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The CTF3 Two-beam Test Stand
Nuclear Instruments and Methods in Physics Research A 729 (2013) 546–553
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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The CTF3 Two-beam Test Stand R. Ruber a,n, V. Ziemann a, T. Ekelöf a, A. Palaia a, W. Farabolini b, R. Corsini c a b c
Uppsala University, Uppsala, Sweden CEA IRFU, Saclay, France CERN, Geneva, Switzerland
art ic l e i nf o
a b s t r a c t
Article history: Received 27 September 2012 Received in revised form 16 July 2013 Accepted 16 July 2013 Available online 24 July 2013
The Two-beam Test Stand (TBTS) has been constructed and operated at the CLIC test facility CTF3 at CERN. The TBTS comprises two parallel and independent electron beam lines and has been designed to demonstrate the feasibility of a two-beam high gradient acceleration concept as proposed for the Compact Linear Collider (CLIC). In the CLIC scheme, the RF power is extracted from a high current drive beam using RF power extraction structures while the main beam is accelerated using this RF power which is fed into high gradient high frequency normal conducting accelerating structures. The Two-beam Test Stand is a unique facility to demonstrate the feasibility of the CLIC two-beam high gradient acceleration concept and to test the individual CLIC components and complete two-beam CLIC modules. The TBTS is particularly well suited to investigate the effects on the beam of RF breakdown in the high gradient accelerating structures. We report on the design, construction and commissioning of the Two-beam Test Stand. & 2013 Elsevier B.V. All rights reserved.
Keywords: Two-beam acceleration RF breakdown CLIC CTF3
1. Introduction During the coming ten years high-energy physics will be dominated by research done at the LHC, but eventually a highenergy lepton collider will be required in order to do precision measurements of the new physics expected to be discovered at the LHC. The CLIC/CTF3 collaboration aims to develop such a linear collider based on a two-beam acceleration scheme that efficiently solves the problem of power generation and distribution to the acceleration structures. Such a scheme was originally proposed using a superconducting drive linac [1], but later adapted to a normal conducting linac [2]. The required radio-frequency (RF) power needed to accelerate the main beam to high energies is generated by decelerating the so-called drive beam, a second, lower energy but higher intensity and higher power density electron beam that runs parallel to the main beam, in the context of CTF3 also called the probe beam. This process is schematically shown in Fig. 1. The RF field is produced in so-called power extraction and transfer structures (PETS) which, as the name implies, also transfer it to the accelerating structures of the main beam. These accelerating structures are normal-conducting, designed for 12 GHz (X-band) operation, and are expected to
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Corresponding author. E-mail address: [email protected] (R. Ruber). URL: http://cern.ch/ctf3-tbts (R. Ruber).
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.07.055
achieve high-accelerating gradients, which is the requirement for keeping the length of the accelerator within reasonable limits. The CTF3 test facility at CERN, which includes the Two-beam Test Stand (TBTS), was built in order to test key technological issues for the future Compact Linear Collider (CLIC) [3]. One of the central issues is the verification of the feasibility of the two-beam acceleration scheme including the PETS power extraction structures and the main beam acceleration structures. This verification and the investigation of various limiting aspects of the scheme is the principal task of the TBTS [4]. The previous CTF1 and CTF2 test facilities have provided the proof of principle for the two-beam acceleration scheme, albeit with low intensity and low energy beams [5,6]. The CTF3 beam intensity is considerable higher than in the previous test facilities, but still lower than in CLIC: The drive beam reaches a final beam intensity of 30 A compared to 100 A in CLIC. The drive beam is generated with fully loaded acceleration and for an appropriate time structure of 12 GHz. The drive and probe beam (main beam in CLIC) energies are in the range of 100 to 200 MeV compared to 2.75 and 2.86 GeV in CLIC [7]. The TBTS uses a prototype CLIC accelerating structure and an extra long PETS structure (to compensate for lower beam current) for the two-beam acceleration feasibility study and addresses these issues at the same RF power level and 100 MV/m accelerating gradient as proposed for CLIC. The CTF3 facility has two electron linacs: a drive beam linac with a thermionic gun, and a probe beam linac with a photoinjector. The drive beam linac is connected to two rings, called the delay loop (DL) and the combiner ring (CR), to create a high power
R. Ruber et al. / Nuclear Instruments and Methods in Physics Research A 729 (2013) 546–553
density beam with 12 GHz bunch repetition frequency. The layout of the overall CTF3 facility with linacs, rings and experimental hall containing the TBTS is shown in Fig. 2. The drive beam linac can be seen stretching from the top left to the top right, subsequently entering the delay loop and the combiner ring. The drive beam is then extracted near the lower right part of the combiner ring with a nominal beam current of 28 A and a pulse length of 140 ns and transported to the CLIC experimental (CLEX) hall where it will be used to generate RF power in the PETS in the TBTS. The RF power is used to accelerate the probe beam which is prepared in the CALIFES linac seen below the drive beam line. The TBTS is the only facility where CLIC type accelerating structures can be tested with beam. It will also be used for an extensive program to investigate the PETS power generation structures and high gradient accelerating structures.
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extraction structure installed in the drive beam transfers RF power to the accelerating structure (ACS) in the probe beam through interconnecting waveguides which are equipped with directional couplers to monitor the RF power flow. The experiment areas of 1.8 m length in the drive beam and 2 m in the probe beam are bordered by vacuum sector valves to allow for changing experiment equipment without affecting the accelerator vacuum [8]. The beam line vacuum tubes are made of 40 mm diameter aluminium pipes. The two parallel beam lines for the drive and the probe beam contain the PETS and ACS which are located in the test area as indicated in Fig. 3. The beam optical design of the two beam lines is rather similar with quadrupole triplets adjacent to the experiment area to adjust the beam size in the experiment area.
2. Overall design The TBTS consists of two parallel beam lines fed with the drive and probe beam, respectively, as shown as schematic in Fig. 3. The actual installation is shown in Figs. 4 and 5. The optics of the two beam lines is similar with differences in the drift spaces to adjust for the physical constraints in the CLEX hall so as to have the test areas for drive and probe beam next to each other while allowing sufficient space for beam instrumentation and other diagnostics including spectrometer lines at the end of both beam lines. In the experiment area, see Fig. 5, large vacuum tanks installed in the drive and probe beam contain the RF structures for power extraction and beam acceleration respectively. The PETS power
transfered wakefield pulse transfer waveguide wakefield pulse
Fig. 3. Two-beam Test Stand layout with the drive and probe beam lines.
main beam bunches
drive beam bunches
Fig. 1. Two-beam acceleration scheme. The RF wakefield created by the drive beam is transferred to the main (probe) beam accelerating structures. Due to differences in the group velocity of the RF wave, the energy density is increased and thereby the electric field magnitude.
Fig. 4. Photo of the Two-beam Test Stand with the drive beam line (left) and probe beam line (right) [22] before installation of PETS and accelerating structure.
Fig. 2. Layout of CTF3 and the CLEX hall with TBTS (courtesy CERN Courier).
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Table 1 Main parameters of the drive and probe beam.
Fig. 5. Photo of the TBTS experiment area with vacuum tanks for PETS (to the right) and accelerating structure (at the left) [23].
Moreover, there are steering magnets to guide the beam. The transverse positions of the beams are observed using beam position monitors (BPM) and the beam size using screens that can be remotely inserted into the beam path and observed with cameras. At the end of each beam line there is a spectrometer dipole magnet that deflects the beam by an amount that depends on its energy and is, in conjunction with a BPM and a screen, used to analyze the energy lost or gained by the respective beams.
3. Drive and probe beam The TBTS uses both the drive beam, generated in the main part of the CTF3 facility, and the probe beam, generated in the same hall as the TBTS. Detailed parameters of the available beams are listed in Table 1. The drive beam is generated in a 70 m long linac followed by two rings where beam manipulations are carried out: a 42 m long delay loop and a 84 m long combiner ring. The two rings form a pulse recombination system that multiplies the bunch frequency and the beam current and simultaneously shortens the macro pulse length [9]. Beam recombination in the delay loop and the combiner ring together creates a 12 GHz beam of around 28 A [10]. Different combinations of using or bypassing the two rings enable different modes for drive beam generation as listed in Table 2. To obtain maximum power extraction efficiency during the drive beam deceleration, the drive beam bunches must be short and correctly spaced at the desired RF frequency for power generation. The probe beam is generated in the 24 m long CALIFES linac which is situated in the same experiment hall as the TBTS. It has been developed to deliver single bunches as well as bunch trains at 1.5 GHz bunch repetition rate [11]. Bunch lengths in the order of 1.4 ps have been measured. In full acceleration mode, in which the buncher cavity is used in acceleration mode, a further acceleration of the probe beam is obtained above its design value from 180 MeV up to 200 MeV, albeit with longer bunch lengths. The energy spread can be tuned to less than 1% RMS. A normalized beam emittance of 10 mm mrad has been achieved [12].
4. Instrumentation A central role in the TBTS is played by the instrumentation to diagnose the behaviour of the beam under normal operation as well as, very important, in the presence of RF discharges (also called RF breakdown) in the accelerating structures that will limit the performance of CLIC and therefore need to be understood.
Parameter name
Drive beam
Probe beam
Energy (MeV) Energy spread (RMS) (%) Pulse length (ns) Bunch frequency (GHz) Bunch charge (nC) Intensity Short pulse (A) Long pulse (A) Repetition rate (Hz) Experiment area Beam height to floor (m) Beam-to-beam distance (m) Length (m) Beam parameters (example values) β (m) ϵ (m) Relative momentum spread Beam size (4s) (mm)
120 2 140–1100 1.5–3–6–9–12–15 up to 3
180 1 0.6–150 1.5 0.05–0.6
28 4 0.8–5
1 0.13 0.8–5
1.35 0.75 1.8
1.36 2.0
1 5.1 10 7 0.01 2.9
1 2.6 10 8 0.014 0.65
Table 2 Different scenarios for drive beam generation. Operation mode
1
2
3
Rings in use Bunch repetition frequency From injector (GHz) Into TBTS (GHz) Current (A) Pulse length (ns)
DL+CR
CR
–
1.5 12 28 140
3 12 14 240
3 3 4 1400
Moreover the diagnostic system is needed to analyze the power transfer from the PETS to the ACS, to investigate the two-beam acceleration and RF breakdown phenomena. An overview of the diagnostic system is shown in Fig. 6. CTF3 operates on a pulsed basis and discharges are just occurring occasionally and are therefore rare events, which imply that all diagnostic devices need to be read out synchronized on a per-pulse basis. Moreover, the beam pulses are rather short, on the order of 100 ns, which implies that fast digitizers are employed also for the beam generated signals, such as those from the BPMs and the RF and breakdown diagnostics instrumentation. Beam energy measurements are done with spectrometer lines equipped with beam position and beam profile monitors. The beam kick studies require the beam lines to be equipped with beam position monitors before and after the structures in the experimental areas. In addition, the study of RF breakdown requires diagnostics to measure breakdown currents emitted from the structures in presence of the beam. Two quadrupole triplets are used to vary and optimise the beam size in the experiment area and on a beam profile monitor screen positioned downstream of a spectrometer dipole in order to maximise the energy resolution. Small beam sizes below 0.2 mm can be reached in the experiment area, see Table 1. A beam profile monitor for emittance measurements is installed after the second quadrupole triplet. The spectrometer lines include one beam position monitor and one video profile monitor with a nonmoveable screen. The downstream quadrupole triplet is tuned as to make an upright elliptical spot on the beam screen in order to maximize the energy resolution. Five inductive beam position monitors (BPM) are installed for intensity and position measurements [13]. They are used in combination with four steering magnets to control the beam position and the incident angle of the beam in the experiment
R. Ruber et al. / Nuclear Instruments and Methods in Physics Research A 729 (2013) 546–553
549
P R P R
P R
F
F
PETS PETS
F
F
drive beam
A
probe beam Flashbox ACS Accelerating Structure
F
F
F
P R
P R
F
P R
F
F
Dipole magnet, orbit corrector
Vacuum pump (ion pump)
Photomultiplier
Quadrupole triplet
Vacuum valve
Faraday cup (moveable)
Variable phase shifter
Beam position monitor (BPM)
Vacuum gauges (Penning, Pirani)
Antenna
Directional coupler
Beam profile monitor (MTV)
P R
A
Variable attenuator
RF waveguide and power load
Beam dump
Fig. 6. Layout of the two beam lines and installed equipment. The drive beam is the upper line, the probe beam is the bottom line.
area. The steering magnets can also adjust the beam trajectory inside the experiment area with a closed bump. The probe beam includes two extra horizontal steering dipoles just before the experiment area. They are used as a small chicane to separate low energy electrons and ions emitted by RF breakdowns in the experiment area from the accelerator beam. This can be used to protect the BPM readout from being overloaded with background. Table 3 lists the basic parameters. Unfortunately the initial resolution of the BPMs in the probe beam line was not satisfactory due to low signal gain and high noise levels. The front end electronics on these BPMs in the probe and drive beam were set-up for the same signal gain, however the drive beam has a much lower current and thus signal level. Therefore the front end electronics on the BPMs in the probe beam were modified to increase the signal gain. Recently (2012) two re-entrant cavity BPMs have been installed in probe beam line in direct connection with the two inductive BPMs behind the experimental area. In the probe beam, these BPMs achieve a ten times higher resolution than the inductive BPMs. Beam profile monitors are used for emittance measurements, measurements of beam energy spread and to optimize the beam conditions. For the emittance measurements, the monitors are placed at the end of the straight line, after the second quadrupole triplet and before the spectrometer dipole. Table 3 lists the basic parameters. The beam profile monitors in the straight line are equipped with an aluminium deposit optical transition radiation (OTR) screen for the drive beam and a YAG screen for the probe beam [14]. For the spectrometer lines, an OTR screen is used in the drive beam, made of aluminium in a parabolic shape to reduce the vignetting effect in the optical system [15]. The probe beam spectrometer line, with its lower beam intensity, has a high sensitivity fluorescent ceramic screen. RF breakdown can be detected from the RF amplitude and phase measurements before and after the accelerating structure by analyzing the signals for reflected and transmitted power and missing energy. In addition RF breakdowns can be detected with photomultipliers sensitive to UV and visible light that are installed viewing one of the PETS and ACS. An insertable Faraday cup is installed on the downstream side of the probe beam experiment area to measure discharges in the
Table 3 Parameters of the beam line instrumentation. Inductive beam position monitor Amount 10 Beam impedance 10 Ω Length electrodes 90 mm Signal bandwidth (after front-end electronics) ΔH, ΔV 0.8 kHz–150 MHz Σ 0.3 kHz–250 MHz Achieved resolution Drive beam 0.06 mm Probe beam 0.3 mm Re-entrant cavity beam position monitor Amount 2 Resonance frequency 5.997 GHz Signal bandwidth 600 MHz Achieved resolution 0.02 mm Beam profile monitor Amount Screen Probe beam, straight line Probe beam, spectrometer Drive beam, straight line Drive beam, spectrometer Camera type Achieved resolution
4 40 40 mm YAG 150 48 mm St.Gobain type AF995R 20 20 mm stainless steel OTR 170 38 mm parabolic aluminium OTR CCD 0.1 mm
ACS in the absence of beam. It can be used to measure both dark and breakdown currents. A so-called Flashbox is installed on the upstream side with which it is possible to detect discharges also if the probe beam is present. It includes sensors for electron and ion detection from possible breakdown currents emitted from the accelerating structure. No measurements had yet been performed with the Flashbox nor Faraday cup when preparing this article.
5. Two-beam acceleration After the first commissioning of the beam lines and the installation of a PETS structure in the drive beam, an external RF power recirculation system was installed that recirculates the RF power produced by the PETS in order to increase the RF power
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inside the PETS and to test the PETS structure more thoroughly and under higher RF power levels. Fig. 7 shows the conceptual set-up of the recirculation loop. Experience gained in running showed that the PETS performed reliably [16,17]. In order to understand the dynamical behaviour of the RF power build-up due to RF power recirculation in the PETS, an analytical model was devised with which the measured RF pulse can be predicted from beam intensity data as measured by a BPM. Fig. 8 shows an example of measured and predicted (reconstructed) RF power level. This analysis model led to a detailed understanding of the energy balance of the entire system with beam and RF power by analyzing the energy lost by the beam and the RF power measured in the RF diagnostics. The lost energy, or deceleration, of the beam can be measured with the drive beam spectrometer or estimated from the RF power production. The results are published in Ref. [18]. During the 2009 run, the PETS produced over 170 MW peak power in full RF recirculation mode, exceeding the 240 ns nominal pulse length and well above the nominal 135 MW foreseen in CLIC. However, many pulses during that run exhibited sudden drops in RF power, a pulse shortening which was linked to RF breakdown in the RF power recirculation system. The performance was limited by this effect, linked to a high power variable splitter and phase shifter. These parts were repaired and improved for the 2010 run. During the 2010 run the first two-beam acceleration of the probe beam was achieved [19]. This first required to carefully synchronize the arrival time of the probe beam bunches to the phase of the RF power generated with a precision of a few degrees.
power output
PETS RF Output
power splitter
PETS RF Input
phase shifter
Drive Beam
PETS
Fig. 7. Layout of a PETS with external RF power recirculation option.
Pulse 7-150
25
This measurement is done by observing the position of the drive beam on a screen in the spectrometer beam line at the end of the TBTS and observing how the position moves while varying the relative phase or arrival time of the probe beam with respect to that of the drive beam. Once the timing is adjusted, the power level in the PETS can be adjusted and it is then possible to actually measure the probe beam acceleration in the ACS. The accelerating gradient and energy gain as function of the RF input power is shown in Fig. 9, and compared to the nominal expectation of this accelerating structure (black line). The nominal CLIC accelerating gradient of 100 MV/m correspond to ΔE ¼ 21:4 MeV in the 22 cm long ACS. The acceleration measurements were done with the beam screen in the spectrometer line while running the probe beam at twice the repetition rate of the drive beam. Thus accelerated and nonaccelerated beam energies can be measured alternatively. Fig. 10 shows an example 23 MeV probe beam acceleration. In Fig. 9 the measured accelerating gradient is consistently below the calculated gradient. We believe that the measured RF power level is overestimated due to the used calibration procedures. Methods are investigated to improve the calibration procedure.
6. Beam trajectory kicks By RF breakdown we denote an electric discharge in vacuum in the presence of the RF field that provides the accelerating gradient in the accelerating structure. RF breakdown is considered to limit the transmission of the RF wave in the travelling wave accelerating structure and thereby limiting the beam acceleration. RF breakdown seems to occur randomly when the structures are driven by RF power pulses and its rate depends among others on the RF power level, the length of the RF power pulses and the length of the run period. RF breakdown and beam induced higher order modes in PETS and accelerating structures can affect the beam, modifying its trajectory and energy. For the design of a stable two-beam accelerator it is important to understand these effects. A beam trajectory kick can be determined using the horizontal and vertical beam position measurements in the BPMs [20]. The layout of the set-up used to measure such beam trajectory kicks comprises five BPMs in the beam line and is shown in Fig. 11. The first measurements of the effects on the beam trajectory by RF breakdown in the accelerating structure were performed in 2011. They are based on measurements of the beam spot with a YAG type beam profile monitor situated just in front of the
BPM 0370S
20
RF forward
|I| [A], P [MW]
Recons. power
Reconstructed power
15
Nominal accelerating gradient RF forward
10
5
0
BPM 0370S
0
50
100
150
200
250
300
350
400
450
500
time [ns]
Fig. 8. Example of a drive beam pulse with measured (marked RF forward) and reconstructed RF power generation from the PETS. RF recirculation gain g ¼0.75 and phase shift ϕ ¼ 181. Recirculation round-trip time τ ¼ 26 ns. For comparison, the BPM pulse is also shown.
Fig. 9. Measured and nominal (upper line) accelerating gradient as function of the RF input power for accelerating structure 12WDSDVG1.8 (2011 run). The corresponding beam energy gain is given on the right hand axis.
R. Ruber et al. / Nuclear Instruments and Methods in Physics Research A 729 (2013) 546–553
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Fig. 10. Two-beam acceleration measurement of the probe beam. The probe beam is operated at double repetition frequency compared to the drive beam and RF power pulse. Thus every other pulse is accelerated (top beam profile monitor snap-shot) while the pulse in between is not (bottom beam profile snap-shot). The bottom graph shows the stability of the probe beam energy with (ACS on, upper line) and without acceleration (ACS off, lower line).
dipole
BPM4 x4
BPM3 x3
corrector dipoles
Flashbox
breakdown current
ICT ion & electron detector
BPM2 x2
BPM1 x1
BPM5 x5
incoming beam x1, x’1
Fig. 11. Scheme of the transverse kick and breakdown current measurements.
spectrometer line dipole because the achieved position resolution was at that time better than with the BPMs. Double spots, of which one example is shown in Fig. 12, were occasionally measured on the same beam pulse. This is interpreted as resulting from a change of the beam trajectory during the pulse. In a preliminary analysis the double spots that appeared on 35 individual breakdown events were considered. They were recorded during 24 h within a period of three days during August 2011. Plotting the difference of the center of gravity of the two spots leads to the distribution shown in the compass plot in Fig. 13. During these measurements the RF power into the accelerating structure was fluctuating between 60 and 85 MW due to drive beam variations.
The probe beam energy was 195 MeV with 30 MeV acceleration in the accelerating structure. Beam pulse length was 132–136 ns with bunch spacing of 0.666 ns (determined by the laser structure), bunch charge of 0.07 nC and beam repetition rate of 0.8 Hz. The magnitude of the measured kicks is about 0.13 mrad, which corresponds to a transverse momentum of about 25 keV/c in the accelerating structure. The number of events is unfortunately too small to draw any further conclusions. Detailed studies are continuing and eventually this analysis will give valuable information about the severity of the disturbance to the beam trajectory that is caused by breakdown events and how much operation of CLIC would be disrupted by this phenomenon [21].
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Fig. 12. Example of a double beam spot detected on a beam screen in correspondence of an RF breakdown in the accelerating structure (right) and the beam under normal operating conditions (left). The image on the right is interpreted as one part of the beam pulse having received a kick during an RF breakdown event.
90
60
25
150
keV/c
CEA Saclay and CERN for their contributions to the construction and operation of the CTF3 facility and the TBTS. We warmly acknowledge the efforts of all our colleagues at the collaborating institutes in CTF3 without whom the facility would not have been built. This work is supported by the Swedish Vetenskapsrådet and the Knut and Alice Wallenberg foundation.
50
120
30
References 180
0
210
330
240
300 270 deg
Fig. 13. Compass plot showing direction and transverse momentum of beam trajectory kicks in correspondence to an RF breakdown in the accelerator structure. The transverse momentum is calculated considering the beam energy and the distance between the screen used for the measurement and the accelerator structure.
7. Conclusions The construction and commissioning of the originally designed TBTS is completed. It is currently used as an active research facility and continues to be upgraded and recommissioned. The facility is a central part of the ongoing effort to demonstrate the feasibility of the two-beam acceleration scheme for CLIC. The achieved 100 MeV/m acceleration gradient is the highlight of TBTS operation so far and represents a decisive milestone for demonstrating the feasibility of two-beam acceleration and the CLIC project. In the continued studies reliability aspects, such as the rate of breakdown events and their effect on the beam, will assume a more central role. Special developed diagnostics such as the Flashbox will be used to investigate the energy and charge profile of the RF breakdown currents.
Acknowledgments The authors would like to thank their colleagues in the CTF3 collaboration for their support, advice and cheerful collaboration, especially Alexey Dubrovskiy. We are indebted to Uppsala University,
[1] W. Schnell, A Two-Stage RF Linear Collider using a Superconducting Drive Linac, CLIC Note 13, CERN, CH-1211 Geneva 23, Switzerland, LEP-RF/86-06, 1986. [2] G. Guignard, A 3TeV eþ e Linear Collider based on CLIC Technology, CERN 2000-008, CERN, CH-1211 Geneva 23, Switzerland, 2000. [3] G. Geschonke, A. Ghigo, CTF3 Design Report, CTF3 Note 047, CERN, CH-1211 Geneva 23, Switzerland, 2002. [4] T. Ekelöf, et al., The Two-beam Test-stand in CTF3, in: Proceedings of EPAC'06, EPAC 2006, European Particle Accelerator Conference, p. 2445, 〈http://www. JACoW.org〉. [5] H. Braun, et al., Results from the CLIC Test Facility, in: Proceedings of EPAC'96, EPAC 1996, June 1996, Sitges, p. 42. 〈http://www.JACoW.org〉. [6] H. Braun, Experimental results and technical research and development at CTFII, in: Proceedings of EPAC'00, EPAC 2000, June 2000, Vienna, p. 48. 〈http:// www.JACoW.org〉. [7] CLIC Collaboration, The CLIC Programme: Towards a Staged eþ e Linear Collider Exploring the Terascale, CERN Yellow Report 2012-005, CERN, CH1211 Geneva 23, Switzerland, 2012. [8] R. Ruber, et al., The CTF3 Two-beam Test-stand: installation and experimental program, in: Proceedings of EPAC'08 EPAC 2008, European Particle Accelerator Conference, p. 2821. 〈http://www.JACoW.org〉. [9] A. Ghigo, et al., Status of the CTF3 frequency multiplication rings, in: Proc. APAC'07, APAC 2007, January–February 2007, Indore, India, p. 217. 〈http:// www.JACoW.org〉. [10] P.K. Skowroński, et al., Progress towards the CLIC feasibility demonstration in CTF3, in: Proceedings of IPAC'10, IPAC 2010, May 2010, Kyoto, Japan, p. 3410. 〈http://www.JACoW.org〉. [11] A. Mosnier, et al., The probe beam linac in CTF3, in: Proceedings of EPAC'06, EPAC 2006, European Particle Accelerator Conference, p. 679. 〈http://www. JACoW.org〉 , 〈http://www.JACoW.org〉. [12] W. Farabolini, et al., CTF3 probe beam Linac commissioning and operation, in: Proceedings of LINAC'10, LINAC 2010, September 2010, Tsukuba, Japan, p. 46. 〈http://www.JACoW.org〉. [13] M. Gasior, An inductive pick-up for beam position and current measurements, in: A. Peters, V. Schaa (Eds.), Proceedings of DIPAC 2003, DIPAC 2003, May 2003, Mainz, p. 53. 〈http://www.JACoW.org〉. [14] W. Farabolini, et al., Video profile monitors development for the CTF3 probe beam linac, in: Proceedings of EPAC'08, EPAC 2008, European Particle Accelerator Conference, 2008, p. 1101. 〈http://www.JACoW.org〉, 〈http://www. JACoW.org〉. [15] C. Welsch, et al., Design alternatives for beam halo monitors in high intensity accelerators, in: Proceedings of DIPAC'05, DIPAC 2005, Lyon, June 2005, p. 54. 〈http://www.JACoW.org〉. [16] I. Syratchev, et al., High power testing of X-band CLIC power generating structures, in: Proceedings of PAC'09, PAC 2009, Vancouver, Canada, May 2009, p. 1873. 〈http://www.JACoW.org〉. [17] I. Syratchev, High power operation with beam of a CLIC PETS equipped with ON/OFF mechanism, in: Proceedings of IPAC'12, IPAC 2012, New Orleans, Louisiana, U.S.A., May 2012, p. 1852. 〈http://www.JACoW.org〉. [18] E. Adli, et al., Physical Review Special Topics—Accelerators and Beams 14 (2011) 081001.
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[19] W. Farabolini, et al., Two beam Test Stand experiments in the CTF3 facility, in: Proceedings of IPAC'11, IPAC 2011, San Sebastián, Spain, September 2011, p. 29. 〈http://www.JACoW.org〉. [20] M. Johnson, Beam-Based Diagnostics of RF-Breakdown in the Two-Beam TestStand in CTF3, CLIC Note 710, CERN, CH-1211 Geneva 23, Switzerland, 2007.
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[21] A. Palaia, et al., Effects of RF breakdown on the beam in a CLIC prototype accelerator structure, arxiv.org/abs/1301.4673, 2013. [22] C. Marcelloni, The Two-beam Test Stand, CERN Photolab, 2008. [23] M. Brice, The CLIC Test Facility, CERN Photolab, 2010.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The CTF3 Two-beam Test Stand R. Ruber a,n, V. Ziemann a, T. Ekelöf a, A. Palaia a, W. Farabolini b, R. Corsini c a b c
Uppsala University, Uppsala, Sweden CEA IRFU, Saclay, France CERN, Geneva, Switzerland
art ic l e i nf o
a b s t r a c t
Article history: Received 27 September 2012 Received in revised form 16 July 2013 Accepted 16 July 2013 Available online 24 July 2013
The Two-beam Test Stand (TBTS) has been constructed and operated at the CLIC test facility CTF3 at CERN. The TBTS comprises two parallel and independent electron beam lines and has been designed to demonstrate the feasibility of a two-beam high gradient acceleration concept as proposed for the Compact Linear Collider (CLIC). In the CLIC scheme, the RF power is extracted from a high current drive beam using RF power extraction structures while the main beam is accelerated using this RF power which is fed into high gradient high frequency normal conducting accelerating structures. The Two-beam Test Stand is a unique facility to demonstrate the feasibility of the CLIC two-beam high gradient acceleration concept and to test the individual CLIC components and complete two-beam CLIC modules. The TBTS is particularly well suited to investigate the effects on the beam of RF breakdown in the high gradient accelerating structures. We report on the design, construction and commissioning of the Two-beam Test Stand. & 2013 Elsevier B.V. All rights reserved.
Keywords: Two-beam acceleration RF breakdown CLIC CTF3
1. Introduction During the coming ten years high-energy physics will be dominated by research done at the LHC, but eventually a highenergy lepton collider will be required in order to do precision measurements of the new physics expected to be discovered at the LHC. The CLIC/CTF3 collaboration aims to develop such a linear collider based on a two-beam acceleration scheme that efficiently solves the problem of power generation and distribution to the acceleration structures. Such a scheme was originally proposed using a superconducting drive linac [1], but later adapted to a normal conducting linac [2]. The required radio-frequency (RF) power needed to accelerate the main beam to high energies is generated by decelerating the so-called drive beam, a second, lower energy but higher intensity and higher power density electron beam that runs parallel to the main beam, in the context of CTF3 also called the probe beam. This process is schematically shown in Fig. 1. The RF field is produced in so-called power extraction and transfer structures (PETS) which, as the name implies, also transfer it to the accelerating structures of the main beam. These accelerating structures are normal-conducting, designed for 12 GHz (X-band) operation, and are expected to
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Corresponding author. E-mail address: [email protected] (R. Ruber). URL: http://cern.ch/ctf3-tbts (R. Ruber).
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.07.055
achieve high-accelerating gradients, which is the requirement for keeping the length of the accelerator within reasonable limits. The CTF3 test facility at CERN, which includes the Two-beam Test Stand (TBTS), was built in order to test key technological issues for the future Compact Linear Collider (CLIC) [3]. One of the central issues is the verification of the feasibility of the two-beam acceleration scheme including the PETS power extraction structures and the main beam acceleration structures. This verification and the investigation of various limiting aspects of the scheme is the principal task of the TBTS [4]. The previous CTF1 and CTF2 test facilities have provided the proof of principle for the two-beam acceleration scheme, albeit with low intensity and low energy beams [5,6]. The CTF3 beam intensity is considerable higher than in the previous test facilities, but still lower than in CLIC: The drive beam reaches a final beam intensity of 30 A compared to 100 A in CLIC. The drive beam is generated with fully loaded acceleration and for an appropriate time structure of 12 GHz. The drive and probe beam (main beam in CLIC) energies are in the range of 100 to 200 MeV compared to 2.75 and 2.86 GeV in CLIC [7]. The TBTS uses a prototype CLIC accelerating structure and an extra long PETS structure (to compensate for lower beam current) for the two-beam acceleration feasibility study and addresses these issues at the same RF power level and 100 MV/m accelerating gradient as proposed for CLIC. The CTF3 facility has two electron linacs: a drive beam linac with a thermionic gun, and a probe beam linac with a photoinjector. The drive beam linac is connected to two rings, called the delay loop (DL) and the combiner ring (CR), to create a high power
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density beam with 12 GHz bunch repetition frequency. The layout of the overall CTF3 facility with linacs, rings and experimental hall containing the TBTS is shown in Fig. 2. The drive beam linac can be seen stretching from the top left to the top right, subsequently entering the delay loop and the combiner ring. The drive beam is then extracted near the lower right part of the combiner ring with a nominal beam current of 28 A and a pulse length of 140 ns and transported to the CLIC experimental (CLEX) hall where it will be used to generate RF power in the PETS in the TBTS. The RF power is used to accelerate the probe beam which is prepared in the CALIFES linac seen below the drive beam line. The TBTS is the only facility where CLIC type accelerating structures can be tested with beam. It will also be used for an extensive program to investigate the PETS power generation structures and high gradient accelerating structures.
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extraction structure installed in the drive beam transfers RF power to the accelerating structure (ACS) in the probe beam through interconnecting waveguides which are equipped with directional couplers to monitor the RF power flow. The experiment areas of 1.8 m length in the drive beam and 2 m in the probe beam are bordered by vacuum sector valves to allow for changing experiment equipment without affecting the accelerator vacuum [8]. The beam line vacuum tubes are made of 40 mm diameter aluminium pipes. The two parallel beam lines for the drive and the probe beam contain the PETS and ACS which are located in the test area as indicated in Fig. 3. The beam optical design of the two beam lines is rather similar with quadrupole triplets adjacent to the experiment area to adjust the beam size in the experiment area.
2. Overall design The TBTS consists of two parallel beam lines fed with the drive and probe beam, respectively, as shown as schematic in Fig. 3. The actual installation is shown in Figs. 4 and 5. The optics of the two beam lines is similar with differences in the drift spaces to adjust for the physical constraints in the CLEX hall so as to have the test areas for drive and probe beam next to each other while allowing sufficient space for beam instrumentation and other diagnostics including spectrometer lines at the end of both beam lines. In the experiment area, see Fig. 5, large vacuum tanks installed in the drive and probe beam contain the RF structures for power extraction and beam acceleration respectively. The PETS power
transfered wakefield pulse transfer waveguide wakefield pulse
Fig. 3. Two-beam Test Stand layout with the drive and probe beam lines.
main beam bunches
drive beam bunches
Fig. 1. Two-beam acceleration scheme. The RF wakefield created by the drive beam is transferred to the main (probe) beam accelerating structures. Due to differences in the group velocity of the RF wave, the energy density is increased and thereby the electric field magnitude.
Fig. 4. Photo of the Two-beam Test Stand with the drive beam line (left) and probe beam line (right) [22] before installation of PETS and accelerating structure.
Fig. 2. Layout of CTF3 and the CLEX hall with TBTS (courtesy CERN Courier).
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Table 1 Main parameters of the drive and probe beam.
Fig. 5. Photo of the TBTS experiment area with vacuum tanks for PETS (to the right) and accelerating structure (at the left) [23].
Moreover, there are steering magnets to guide the beam. The transverse positions of the beams are observed using beam position monitors (BPM) and the beam size using screens that can be remotely inserted into the beam path and observed with cameras. At the end of each beam line there is a spectrometer dipole magnet that deflects the beam by an amount that depends on its energy and is, in conjunction with a BPM and a screen, used to analyze the energy lost or gained by the respective beams.
3. Drive and probe beam The TBTS uses both the drive beam, generated in the main part of the CTF3 facility, and the probe beam, generated in the same hall as the TBTS. Detailed parameters of the available beams are listed in Table 1. The drive beam is generated in a 70 m long linac followed by two rings where beam manipulations are carried out: a 42 m long delay loop and a 84 m long combiner ring. The two rings form a pulse recombination system that multiplies the bunch frequency and the beam current and simultaneously shortens the macro pulse length [9]. Beam recombination in the delay loop and the combiner ring together creates a 12 GHz beam of around 28 A [10]. Different combinations of using or bypassing the two rings enable different modes for drive beam generation as listed in Table 2. To obtain maximum power extraction efficiency during the drive beam deceleration, the drive beam bunches must be short and correctly spaced at the desired RF frequency for power generation. The probe beam is generated in the 24 m long CALIFES linac which is situated in the same experiment hall as the TBTS. It has been developed to deliver single bunches as well as bunch trains at 1.5 GHz bunch repetition rate [11]. Bunch lengths in the order of 1.4 ps have been measured. In full acceleration mode, in which the buncher cavity is used in acceleration mode, a further acceleration of the probe beam is obtained above its design value from 180 MeV up to 200 MeV, albeit with longer bunch lengths. The energy spread can be tuned to less than 1% RMS. A normalized beam emittance of 10 mm mrad has been achieved [12].
4. Instrumentation A central role in the TBTS is played by the instrumentation to diagnose the behaviour of the beam under normal operation as well as, very important, in the presence of RF discharges (also called RF breakdown) in the accelerating structures that will limit the performance of CLIC and therefore need to be understood.
Parameter name
Drive beam
Probe beam
Energy (MeV) Energy spread (RMS) (%) Pulse length (ns) Bunch frequency (GHz) Bunch charge (nC) Intensity Short pulse (A) Long pulse (A) Repetition rate (Hz) Experiment area Beam height to floor (m) Beam-to-beam distance (m) Length (m) Beam parameters (example values) β (m) ϵ (m) Relative momentum spread Beam size (4s) (mm)
120 2 140–1100 1.5–3–6–9–12–15 up to 3
180 1 0.6–150 1.5 0.05–0.6
28 4 0.8–5
1 0.13 0.8–5
1.35 0.75 1.8
1.36 2.0
1 5.1 10 7 0.01 2.9
1 2.6 10 8 0.014 0.65
Table 2 Different scenarios for drive beam generation. Operation mode
1
2
3
Rings in use Bunch repetition frequency From injector (GHz) Into TBTS (GHz) Current (A) Pulse length (ns)
DL+CR
CR
–
1.5 12 28 140
3 12 14 240
3 3 4 1400
Moreover the diagnostic system is needed to analyze the power transfer from the PETS to the ACS, to investigate the two-beam acceleration and RF breakdown phenomena. An overview of the diagnostic system is shown in Fig. 6. CTF3 operates on a pulsed basis and discharges are just occurring occasionally and are therefore rare events, which imply that all diagnostic devices need to be read out synchronized on a per-pulse basis. Moreover, the beam pulses are rather short, on the order of 100 ns, which implies that fast digitizers are employed also for the beam generated signals, such as those from the BPMs and the RF and breakdown diagnostics instrumentation. Beam energy measurements are done with spectrometer lines equipped with beam position and beam profile monitors. The beam kick studies require the beam lines to be equipped with beam position monitors before and after the structures in the experimental areas. In addition, the study of RF breakdown requires diagnostics to measure breakdown currents emitted from the structures in presence of the beam. Two quadrupole triplets are used to vary and optimise the beam size in the experiment area and on a beam profile monitor screen positioned downstream of a spectrometer dipole in order to maximise the energy resolution. Small beam sizes below 0.2 mm can be reached in the experiment area, see Table 1. A beam profile monitor for emittance measurements is installed after the second quadrupole triplet. The spectrometer lines include one beam position monitor and one video profile monitor with a nonmoveable screen. The downstream quadrupole triplet is tuned as to make an upright elliptical spot on the beam screen in order to maximize the energy resolution. Five inductive beam position monitors (BPM) are installed for intensity and position measurements [13]. They are used in combination with four steering magnets to control the beam position and the incident angle of the beam in the experiment
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P R P R
P R
F
F
PETS PETS
F
F
drive beam
A
probe beam Flashbox ACS Accelerating Structure
F
F
F
P R
P R
F
P R
F
F
Dipole magnet, orbit corrector
Vacuum pump (ion pump)
Photomultiplier
Quadrupole triplet
Vacuum valve
Faraday cup (moveable)
Variable phase shifter
Beam position monitor (BPM)
Vacuum gauges (Penning, Pirani)
Antenna
Directional coupler
Beam profile monitor (MTV)
P R
A
Variable attenuator
RF waveguide and power load
Beam dump
Fig. 6. Layout of the two beam lines and installed equipment. The drive beam is the upper line, the probe beam is the bottom line.
area. The steering magnets can also adjust the beam trajectory inside the experiment area with a closed bump. The probe beam includes two extra horizontal steering dipoles just before the experiment area. They are used as a small chicane to separate low energy electrons and ions emitted by RF breakdowns in the experiment area from the accelerator beam. This can be used to protect the BPM readout from being overloaded with background. Table 3 lists the basic parameters. Unfortunately the initial resolution of the BPMs in the probe beam line was not satisfactory due to low signal gain and high noise levels. The front end electronics on these BPMs in the probe and drive beam were set-up for the same signal gain, however the drive beam has a much lower current and thus signal level. Therefore the front end electronics on the BPMs in the probe beam were modified to increase the signal gain. Recently (2012) two re-entrant cavity BPMs have been installed in probe beam line in direct connection with the two inductive BPMs behind the experimental area. In the probe beam, these BPMs achieve a ten times higher resolution than the inductive BPMs. Beam profile monitors are used for emittance measurements, measurements of beam energy spread and to optimize the beam conditions. For the emittance measurements, the monitors are placed at the end of the straight line, after the second quadrupole triplet and before the spectrometer dipole. Table 3 lists the basic parameters. The beam profile monitors in the straight line are equipped with an aluminium deposit optical transition radiation (OTR) screen for the drive beam and a YAG screen for the probe beam [14]. For the spectrometer lines, an OTR screen is used in the drive beam, made of aluminium in a parabolic shape to reduce the vignetting effect in the optical system [15]. The probe beam spectrometer line, with its lower beam intensity, has a high sensitivity fluorescent ceramic screen. RF breakdown can be detected from the RF amplitude and phase measurements before and after the accelerating structure by analyzing the signals for reflected and transmitted power and missing energy. In addition RF breakdowns can be detected with photomultipliers sensitive to UV and visible light that are installed viewing one of the PETS and ACS. An insertable Faraday cup is installed on the downstream side of the probe beam experiment area to measure discharges in the
Table 3 Parameters of the beam line instrumentation. Inductive beam position monitor Amount 10 Beam impedance 10 Ω Length electrodes 90 mm Signal bandwidth (after front-end electronics) ΔH, ΔV 0.8 kHz–150 MHz Σ 0.3 kHz–250 MHz Achieved resolution Drive beam 0.06 mm Probe beam 0.3 mm Re-entrant cavity beam position monitor Amount 2 Resonance frequency 5.997 GHz Signal bandwidth 600 MHz Achieved resolution 0.02 mm Beam profile monitor Amount Screen Probe beam, straight line Probe beam, spectrometer Drive beam, straight line Drive beam, spectrometer Camera type Achieved resolution
4 40 40 mm YAG 150 48 mm St.Gobain type AF995R 20 20 mm stainless steel OTR 170 38 mm parabolic aluminium OTR CCD 0.1 mm
ACS in the absence of beam. It can be used to measure both dark and breakdown currents. A so-called Flashbox is installed on the upstream side with which it is possible to detect discharges also if the probe beam is present. It includes sensors for electron and ion detection from possible breakdown currents emitted from the accelerating structure. No measurements had yet been performed with the Flashbox nor Faraday cup when preparing this article.
5. Two-beam acceleration After the first commissioning of the beam lines and the installation of a PETS structure in the drive beam, an external RF power recirculation system was installed that recirculates the RF power produced by the PETS in order to increase the RF power
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inside the PETS and to test the PETS structure more thoroughly and under higher RF power levels. Fig. 7 shows the conceptual set-up of the recirculation loop. Experience gained in running showed that the PETS performed reliably [16,17]. In order to understand the dynamical behaviour of the RF power build-up due to RF power recirculation in the PETS, an analytical model was devised with which the measured RF pulse can be predicted from beam intensity data as measured by a BPM. Fig. 8 shows an example of measured and predicted (reconstructed) RF power level. This analysis model led to a detailed understanding of the energy balance of the entire system with beam and RF power by analyzing the energy lost by the beam and the RF power measured in the RF diagnostics. The lost energy, or deceleration, of the beam can be measured with the drive beam spectrometer or estimated from the RF power production. The results are published in Ref. [18]. During the 2009 run, the PETS produced over 170 MW peak power in full RF recirculation mode, exceeding the 240 ns nominal pulse length and well above the nominal 135 MW foreseen in CLIC. However, many pulses during that run exhibited sudden drops in RF power, a pulse shortening which was linked to RF breakdown in the RF power recirculation system. The performance was limited by this effect, linked to a high power variable splitter and phase shifter. These parts were repaired and improved for the 2010 run. During the 2010 run the first two-beam acceleration of the probe beam was achieved [19]. This first required to carefully synchronize the arrival time of the probe beam bunches to the phase of the RF power generated with a precision of a few degrees.
power output
PETS RF Output
power splitter
PETS RF Input
phase shifter
Drive Beam
PETS
Fig. 7. Layout of a PETS with external RF power recirculation option.
Pulse 7-150
25
This measurement is done by observing the position of the drive beam on a screen in the spectrometer beam line at the end of the TBTS and observing how the position moves while varying the relative phase or arrival time of the probe beam with respect to that of the drive beam. Once the timing is adjusted, the power level in the PETS can be adjusted and it is then possible to actually measure the probe beam acceleration in the ACS. The accelerating gradient and energy gain as function of the RF input power is shown in Fig. 9, and compared to the nominal expectation of this accelerating structure (black line). The nominal CLIC accelerating gradient of 100 MV/m correspond to ΔE ¼ 21:4 MeV in the 22 cm long ACS. The acceleration measurements were done with the beam screen in the spectrometer line while running the probe beam at twice the repetition rate of the drive beam. Thus accelerated and nonaccelerated beam energies can be measured alternatively. Fig. 10 shows an example 23 MeV probe beam acceleration. In Fig. 9 the measured accelerating gradient is consistently below the calculated gradient. We believe that the measured RF power level is overestimated due to the used calibration procedures. Methods are investigated to improve the calibration procedure.
6. Beam trajectory kicks By RF breakdown we denote an electric discharge in vacuum in the presence of the RF field that provides the accelerating gradient in the accelerating structure. RF breakdown is considered to limit the transmission of the RF wave in the travelling wave accelerating structure and thereby limiting the beam acceleration. RF breakdown seems to occur randomly when the structures are driven by RF power pulses and its rate depends among others on the RF power level, the length of the RF power pulses and the length of the run period. RF breakdown and beam induced higher order modes in PETS and accelerating structures can affect the beam, modifying its trajectory and energy. For the design of a stable two-beam accelerator it is important to understand these effects. A beam trajectory kick can be determined using the horizontal and vertical beam position measurements in the BPMs [20]. The layout of the set-up used to measure such beam trajectory kicks comprises five BPMs in the beam line and is shown in Fig. 11. The first measurements of the effects on the beam trajectory by RF breakdown in the accelerating structure were performed in 2011. They are based on measurements of the beam spot with a YAG type beam profile monitor situated just in front of the
BPM 0370S
20
RF forward
|I| [A], P [MW]
Recons. power
Reconstructed power
15
Nominal accelerating gradient RF forward
10
5
0
BPM 0370S
0
50
100
150
200
250
300
350
400
450
500
time [ns]
Fig. 8. Example of a drive beam pulse with measured (marked RF forward) and reconstructed RF power generation from the PETS. RF recirculation gain g ¼0.75 and phase shift ϕ ¼ 181. Recirculation round-trip time τ ¼ 26 ns. For comparison, the BPM pulse is also shown.
Fig. 9. Measured and nominal (upper line) accelerating gradient as function of the RF input power for accelerating structure 12WDSDVG1.8 (2011 run). The corresponding beam energy gain is given on the right hand axis.
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Fig. 10. Two-beam acceleration measurement of the probe beam. The probe beam is operated at double repetition frequency compared to the drive beam and RF power pulse. Thus every other pulse is accelerated (top beam profile monitor snap-shot) while the pulse in between is not (bottom beam profile snap-shot). The bottom graph shows the stability of the probe beam energy with (ACS on, upper line) and without acceleration (ACS off, lower line).
dipole
BPM4 x4
BPM3 x3
corrector dipoles
Flashbox
breakdown current
ICT ion & electron detector
BPM2 x2
BPM1 x1
BPM5 x5
incoming beam x1, x’1
Fig. 11. Scheme of the transverse kick and breakdown current measurements.
spectrometer line dipole because the achieved position resolution was at that time better than with the BPMs. Double spots, of which one example is shown in Fig. 12, were occasionally measured on the same beam pulse. This is interpreted as resulting from a change of the beam trajectory during the pulse. In a preliminary analysis the double spots that appeared on 35 individual breakdown events were considered. They were recorded during 24 h within a period of three days during August 2011. Plotting the difference of the center of gravity of the two spots leads to the distribution shown in the compass plot in Fig. 13. During these measurements the RF power into the accelerating structure was fluctuating between 60 and 85 MW due to drive beam variations.
The probe beam energy was 195 MeV with 30 MeV acceleration in the accelerating structure. Beam pulse length was 132–136 ns with bunch spacing of 0.666 ns (determined by the laser structure), bunch charge of 0.07 nC and beam repetition rate of 0.8 Hz. The magnitude of the measured kicks is about 0.13 mrad, which corresponds to a transverse momentum of about 25 keV/c in the accelerating structure. The number of events is unfortunately too small to draw any further conclusions. Detailed studies are continuing and eventually this analysis will give valuable information about the severity of the disturbance to the beam trajectory that is caused by breakdown events and how much operation of CLIC would be disrupted by this phenomenon [21].
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Fig. 12. Example of a double beam spot detected on a beam screen in correspondence of an RF breakdown in the accelerating structure (right) and the beam under normal operating conditions (left). The image on the right is interpreted as one part of the beam pulse having received a kick during an RF breakdown event.
90
60
25
150
keV/c
CEA Saclay and CERN for their contributions to the construction and operation of the CTF3 facility and the TBTS. We warmly acknowledge the efforts of all our colleagues at the collaborating institutes in CTF3 without whom the facility would not have been built. This work is supported by the Swedish Vetenskapsrådet and the Knut and Alice Wallenberg foundation.
50
120
30
References 180
0
210
330
240
300 270 deg
Fig. 13. Compass plot showing direction and transverse momentum of beam trajectory kicks in correspondence to an RF breakdown in the accelerator structure. The transverse momentum is calculated considering the beam energy and the distance between the screen used for the measurement and the accelerator structure.
7. Conclusions The construction and commissioning of the originally designed TBTS is completed. It is currently used as an active research facility and continues to be upgraded and recommissioned. The facility is a central part of the ongoing effort to demonstrate the feasibility of the two-beam acceleration scheme for CLIC. The achieved 100 MeV/m acceleration gradient is the highlight of TBTS operation so far and represents a decisive milestone for demonstrating the feasibility of two-beam acceleration and the CLIC project. In the continued studies reliability aspects, such as the rate of breakdown events and their effect on the beam, will assume a more central role. Special developed diagnostics such as the Flashbox will be used to investigate the energy and charge profile of the RF breakdown currents.
Acknowledgments The authors would like to thank their colleagues in the CTF3 collaboration for their support, advice and cheerful collaboration, especially Alexey Dubrovskiy. We are indebted to Uppsala University,
[1] W. Schnell, A Two-Stage RF Linear Collider using a Superconducting Drive Linac, CLIC Note 13, CERN, CH-1211 Geneva 23, Switzerland, LEP-RF/86-06, 1986. [2] G. Guignard, A 3TeV eþ e Linear Collider based on CLIC Technology, CERN 2000-008, CERN, CH-1211 Geneva 23, Switzerland, 2000. [3] G. Geschonke, A. Ghigo, CTF3 Design Report, CTF3 Note 047, CERN, CH-1211 Geneva 23, Switzerland, 2002. [4] T. Ekelöf, et al., The Two-beam Test-stand in CTF3, in: Proceedings of EPAC'06, EPAC 2006, European Particle Accelerator Conference, p. 2445, 〈http://www. JACoW.org〉. [5] H. Braun, et al., Results from the CLIC Test Facility, in: Proceedings of EPAC'96, EPAC 1996, June 1996, Sitges, p. 42. 〈http://www.JACoW.org〉. [6] H. Braun, Experimental results and technical research and development at CTFII, in: Proceedings of EPAC'00, EPAC 2000, June 2000, Vienna, p. 48. 〈http:// www.JACoW.org〉. [7] CLIC Collaboration, The CLIC Programme: Towards a Staged eþ e Linear Collider Exploring the Terascale, CERN Yellow Report 2012-005, CERN, CH1211 Geneva 23, Switzerland, 2012. [8] R. Ruber, et al., The CTF3 Two-beam Test-stand: installation and experimental program, in: Proceedings of EPAC'08 EPAC 2008, European Particle Accelerator Conference, p. 2821. 〈http://www.JACoW.org〉. [9] A. Ghigo, et al., Status of the CTF3 frequency multiplication rings, in: Proc. APAC'07, APAC 2007, January–February 2007, Indore, India, p. 217. 〈http:// www.JACoW.org〉. [10] P.K. Skowroński, et al., Progress towards the CLIC feasibility demonstration in CTF3, in: Proceedings of IPAC'10, IPAC 2010, May 2010, Kyoto, Japan, p. 3410. 〈http://www.JACoW.org〉. [11] A. Mosnier, et al., The probe beam linac in CTF3, in: Proceedings of EPAC'06, EPAC 2006, European Particle Accelerator Conference, p. 679. 〈http://www. JACoW.org〉 , 〈http://www.JACoW.org〉. [12] W. Farabolini, et al., CTF3 probe beam Linac commissioning and operation, in: Proceedings of LINAC'10, LINAC 2010, September 2010, Tsukuba, Japan, p. 46. 〈http://www.JACoW.org〉. [13] M. Gasior, An inductive pick-up for beam position and current measurements, in: A. Peters, V. Schaa (Eds.), Proceedings of DIPAC 2003, DIPAC 2003, May 2003, Mainz, p. 53. 〈http://www.JACoW.org〉. [14] W. Farabolini, et al., Video profile monitors development for the CTF3 probe beam linac, in: Proceedings of EPAC'08, EPAC 2008, European Particle Accelerator Conference, 2008, p. 1101. 〈http://www.JACoW.org〉, 〈http://www. JACoW.org〉. [15] C. Welsch, et al., Design alternatives for beam halo monitors in high intensity accelerators, in: Proceedings of DIPAC'05, DIPAC 2005, Lyon, June 2005, p. 54. 〈http://www.JACoW.org〉. [16] I. Syratchev, et al., High power testing of X-band CLIC power generating structures, in: Proceedings of PAC'09, PAC 2009, Vancouver, Canada, May 2009, p. 1873. 〈http://www.JACoW.org〉. [17] I. Syratchev, High power operation with beam of a CLIC PETS equipped with ON/OFF mechanism, in: Proceedings of IPAC'12, IPAC 2012, New Orleans, Louisiana, U.S.A., May 2012, p. 1852. 〈http://www.JACoW.org〉. [18] E. Adli, et al., Physical Review Special Topics—Accelerators and Beams 14 (2011) 081001.
R. Ruber et al. / Nuclear Instruments and Methods in Physics Research A 729 (2013) 546–553
[19] W. Farabolini, et al., Two beam Test Stand experiments in the CTF3 facility, in: Proceedings of IPAC'11, IPAC 2011, San Sebastián, Spain, September 2011, p. 29. 〈http://www.JACoW.org〉. [20] M. Johnson, Beam-Based Diagnostics of RF-Breakdown in the Two-Beam TestStand in CTF3, CLIC Note 710, CERN, CH-1211 Geneva 23, Switzerland, 2007.
553
[21] A. Palaia, et al., Effects of RF breakdown on the beam in a CLIC prototype accelerator structure, arxiv.org/abs/1301.4673, 2013. [22] C. Marcelloni, The Two-beam Test Stand, CERN Photolab, 2008. [23] M. Brice, The CLIC Test Facility, CERN Photolab, 2010.