(INFN – Laboratori Nazionali di Frascati) CORA: Design and Construction of a RF Compressor for High Brightness Electron Beams Franco Alessandria, Alberto Bacci, Carlo De Martinis, Dario Giove, Marco Mauri, Luca Serafini (INFN - sezione di Milano e Università di Milano) David Alesini, Massimo Ferrario, Alessandro Gallo, Fabio Marcellini (INFN – Laboratori Nazionali di Frascati) SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

Introduction The project CORA has been an R&D activity related to the design and the construction of a prototype RF structure able to test the slow-wave velocity bunching theory(1). The parameter which controls the compression process is the phase velocity (vf) of the structure. As it is shown in the formula below the phase velocity can be changed properly detuning the structure. In the CORA scheme the required detuning is obtained controlling the cavity temperature. This makes possible to modify “on-line” the bunch compression factor and find the optimum working point. A compression factor of the order of 10 has been taken as a reasonable goal. This reflects in a change of the vf of the order of 0.1%. The above formula shows that increasing the group velocity of the structure, the phase velocity becomes less sensitive to detuning and, as a consequence, to temperature fluctuations. (1) L. Serafini et al. “Ultra-short Bunch Generation with a Rectlinear Compressor”, PAC2001, Chicago, June 2001, p. 2242. SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

RF compressor main parameters Introduction Main parameters Comparison between a constant-impedance SLAC cavity and the proposed RF compressor cavity MarkIV (SLAC) Alma5 Cavity radius [cm] 4.124 4.248 Iris radius [cm] 1.130 1.54 Septum thickness [cm] 0.584 0.45 Cell length [cm] 3.5 Mode 2/3 Frequency [MHz] 2856 Q 13200 13205 Shunt imp. [MOhm/m] 53 41 Vg/c 0.012 0.034 T [°C] 0.6 1.9 Temperature variation needed to change the phase velocity of 0.1% RF compressor main parameters Frequency of the TW structure 2856 MHz Accelerating gradient 20 MV/m Electrons injection energy 6 MeV Electrons extraction energy 16 Mev Bunch length at injection 10 ps Compression factor 7 RF power 66 MW SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

Introduction Cora started in 2003 with the EM and mechanic design of the structure (using the codes SUPERFISH and ANSYS). After a few months we received from UCLA a 3m-long SLAC structure that was used to carry out RF measurements, to test temperature control schemes and to acquire lab experience. Using this structure we developed all the tools that will be used for the RF compressor. On November and December 2004 in cooperation with the “Soltan Institute for Nuclear Studies” of Warsaw the complete (EM and mechanical) design of the power couplers has been performed. During 2004, a prototype of the RF compressor (made of 8 cells plus 2 end half-cells) was machined in Italy by CINEL. The structure has been measured in our lab and then delivered to CERN for brazing. The brazed 10-cell structure was sent back to Milan (LASA) on October 2005. Topics covered in the following slides: Improvements on the thermal control system Preliminary tests on the 10-cells model structure (Vg=0.034c) Power couplers design and construction SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

1.Improvements on the thermal control system System set up PC SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

1.Improvements on the thermal control system System set up RF Cavity The temperature of the cavity has been mapped using thermocouples and platinum resistance thermometers, A Network Analyzer has been used to monitor the resonance frequency of the cavity Temperatures and frequency were remotely read and stored using LabView. SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

1.Improvements on the thermal control system System set up Chiller Model: Neslab HX-150 Cooling capacity  4 kW A commercial available chiller by Thermo Neslab has been chosen after a deep analysis of the performances with respect to the experimental requirements. The 2 internal PID loops had to be tuned in order to obtain the required thermal stability (fluctuations below 0.1°) and to reduce the response time of the system. SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

1.Improvements on the thermal control system System set up Heater We introduced a heater in the system in order to simulate the thermal load due to RF. With a repetition rate of 10 Hz and pulse length of 5ms we expect to have a dissipation in the cavity of about 1.1kW. The heater can deliver up to 1.5 kW and is remotely controlled. SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

Slide presented at the SPARC Review Committee Meeting - LASA April 2004 SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

1.Improvements on the thermal control system Results The chiller temperature set point has been fixed first at 40.0 °C then moved to 39.7 °C and at the end back to 40.0 °C. During these steps the cavity temperature and cavity RF frequency resonance have been measured as probes of the behaviour of the whole setup. Frequency Df ~ +15 kHz DT ~ -0.3 °C Temperature Main experimental results: thermal stability under 1.5 kW load: better than 0.1 °C (confirmed both from RTD temperature probe and RF measure (0.1 °C  5 kHz) time required to reach a new stability condition: for small change ( less than 3.0 °C) in temperature set point : less than 500 sec. for large change ( more than 25.0 °C) in temperature set point : nearly 1200 sec. SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

1.Improvements on the thermal control system Results The 3 meter long SLAC cavity (thermally insulated) was in series with the chiller (using a 10 meters long pipeline) and the 1.5 kW thermal load. A stability better than ±0.1° has been achieved with a fixed temperature set point (40°C). The plot shows the temperature fluctuations during measurement of about 4 hours. Reaching the steady state condition from a temperature of 26°C took about half an hour. SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

1.Thermal control system test results Results Few tests have been carried out using a CW 100 W RF generator SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

A 10 cells prototype has been designed and built in order to test: 2.Preliminary tests on 10-cells model structure (Vg=0.034c) Introduction A 10 cells prototype has been designed and built in order to test: the EM design of the cells (made with SUPERFISH and tested with Microwave Studio) the mechanical design (in particular with respect to the brazing process) the control of the phase velocity (using the temperature based mechanism) Cooling channels RF probe hole Disk Dinging hole Cell SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

2. Preliminary tests on 10-cells model structure (Vg=0 2.Preliminary tests on 10-cells model structure (Vg=0.034c) Measurements on single cells Measuring the single cells it has been possible to verify that all the cells were machined with a very good repeatability (~ 150 kHz) and in good agreement with the EM simulations (~ 350kHz) SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

2. Preliminary tests on 10-cells model structure (Vg=0 2.Preliminary tests on 10-cells model structure (Vg=0.034c) Measurements on the brazed structure SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

2. Preliminary tests on 10-cells model structure (Vg=0 2.Preliminary tests on 10-cells model structure (Vg=0.034c) Measurements on the brazed structure The resonance frequencies of the brazed structure had been measured in air (structure temperature: 40°C). Considering the 2/3 mode (working mode of the RF compressor) and the air-to-vacuum correction (1) the structure is out of tune of about 1 MHz. By means of the tuning system (dinging) the frequency will be corrected to nominal value (2856 MHz)   Measured Frequency Mode [MHz] 1 2773.67 2 2778.50 3 2788.42 4 2802.22 5 2819.42 6 2837.41 7 2853.88 8 2867.75 9 2876.48 10 2880.38 (1) About +1.1 MHz, evaluated with FSCALE from LLNL SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

2. Preliminary tests on 10-cells model structure (Vg=0 2.Preliminary tests on 10-cells model structure (Vg=0.034c) Measurements on the brazed structure Overshoot due to the PID Transient: t = +0.2 °C in 7 min SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

2. Preliminary tests on 10-cells model structure (Vg=0 2.Preliminary tests on 10-cells model structure (Vg=0.034c) Measurements on the brazed structure SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

Phase velocity measurements 2.Preliminary tests on 10-cells model structure (Vg=0.034c) Measurements on the brazed structure Phase velocity measurements A preliminary set of measurements have been carried out on the brazed structure thermally conditioned. The first results show that the target value of 0.1% change in phase velocity may be obtained changing the cavity temperature by 2.1 °C (the design value was 1.9 °C). Further measurements will be carried out to deeper investigate the behavior. SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

3. Power couplers design and construction The input and output couplers of the RF compressor have been designed in order to ensure a good impedance match in the bandwidth of interest minimizing the detuning effects on the structure. The geometry of a 10-cells structure with couplers has been modeled using a three dimensional computer code to optimize the length of the tapered waveguide (from the WR284 to the cavity), the dimensions of the coupling slot, and the coupling cell diameter Criteria of quality : S11 minimization, proper phase advance per cell at working mode, proper E-field pattern on n cells structure. SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

3. Power couplers design and construction Optimal case: S11 = -50 dB (VSWR < 1.01) @ 2856MHz Phase advance per cell: 120°±2° SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

3. Power couplers design and construction SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

Full EM characterization and tuning of the 10-cells cavity Conclusions By the end of the year: Full EM characterization and tuning of the 10-cells cavity 4-cells model with couplers will be finished and measured (tests with a 100W CW RF amplifier) SPARC Review Committee Meeting - 16.11.05 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams