A HIGH EFFICIENCY LONG PULSE MULTI BEAM KLYSTRON FOR THE
TESLA LINEAR COLLIDER
A. Beunas, G. Faillon, THOMSON TTE, FranceS. Choroba, A. Gamp, DESY, Germany

Abstract
The Tesla linear collider requires 600 pieces of 10MW
L-Band klystrons operating at a pulse duration of 1.5ms
and a repetition rate of up to 10Hz with an efficiency ofthe order of 70%. Since these performances are notachievable with a single beam klystron, a multi beamklystron (MBK), which uses many low perveance
electron beams in parallel in one vacuum envelope, wasdeveloped by THOMSON TUBES ELECTRONIQUES (TTE).
The advantage of this solution is the low voltage required
and the high efficiency compared with a single beamklystron. THOMSON TTE has developed and manufactured
the TH1801. This MBK is in operation at the Tesla Test
Facility at DESY, where it was tested at full pulse
duration of 1.5ms. It reached 10MW at 117kV and 131A
with an efficiency of 65%. This paper reports on thetubes, the test results and the operation experience of theTH1801 multi beam klystrons built until now.

1 THE TESLA LINEAR COLLIDERThe TESLA Linear Collider is a 500 to 800 GeVcm
e+/e- linear collider with integrated Free Electron Laser(FEL) facility. The electrons and positrons areaccelerated in two ~15km long opposing linacs using~10000 superconducting 1.3GHz 9-cell cavities per linac.
The accelerating gradient is 23.4MV/m for the 500GeVand 35MV/m for the 800GeV collider respectively. In thefollowing we will restrict ourselves on the parameters ofthe 500GeV collider.
The macro beam pulse consisting of 2820 buncheswith a spacing 337ns has a duration of 950µs. The
collider is operated with a beam pulse repetition rate of
5Hz with the exception of about 3km of the electron linac
operating at 10Hz, where every second pulse is used for
FEL operation. In order to achieve the accelerating
gradient of 23.4MV/m at a beam current of 9.5mA
231kW of RF input power per cavity are required.
Additional power of 6% must be foreseen for waveguidelosses and another 10% as regulation reserve. The RF
pulse duration of 1.37ms is given as the sum of the beampulse duration of 950µs and the cavity filling time of
420µs. More detailed information can be found in [1].

2 RF SOURCE REQUIREMENTS
In order to minimize the cost of the entire RF system
and in order to achieve high reliability the number of RFstations, which convert AC line power into RF power forthe superconducting cavities, and hence the number of
RF sources should be chosen as small as possible. The
RF power generated by each RF source must then be
distributed to a number of cavities. This results in thedemand of the highest output power, which can begenerated reliably by one RF source.
Some years ago the THOMSON TH2104 was theklystron with the highest output power at long pulse
duration at 1.3GHz. 5MW could be generated with thisklystron. About 1200 klystrons of this type would berequired to equip the TESLA collider. In case of
doubling the output power from 5MW to 10MW thenumber of klystrons could be reduced to about 600 eachsupplying RF power to 36 superconducting cavities.
Besides cost and reliability, efficiency is anotherimportant item, since the power demand of the RF
sources determines to a big extent the power demand of
the entire collider.
One of the parameters which determines the efficiency
of a klystron is the perveance defined as p=I/U3/2, where
U is the klystron cathode voltage and I the klystroncurrent. Comparison of various klystrons shows that theklystron efficiency increases with lower perveance [2, 3].
For a perveance of 2.0.10-6A/V3/2 the efficiency typicallyis 45%, whereas at 0.5.10-6A/V3/2 70% seems to befeasible. The increase in efficiency is due to the effectthat a low perveance results in a low space charge density
which allows easier bunching and therefore better energy
transfer from the DC to the AC component of theklystron electron beam.
The TH2104 has an RF output power of 5MW at avoltage of 128kV and a current of 88A, hence theperveance is 1.92.10-6A/V3/2 and the efficiency 44%.
The attempt to construct a klystron with an efficiencyof 70% and an output power of 10MW based on a singleklystron electron beam with a perveance of 0.5.10-6
A/V3/2 would result in a klystron beam voltage of 241kV.
Since the required pulse duration is 1.5ms theconstruction of a klystron meeting these parameters
seems to be impossible.

The solution is the use of several low perveance beams
in parallel at lower voltage in one vacuum vessel. Thistechnology is utilized in the multi beam klystron.

3 THE TH1801
THOMSON TUBES ELECTRONIQUES has developed andbuilt the TH1801, a long pulse high efficiency multibeam klystron [4]. It uses seven electron beams inparallel, which are produced by the klystron cathodeshown in figure 1.

Figure 1: Cathode of the TH1801
The perveance per beam is 0.5.10-6A/V3/2. The klystronhas 6 cavities including input and output cavity. Thebeams share the cavities but have independent drift tubes.
After RF extraction in the output cavity the spent beams
are absorbed in the collector. Two output waveguides oftype WR650 with two pillbox windows are used tohandle the output power of two times 5MW at 1.5ms
pulse duration. Figure 2 shows the TH1801 without
electromagnet. The total height is ~2.5m.

Figure 2: The TH1801 multi beam klystronThe design parameters are summarized in table 1. The
design cathode voltage of 110kV is lower than thecathode voltage required for the TH2104 5MW klystron,
thus allowing the installation of a 10MW MBK in thesame modulator only requiring slight modifications forthe higher klystron current and for a different filament

power. The 10MW tube requires only ~360W offilament power instead of ~560W required for the 5MWtubes.
Table 1: Design parameters of the TH1801
DesignOperation Frequency
1300MHzRF Pulse Duration
1.5ms
Repetition Rate
10Hz
Cathode Voltage
110kV
Beam Current
130A
HV Pulse Duration
1.7ms
No. of Beams
7Total Perveance3.5.10-6 A/V3/2
No. of Cavities
6Max. RF Peak Power10MW
RF Average Power
150kW
Efficiency
70% goalGain
48dB
Solenoid Power
4kW goal

4 TEST RESULTS
Two MBKs have been produced until the end of 2000.
Both have completed the factory acceptance test at
THOMSON, where they were tested to full RF power at
shorter pulse duration of 0.5ms but higher repetition rateof 30Hz, since no modulator for a pulse duration of morethan 0.5ms exists at THOMSON. The first klystron wasinstalled in May 2000 in one of the HV modulators at theTESLA Test Facility (TTF) at DESY and thenconditioned and tested to full RF power at full pulse
width of 1.5ms, but lower repetition rate of 5Hz. It is
now in use for the operation of TTF. The second MBK
will be tested at full pulse duration in one of the new HVmodulators now being installed at DESY.
Figure 3 shows output power of the first MBK as
function of the input power for various beam voltages,pulse durations and operation conditions. The RF output
power was determined from calorimetric data taken fromRF loads. The measurements at 0.5ms performed at
THOMSON and at 1.5ms performed at DESY
respectively are in good agreement for the same voltages.
The measurements were done using water loads atTHOMSON and water cooled ferrite loads at DESY
under matched load conditions with a VSWR smaller
than 1.1. These measurements are indicated as Mode A.
Additional measurements with mismatched water loads
with a VSWR of 1.2 at optimum phase conditions wereperformed at THOMSON, indicated as Mode B. Under
these conditions the RF output power is higher than for
perfectly matched load conditions. The efficiency using
matched loads is 65%, whereas with mismatched loads
an efficiency of 68% could be achieved at 117kV.
The high gain of 48dB allows the use of
semiconductor drive amplifiers. Figure 4 shows thewaveforms recorded during the test at DESY. The
perveance of 3.27.10-6A/V3/2 is lower than design, thus

together with the slightly lower efficiency leading tohigher beam voltages. This klystron was operated at 5Hz
repetition rate to full power under all conditions. At
10Hz we were restricted to 105kV operation voltage or7MW output power respectively, because of main powerlimitations.

MBKs. The perveance of 3.44.10-6A/V3/2 is close to thedesign perveance.
Both MBKs require a slightly higher solenoid power of
5.5kW than the design goal of 4kW.

Figure 5: RF output power as function of RF drive powerof the second MBK at 0.5ms pulse duration for different
operation conditions

5 SUMMARY
Figure 3: RF output power as function of RF drive powerof the first MBK for different operation conditions
The 1dB bandwidth is more than ±5MHz. We alsomeasured a phase sensitivity of this klystron of 14.3°/kV.
After completion of the site acceptance test the first
MBK became one of the klystrons used for TTF
operation. Since only 16 superconducting cavities areconnected, the klystron is operated at less than 5MW of
total RF output power, typically at 3 to 4MW.

THOMSON TTE has successfully developed and built
a long pulse high efficiency multi beam klystron. All
tubes, which have been built and tested up to now, areclose to the design goals. Additional tubes and further
testing at long pulse duration is required and will bedone at TTF at DESY, where these klystrons will be
installed and used for the operation of TTF. For theTESLA linear collider additional modifications arerequired, in order to meet the infrastructure requirementsof the tunnel installation, for instance the horizontal
mounting position.

6 ACKNOWLEDGEMENTS
The authors would like to acknowledge the work of T.
Böckmann, T. Grevsmühl, J. Kahl, F.-R. Kaiser, V.
Katelev, F. Lu, G. Möller, S. Simrock, I. Sokolov, H.
Timm and U. Zobjack during the installation and testingof the first MBK at DESY.

REFERENCESFigure 4: Waveforms at 10MW output power recorded at
the first MBK during the long pulse test at DESY
The test results of the second MBK at 0.5ms pulse
duration are shown in figure 5. The maximum efficiencyof 63% for Mode A and 66% for Mode B at 116kV isslightly lower than for the first klystron. This is due to a
detuning of +4MHz of the output cavity, which occurred
during tube bakeout. Moreover, the output cavity had a
low Qx. Measures to avoid this will be taken for the next

[1] R. Brinkmann, “TESLA Linear Collider Design andR&D Status”, Proceedings of the XX International
Linac Conference, LINAC2000, Monterey, USA,
August 21-25, 2000
[2] C. Bearzatto, M. Bres, G. Faillon, “Advantages ofMultiple Beam Klystrons”, ITG GarmischPartenkirchen, Germany, May 4 -5, 1992
[3] R. Palmer, “Introduction to Cluster Klystrons”,
Proceedings of the International Workshop on
Pulsed RF Power Sources For Linear Colliders,RF93, Dubna, Protvino, Russia, July 5-9,1993[4] A. Beunas, G. Faillon, “10 MW/1.5 ms, L-band
multi-beam klystron”, Proc. Conf. Displays andVacuum
Electronics,Garmisch-Partenkirchen,Germany, April 29-30, 1998