKB FREQUENCY
Klystron characteristics at KEKB frequency, 508.887
MHz, are illustrated in Fig. 1. This tube, T41, works
with high efficiency (64-67%) and large gain (53-58 dB).
The highest rf output was 1.3 MW, which is the world
record in the UHF CW klystrons. Constant collector loss
curves (in kW) are shown up to 1000 kW. This tube
belongs to the category of Type 2, which shows morecollector loss on the way of increasing the modulating
anode voltage [3].

GENERAL RF PERFORMANCE
Figure 2 shows the measured saturation output power,
the efficiency and the phase shift as a function of thebeam voltage with the modulating anode voltage adjusted
for a µbeam perveance of 0.75. The maximum efficiency
is 65.5% at a beam voltage of 85.2 kV for a beam currentof 18.3 A. The highest rf output power is 1.3 MW for abeam voltage of 94.1 kV with an efficiency of 64.8%. A
four port

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

∆φphase-shift circulator and an improved version ofslanted-tube water load [1] have performed well (VSWR1.07) under such super-high power rf conditions. Figure
3 shows the variation of the saturation output power, theefficiency and the phase shift as the modulation anodevoltage is varied. An example of the frequency
dependence of the output power, the phase shift and thegroup delay, tg = − dφ /dω, are shown in Fig. 4 for thebeam voltage of 90.1 kV.

508.887MHz

1400

600

1200

Beam Perveance= 0.75 x 10-6 A / V 3/2
Focusing = const.

∆φPo

1000

500

400

300

η
80

800

η

600

60

400

40100

200

2000200

020406080

100

SAT. EFFICIENCY (%)

SAT. OUTPUT POWER (kW)PHASE SHIFT (Degrees)

Po7000BEAM VOLTAGE (kV)

Figure 2: Performance of T41 as a function of beamvoltage.
∆φPo508.887MHz

701400Beam Voltage = 90 kV

601200403020Po1000

η
80800

η

6006040400∆φ10200

20000020406080100SAT. EFFICIENCY (%)

50SAT. OUTPUT POWER (kW)PHASE SHIFT (Degrees)

Focusing = const.

ANODE VOLTAGE (kV)

Figure 3: Performance as a function of anode voltage.

HYBRID COOLING OF OUTPUT WINDOW
Ceramic windows now adopted for 1.2 MW tubes areof disk type made of 95% aluminum oxide. Both innerand outer peripheries of the disk are cooled by water andthe flat face is by filtered forced air. The vacuum-side
surface of the disk and the 152D part of the coaxial
waveguide are coated with titanium nitride about 10 nm
thick to avoid abnormal temperature rise due tomultipactoring [2,4-6]. In the test field 10% ethylene
glycol water is used as a coolant, which is circulated by achiller, CFT-75. The pressure drop ∆ p across a known
disk-type iris in a sufficiently long straight pipe (DIN1952) was measured by using a U-tube manometercontaining water or silicon oil. The flow rate of air canbe obtained as a function of ∆ p by using a working curve
Q = f (∆ p). Strictly speaking, the temperature difference∆T of air depends on the air inlet temperature, Tin. To
eliminate the changes due to this dependence, aregression line was induced by way of regression analysis
and 47.1˚C was selected as a base air inlet temperature.
In the plots, this compensated temperature rise is used.
-11∆T = Tout -Tin (K)

800

AIR
0.7 m3 /min-12-13-14-15: POWER INCREASED-16: POWER DECREASED-17020406080100120

OUTPUT POWER (kW)Figure 5: Compensated temperature rise of air as afunction of klystron output power.

Figure 4: An example of instantaneous bandwidthcharacteristics of T41. The beam voltage, Vk, is 90.1 kV.

As shown in Fig. 5 and 6, the behaviour of thetemperature rise is linear and no abnormal increase canbe seen, indicating heating is caused mainly by dielectricloss and one-side multipactoring is suppressed by thetitanium nitride coating. Around 800 kW one can see aslight decrease of slope the origin of which is for the time
being not definitely clear. Possible explanations are as
follows. With growing rf electric field, the effective total
secondary electron yield; 1) gets smaller than unity; 2)gets limited by the finite dimension of the ceramic disc.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Figure 6: Temperature rise of (a) cooling air and (b)chiller cooling water containing 10% ethylene glycol as afunction of klystron output power up to 1.3 MW.
From the flow rate Q (l/min) and the temperature rise
∆T (K) we can estimate the dissipated power P (kW) as

stage, it would take long ageing time. Even if the superpower as high as 1.3 MW is capable, such a tube could
not be used for KEKB machine operation practically.
All these phenomena can be accounted for just by theexistence of small quantity, 5.24 ppm by volume, ofhelium in the atmospheric air. If the leak hole is very
narrow, only the helium can selectively diffuse into thetube owing to its smaller atomic dimension (1.95 Å indiameter) and higher permeability. The narrower theaisle, the higher the selectivity of helium. After a longterm storage, it can gradually concentrate andaccumulate in the tube. Due to its weak binding toelectrode surfaces helium can be easily desorbed with low
energy electrons below 100 eV but never pumped out bygetter ion pumps and can return back to gun and rf
output area. For a complete repair, such a tube must beopened and baked in a furnace again. We could evacuate
the residual inert gas like helium only by this process.

follows:
P = Q ·∆T / (14.35· G), [G = C kg⋅K ⋅ ρkcal

CONCLUSION]

kg −1
,
"

where C is the specific heat and ρ the specific weight.
In our case, as the factor G, 1.02 was used for water and3578 for air. Figure 7 summarises power balance of thesystem. At the klystron output of 1.3 MW the total powerfrom the window is 1.63 kW, while the body loss
amounts to 19.4 kW. About 26% of heat from thewindow is removed by air and 74% by water. The ratio
by air increases with rf power.
µ WAVE OUTPUTP0 = 621.8 kW

P0 = 0 kW
10% Glycol
WATER
8.4 l/min∆T=0.3K
22.7°CAIR
0.7m3/min
30.7°CP0 = 1.3 MW
8.4 l/min

0.7m3/min
80.8°C

58.0°C∆T=-17.0K ∆T=1.1K∆T=8.6K∆T=2.1K∆T=31.0K

24.0°C

49.7°C

25.4°C

49.8°C

47.6°CREFERENCESOUTPUT
WINDOW

PW = 0.63 kW (84%)
PA = 0.12 kW (16%)

WATER : PW = 0.17 kW
AIR

Small leak from air occurred in one of the high power
klystron tubes. Grain boundary corrosion of welded parts
made of stainless steel was the cause of that leak. Thetube was successfully in-house repaired and aged torecover its original rf specification and even more.
Continuous operation at the world’s highest power levelof 1.3 MW became possible. The hybrid cooling system
for the rf output window worked well. This tube cannot
be, however, used on active service as it soon loses theaged memory and emits a lot of gas again. Helium can bea most probable gas accumulated in such a tube.
Selective diffusion and ease of desorption of this inert gas
can account for the tube behaviour losing its dc- and rfaged memory.

: PA =-0.23 kW

TOTAL : PW + PA = -0.06 kW

0.75 kWKLYSTRON BODY LOSS :∆TB 1.2 K, 8.6 kW

PW =1.21 kW (74%)
PA = 0.42 kW (26%)
1.63 kW∆TB 2.7 K, 19.4 kW

Figure 7:Hybrid cooling system of klystron output window.

DISCUSSION
Leak hunting, repairing, dc ageing and rf ageing weresuccessfully done on one of the leaked KEKB tubes (T41).
After cured, the tube could be operated continuously
above 1 MW for 9 h, above 1.2 MW for 1.5 h, and above1.3 MW for 15 minutes without any vacuum or rf
troubles.
Gas bursts accompanying the multipactoring effects inthe coaxial waveguide around the power levels of 36.5
kW, 132.4 kW and 267.3 kW, respectively, werecarefully suppressed by long term on-site rf ageing. If
stored, however, this aged tube showed a tendency to lose
its memory of having been aged. Although we know theway to conquer large gas bursts happening at the early[1] S. Isagawa, A. Miura and K. Suzuki, “New Klystron Test
Fields and Improved Version of 1.2 MW Water Load,”
EPAC’00, Vienna, Austria, June 2000, p.2426.
[2] K. Ohya and Y. Kawakami: Japan Patent 1860949 (1986),
U.S.Patent 4,734,666 (1988).
[3] M. Yoshida, S. Isagawa, Yo.Takeuchi, M. Ono, S.
Yoshimoto, K. Itoh, K. Kobayashi and K. Ohya,“Characteristics and Anode Current Stabilization ofTRISTAN 1.2 MW CW Klystron,” SAST’93, Tsukuba,Japan, Aug 1993, p.131.
[4] S. Isagawa, Yo. Takeuchi, H. Baba, J. Tanaka, K. Ohya, Y.
Kawakami and S. Hosoi, “Development of High Power CWKlystrons for TRISTAN,” IEEE PAC, Washington, D.C.,
U.S.A., March 1987, p1934.
[5] S. Isagawa and Yo. Takeuchi, “Suppression ofMultipactoring on High Power rf Ceramic Windows byTitanium Nitride Coatings,” 84th Lecture Meeting of theSurface Finishing Society of Japan, Tsukuba, Nov 1991,
p.97. (in Japanese).
[6] S. Isagawa, M. Yoshida, Yo. Takeuchi, K. Itoh, K. Ohya, K.
Kobayashi and T. Higuchi, “Coating Techniques andImprovement of High Power CW Klystrons for TRISTAN,”
EPAC’94, London, U.K., June-July 1994, p.1912.