Experimental Results and Technical Results and Development at CTFII

The second phase of the Compact Linear Collider Test Facility (CTF II) has demonstrated the feasibility of two key ingredients of the Compact Linear Collider scheme (CLIC) [1], namely the acceleration with a 30 GHz normal conducting linac and the 30 GHz RF power production by a tightly-bunched, high-charge drive beam running parallel to the main beam. This beam is produced and accelerated with a 3 GHz linac using an RF-photo-injector and two travelling-wave sections, all specially developed for handling very high charges. A magnetic chicane compresses the micro-bunches to their nominal length. A mm-wave spectrometer, coupled to the beam pipe, allows non-destructive measurements of bunch length. So far a total acceleration of 60 MeV has been obtained using a string of five accelerating structures with a total active length of 1.4 m. The corresponding drive-beam deceleration is 6 MeV. The flexibility and extensive beam instrumentation allows a variety of other experiments, such as measurements of emittance growth and energy loss in bunch compressors due to coherent synchrotron radiation, high-gradient tests in single-cell 30 GHz cavities, high-power tests of a planar 30 GHz RF structure and tests of beam position monitor prototypes

Achievements and Future Plans of CLIC Test Facilities

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 high gradient testing of CAS's and 30 GHz single cell cavities (SCC). With these modifications, it is now possible to provide 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 been launched to advance the understanding of RF breakdown processes, to improve surface properties, investigate new materials 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 acceler-ate a 1.54 ms long electron pulse of 3.5 A in a fully beam-loaded 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 nomi-nal CLIC parameters. For this reason the drive beam extracted from the combiner ring is transported through a string of 30 GHz power extraction cavities feeding CAS's. The 30 GHz acceleration is measured with a probe beam provided by a small, separate S-band linac

Non-Intercepting Bunch Length Monitor for Picosecond Electron Bunches

A bunch length monitor for very short electron bunches, r.m.s. > 1 ps, has been implemented in the CLIC (Compact Linear Collider) Test Facility (CTF) at CERN. It consists of a long, rectangular wavegu ide connected at one end to the beam pipe and a detection system at the other end. Information on the bunch length is obtained by frequency-domain analysis of the signal excited by the beam in the wav eguide. The signal can be detected either by a fast diode detector or by a RF mixer in combination with a RF sweep oscillator. With the diode detector, single-shot measurements can be performed, while the mixer set-up allows quantitative bunch length measurements for single bunches and for bunch-trains. The design and installation of two monitors and detection systems operating in two different fre quency bands, Ka (26.5--40 GHz) and E (60--90 GHz), are described. Results obtained with the new monitor are presented and compared with bunch length measurements that were performed simultaneously wi th a streak camera recording Cerenkov Radiation

Two frequency beam-loading compensation in the drive-beam accelerator of the CLIC Test Facility

The CLIC Test Facility (CTF) is a prototype two-beam accelerator, in which a high-current "drive beam" is used to generate the RF power for the main-beam accelerator. The drive-beam accelerator consists of two S-band structures which accelerate a bunch train with a total charge of 500 nC. The substantial beam loading is compensated by operating the two accelerating structures at 7.81 MHz above and below the bunch repetition frequency, respectively. This introduces a change of RF phase from bunch to bunch, which leads, together with off-crest injection into the accelerator, to an approximate compensation of the beam loading. Due to the sinusoidal time-dependency of the RF field, an energy spread of about 7% remains in the bunch train. A set of idler cavities has been installed to reduce this residual energy spread further. In this paper, the considerations that motivated the choice of the parameters of the beam-loading compensation system, together with the experimental results, are presented

Lifetime limit from nuclear intra-bunch scattering for high-energy hadron beams

We discuss the possibility and importance of nuclear scattering processes inside a bunched hadron beam. Estimates are presented for the LHC

Nulling Emittance Measurement Technique for CLIC Test Facility

In order to test the principle of Two-Beam-Acceleration (TBA), the CLIC Test Facility utilizes a high-intensity drive beam of 640 to 1000 nC to generate 30 GHz accelerating fields. To ensure that the beam is transported efficiently, a robust measurement of beam emittance and Twiss parameters is required. This is accomplished by measuring the beam size on a profile monitor, while scanning five or more upstream quadrupoles in such a fashion that the Twiss parameters at the profile monitor remain constant while the phase advance through the beam line changes. In this way the beam size can be sampled at different phases while a near-constant size is of such measurement devices, especially those associated with limited dynamic range. In addition, the beam size is explicitly constant for a matched beam, which provides a ``nulling'' measurement of the match. Details of the technique, simulations, and results of the measurements are discussed

CLIC Final Focus Studies

The CLIC final focus system has been designed based on the local compensation scheme proposed by P. Raimondi and A. Seryi. However, there exist important chromatic aberrations that deteriorate the performance of the system. This paper studies the optimization of the final focus based on the computation of the higher orders of the map using MAD-X and PTC. The use of octupole tail folding to reduce the size of the halo in the locations with aperture limitations is also discussed

High power test of a 30 GHz planar accelerator

A 30-GHz muffin-tin, traveling-wave accelerating structure consisting of 37 cells was tested at high power using the CTF2 at CERN. The structure was fabricated with conventional milling and brazing, including tuning holes at cavity roofs. No special surface preparation or treatment was done to the structure. A maximum peak power in excess of 100 MW at a pulse width of 4 ns was transported through the structure before electron bursts were initiated. A maximum accelerating gradient of 60 MV/m was achieved with a peak RF power of 40 MW at a pulse width of 8 ns

Status of CLIC High-gradient Studies

The recent RF structure testing program carried out in the CLIC Test Facility, CTF II, is described. The main objectives of the testing program have been to gain an insight into the physical processes involved in breakdown and damage, to isolate parameters that influence breakdown and damage, and to determine gradient limits for 30 GHz structures. The layout of CTFII in the new 'Test Stand' configuration, the instrumentation used to study breakdown and the experimental results are summarised. The new results are compared to previously published results at 11, 30 and 33 GHz produced in the context of the CLIC study