Achievements and Future Plans of CLIC Test Facilities
H.H. Braun for the CLIC study team
CERN, Geneva, Switzerland
Abstract
CTF2 was originally designed to demonstrate the feasibility of two-beam acceleration with high current drive
beams and a string of 30 GHz CLIC accelerating structure prototypes (CAS). This goal was achieved in 1999
and the facility has since been modified to focus on highgradient testing of CAS’s and 30 GHz single cell cavities(SCC). With these modifications, it is now possible toprovide 30 GHz RF pulses of more than 150 MW and an
adjustable pulselength from 3 to 15 ns. While the SCC
results are promising, the testing of CAS’s revealed
problems of RF breakdown and related surface damage.
As a consequence, a new R&D program has beenlaunched to advance the understanding of RF breakdown
processes, to improve surface properties, investigate newmaterials and to optimise the structure geometries of the
CAS’s. In parallel the construction of a new facility
named CTF3 has started. CTF3 will mainly serve two
purposes. The first is the demonstration of the CLIC
drive beam generation scheme. CTF3 will accelerate a1.54 µs long electron pulse of 3.5 A in a fully beamloaded S-band linac. The linac beam pulse is compressed
in isochronous rings to 140 ns pulse-length, 35 A beam
current and a micro-bunch repetition rate of 15 GHz. The
second purpose of CTF3 is to test CAS’s with nominal
CLIC parameters. For this reason the drive beam
extracted from the combiner ring is transported througha string of 30 GHz power extraction cavities feeding
CAS’s. The 30 GHz acceleration is measured with aprobe beam provided by a small, separate S-band linac.

1 EXPERIMENTS WITH CTF2
CTF2 has been constructed to demonstrate the feasibility of the CLIC [1] two beam acceleration scheme. The
layout of CTF2 as operated in 1999 is shown in figure 1a. The drive beam generates 30 GHz power, while
the probe beam measures the accelerating field in the
30 GHz accelerator. Both beams are generated by S-band
RF photo-injectors. The RF photo-injectors cathodes are
illuminated by a short pulse (8 ps FWHM) laser. The
maximum drive beam charge obtained is 750 nC
distributed among 48 bunches with a bunch spacing of
10 cm. A magnetic chicane compresses the bunches to≤ 5 ps for efficient 30 GHz power production. Aftercompression, the beam is injected into the 30 GHz
decelerator, where a part of its energy is converted into
30 GHz power. A downstream spectro-meter magnet
measures the energy of the beam after power extraction.

The probe beam operates with a single bunch of ≤1 nC
charge. Before being injected into the 30 GHz
accelerator, the probe beam is accelerated to 46 MeV in
an S-band travelling wave structure. This is necessary toobtain a small enough geometric emittance to fit into the
4mm ∅ aperture of the 30 GHz accelerating structures.
Magnetic spectrometers before and after the 30 GHz
accelerator are used to measure the beam energy.

Two beam acceleration
With the 1999 CTF2 configuration two beam
acceleration was successfully demonstrated with four
30 GHz power extraction structures decelerating thedrive beam and a string of five 30 GHz accelerating
structures, accelerating the probe beam by 60 MeV. Moreinformation and references about techniques developped
for CTF2 and two beam acceleration results can be foundin [2].

CTF2 used as 30 GHz high power test standObservation of RF induced damage in X-band structuresat SLAC triggered the inspection of 30 GHz accelerating
structures used in CTF1 [3] and CTF2. It was found thatthose structures that had been run at mean accelerating
fields in excess of 60 MV/m showed signs of damage on
the beam iris of the input coupler cell. This came as asurprise since the usual indication of RF breakdown, fullreflection of power, was never observed. As aconsequence of these observations it was decided toabandon the original plan of dismantling CTF2 andinstead CTF2 was converted into a high power test standfor studying RF breakdown phenomena. A 1 m long,
30 GHz power extraction structure was constructed andinstalled in the drive beam line and the existing four
0.5 m structures were dismounted (fig. 1b). With thisconfiguration 150 MW 30 GHz power is produced in asingle WR28 waveguide. This allows to test CLIC
prototype accelerating structures at mean accelerating
fields up to 155 MV/m. On the probe beam side thestring of 30 GHz accelerating structures was taken out
and a testbed for one 30 GHz accelerating structure wasinstalled. This testbed is equipped for measurements of:− dark current charge and energy− incident, reflected and transmitted RF power− acoustic vibrations of the structure− visible light emitted from the structure− acceleration of the probe beam
− vacuum pressure

A

B

Figure 1

CTF2 configurations

A: for two beam acceleration with a string of 30 GHz structures
B: for high power testing of 30 GHz structures

Three different structures have been tested so far. Two
are disk-loaded waveguides of 28.5 cm active length andconstant impedance. The main difference between thesetwo CAS is the coupler design, which is the standard
CTF single feed design in one case and a prototype
double feed design in the other. The third structure was a
12.3 cm long planar structure build at the Technische
Universität of Berlin/Germany. The single port disk
loaded structure was breakdown limited at a mean
accelerating gradient of 60 MV/m corresponding to apeak surface field of 265 MV/m. The two port structure
reached a mean accelerating gradient of 70 MV/mcorresponding to a peak surface field of 266 MV/m. Bothstructures have the maximum surface field on the iris
between the coupling cell and the first regular accelerating cell. Post-mortem inspection of both structures
showed damaged surfaces at only this location. The
planar structure reached an accelerating gradient of
50 MV/m. The peak surface field for this structure isestimated to be ≈300 MV/m. Post-mortem inspection
showed slight damage in the coupler region for thisstructure as well. All field values quoted above are for
15 ns pulse length. Accelerating fields of 140 MV/m andsurface fields of 620 MV/m have been reached for 3 ns
pulses. A large quantity of data has been collected tocharacterise the RF breakdown process. One important
observation is that RF breakdowns in the CTF2
parameter regime hardly give reflected power as in moreconventional RF cavities. This explains why we didn’t
observe these breakdown events in our past experiments.
However, breakdowns produce intense electron bursts.
The intensity of the electron bursts is strongly correlatedwith a reduction of the RF power transmitted through thestructure. Another fascinating feature which has been
observed are light pulses emitted during RF breakdowns
which have a duration of few hundreds of ns, thus much
longer than the RF pulse itself. First experiments withaccelerometers show the presence of acoustic waves withamplitude proportional to the RF power. This amplitude
increases strongly when a breakdown occurs. This system
will be further improved with the goal to localize
breakdown events in the structures.

30 GHz, single cell, high gradient experiments
High gradient experiments have been performed with asingle 30 GHz cell powered directly by the CTF2 drive
beam. After conditioning the cavity with 5⋅105 pulses theonset of breakdowns was observed for ESURF τ380 MV/m.
The cavity was powered while its temperature was varied
from 100 K to 500 K. No significant influence oftemperature on the breakdown threshold was observed.
The observation of a positive shift of the resonant
frequency by about 200 MHz and the occurrence ofdelayed breakdown as seen in an earlier experiment withthis cavity [4] was confirmed (unfortunately, due to acalibration error, all field values quoted in [4] have been
too large by a factor π ). During this year we plan tocontinue this kind of experiments with cavities operating
at 21, 30 and 39 GHz to gather information about thefrequency scaling of RF breakdowns.

Structure R&D program
To overcome the RF breakdown and damage problems
with the 30 GHz structures an R&D program has been
established to find improved solutions. This program has
two approaches:− Improve the geometry of the structures. Since ourpresent data seem to indicate that the peak surface
field is so far our main limitation we try to reduce theratio ÊSURFACE/EACCEL . A gain of almost a factor 2 ingradient without compromising much on theacceleration efficiency and transverse wakefield
amplitudes is expected.
− Investigate new materials. A hybrid tungsten/copper
structure will be built with tungsten in the regions
where the electric field is highest.
Construction and testing of prototypes using theseapproaches is planned in CTF2 for 2001 and 2002.

Coherent synchrotron radiation experiments
Experiments have been performed with the CTF2
drive beam bunch compressor to measure the influence ofcoherent synchrotron radiation on beam emittance andenergy. These experiments are used to benchmark CSR
simulation programs. Details are found in [5].

Drive Beam
Injector

Two-Beam
SectionFigure 2

Drive Beam
Accelerator

3.5 A - 152 MeV
1.54 µs - 20 cm
bunch spacing

High Power
Test Stand

Layout of CTF32

CTF3, GOALS AND PLANS

The construction of a third phase of CLIC test
facilities has started recently [6]. CTF3 will mainly serve
two purposes. The first is the demonstration of the CLIC
drive beam generation scheme [7]. CTF3 will accelerate
a 1.54 µs long electron pulse of 3.5 A in a fully beamloaded S-band linac. The linac beam pulse is compressed
in isochronous rings [8] to 140 ns pulse-length, 35 A
beam current and a micro-bunch repetition rate of
15 GHz. The second purpose of CTF3 is to test CAS’swith nominal CLIC RF parameters for power and pulselength. For this reason the drive beam extracted from the
combiner ring is transported through a 30 GHz
decelerator feeding CAS’s. The 30 GHz acceleration ismeasured with a probe beam provided by a small,separate S-band linac. A schematic layout of CTF3 isshown in Fig. 2.

Drive beam injector
The drive beam injector [9] is built in collaboration
with LAL/Orsay and SLAC, with SLAC providing the
gun triode and the beam dynamics design; and LAL
providing the gun electronics circuitry and the 3 GHz
pre-bunchers. The injector features a 140 kV, 9 A
thermionic triode gun, a set of 1.5 GHz subharmonic prebunchers with fast 1800 phase switching every 140 ns asneeded for the phase coding described in [7], 3 GHz prebunchers, a graded-β travelling wave buncher withhigher order mode dampers and two TW accelerating
structures (see below). The buncher and the accelerator
structures are embedded in a long solenoid with amaximum on-axis field of 2 kG. A magnet chicane withcollimators downstream of the injector will be used toeliminate low energy beam tails produced during thebunching process.

×2
Delay Loop
C=42 mProbe Beam
Pre-Accelerator

35 A - 140 ns
2 cm bunch spacing

Bunch
Compression

× 5 Combiner Ring
C=84 mRF photo-injector option

Drive beam accelerator

As an alternative to the thermionic injector discussed
above an RF photo-injector for the drive beam is studied
as a potential later upgrade for CTF3. The advantages ofsuch an solution are smaller beam emittances in all threephase space planes, absence of low charge parasitebunches in every second 3 GHz bucket, no phase/energy
tails as produced in conventional bunching systems, andeasier tailoring of the 1800 phase switching. The designof the RF gun for such an injector can be based on
existing CTF2 guns [10] with moderate modifications,but producing the long drive beam train requires a newlaser design. Moreover, the average cathode currents
exceeds by far values achieved with RF photo-injectors
presently in operation. The specification of the beam for
the CTF3 photo-injector leads to the design requirementsof the laser system:• 1.5 GHz oscillator frequency,• 1.54 µs burst of 2310 pulses,• 15 µJ/ pulse, 35 mJ/ train,• pulse train intensity stability better than 0.3 %,• synchronisation to better than 1 ps,
In order to meet the specifications RAL/UK has madea study of a laser system as needed for a CTF3/CLIC
drive beam photo-injector [11]. It is concluded that sucha laser is feasible with heavily saturated, diode-pumped
ND:YLF laser amplifiers. When these are operated at thehigh bunch repetition frequency of the CTF3 beam,
33 kW of instantenous diode pump power will be neededin the final amplifier to maintain equilibrium conditions
during the train. Tests are underway at RAL to prove thevalidity of the proposed system including the construction of a 5 kW amplifier section. The cw laser oscillator
needed for injection into the amplifier should be highpower in order to reduce the gain requirement on theamplifier. Yet the high frequency (1.5 GHz) implies ashort cavity of 667 ps round trip time. Such high power,
mode-locked laser oscillators are studied at the Instituteof Photonics in Glasgow/UK with especial attention to
the CTF3 specifications. Tests are conducted on a 200 W
CW oscillator based on a “thin disk” configuration where
the active laser material is only 0.2 mm thick.
The feasibility of photo-cathodes with the required
performance for CTF3 has recently been demonstrated
with an experiment in the CTF photo-cathode laboratory.
In this test a high power laser with 100 ns pulselengthand λ=262 nm was used to illuminate Cs2Te cathodes ina 80 kV electron gun. The spot size of the laser beam on
the cathode was 6 mm FWHM. Vacuum levels weretypically 10-9 mbar. During these tests quantum
efficiencies of better than 1.5% were maintained for morethan 2 weeks of operation with mean beam currents of
0.7 mA. This performance exceeds by far the CTF3
requirements and brings in reach the use of an RF photoinjector for the CLIC drive beam.

The drive beam accelerator will consist of 8 modulesof 4.5 m length. Each module consists of two accelerating structures , a BPM, a quadrupole triplet and a pair ofsteering magnets. The structures are fed by one 35 MW
klystron with RF pulse compression. The pulse compression uses a programmed phase ramp to get an almost
rectangular RF pulse with a power multiplication of ≈2.
Extensive beam dynamics simulations have shown that
this configuration conserves a small emittance along thelinac despite of the high beam current. However, HOM
suppression techniques as described below are a must.

3 GHz drive beam accelerating structures
The requirements of fully loaded operation with highRF to beam power efficiency and a beam current of 3.5 A
lead to a 2π/3 mode travelling wave structure design with
100 ns filltime. The active length is 1.13 m and theenergy gain with full beam loading is 8 MeV. The highbeam current together with the long beam pulse length of
1.54 µs requires an effective suppression of higher ordermodes (HOM’s). Two different structure designs havebeen developped to cope with the HOM’s. The first
design is derived from the 30 GHz taperedA

B
Figure 3

A) The TDS prototype structureB) A SICA protoype cell

damped structure (TDS) of the CLIC main beam [12]using four waveguides with wideband SiC RF-loads per
accelerating cell. The cut-off frequency of these waveguides is in-between the accelerating mode and the first
HOM, thus acting as a high pass filter. With this method
the Q value of the first HOM is reduced to about 18.
Further reduction of long range wakes is achieved byspreading the HOM frequencies by varying the aperture
diameter along the structure from 34 mm to 26.6 mm. A
full prototye of this structure has been built and power
tested up to 40 MW (fig. 3a).
The second approach, called SICA for slotted iris constant aperture, uses four radial slots in the iris to couple
the HOM’s to SiC RF-loads (fig 3b). In this approach theselection of modes coupled to the loads is due to themode symmetry, thus all dipole modes are damped. The
SICA structure has a Q value of the first HOM of ≈5. Asin the TDS the HOM mode frequencies are spread along
the structure. The spreading is achieved by a nose cone ofvariable length keeping the aperture constant at 34 mm.
A prototype for testing is under construction

Delay loop and combiner ring
The design of the delay loop, combiner ring and related beam transfer lines is done by INFN/Frascati [8].
Magnet lattices isochronous to 2nd order have beendevelopped (fig. 4). Wiggler magnet chicanes areforeseen for adjusting the ring circumference precisely toa N + 15 multiple of the bunch spacing. Prototypes ofthese wigglers are under construction. The design workfor the 3 GHz RF deflectors used as injection kickers in
the ring is under progress at Frascati as well. The shortbunch length of σZ≈2mm and the high bunch charge of
2.33 nC puts stringent requirements on vacuum chamber
impedance and makes coherent synchrotron radiationenergy loss a serious issue.
Before the assembly of the different parts of CTF3 a
prove of principle experiment at low beam current isplanned to demonstrate bunch frequency multiplication
by injection with RF deflector cavities in an isochronous

REFERENCES[1] G. Guignard (ed.), “A 3 TeV e+/e- Linear Collider
Based on CLIC Technology,” CERN rep. 2000-008[2] H.H. Braun for the CTF team, “Exp. Results andTechnical R&D at CTF2,” EPAC, Vienna, 2000
[3] H.H. Braun and 16 co-authors, “Results from the
CLIC Test Facility,” EPAC, Sitges, 1996
[4] H.H. Braun, M. Luong, I. Wilson, W. Wuensch, “Avery High Gradient Test of a 30 GHz Single Cell
Resonant Cavity,” EPAC, Vienna, 2000
[5] L. Groening and 7 co-authors, “Emittance Growthand Energy Loss due to CSR in the Bunch
Compressor of CTF2,” EPAC, Vienna, 2000
[6] CLIC Study Team, ”Proposals for Future CLIC
Studies and a New CLIC Test Facility (CTF3),”
CLIC note 402, 1999

Figure 4 Optical functions of one combiner ring quarter.
ring [13]. This experiment will make use of the former
LEP electron injector linac and accumulator ring withsome appropriate modifications. A new 80 kV electron,
necessary to get the right time structure for this experiment, has been designed and constructed at LAL/Orsay.

30 GHz test area
The beam extracted from the combiner ring consists ofa 140 ns bunch train, with a bunch repetition rate of
15 GHz and a beam current of 35 A. This beam can beused to drive a single 30 GHz decelerator structure optimised for the given beam energy/current combination.
This RF source will be used to test the performance andlimitations of CLIC prototype RF structures andwaveguide components at nominal power and beyond. It
will allow to perform these tests with a pulse-lengthcomparable to the nominal CLIC value of 120 ns, which
is not possible with CTF2. Alternatively the extracted
beam can drive a string of decelerating structures topower a representative section of the CLIC main linac
and to accelerate a probe beam. This set-up will allow tosimulate realistic operating conditions for the mainbuilding blocks of the CLIC linac.
[7] R. Corsini (ed.), “The CLIC RF Power Source: aNovel Scheme of Two-Beam Acceleration for Electron-Positron Linear Colliders,” CERN rep. 1999-06[8] C. Biscari and 12 co-authors, “CTF3: Design ofDrive Beam Combiner Ring,” EPAC, Vienna, 2000
[9] H. Braun, R. Pittin, L. Rinolfi, F. Zhou, B. Mouton,
R. Miller, D. Yeremian, “An Injector for the CLIC
Test Facility (CTF3),” LINAC, Monterey, 2000
[10] R. Bossart, M. Dehler, “Design of a RF Gun for
Heavy Beam Loading,” LINAC, Chicago, 1998[11] I.N.Ross, “Feasibility Study for the CERN CLIC
Photo-Injector Laser System,” CLIC note 462, 2000
[12] M.Dehler, I.Wilson, W.Wuensch, “A Tapered
Damped Accelerating Structure for CLIC”, LINAC,
Chicago, 1998
[13] R. Corsini, A. Ferrari, L. Rinolfi, T. Risselada,
P. Royer, F. Tecker, “Beam dynamics for the CTF3
preliminary phase,” CLIC Note 470, 2001