1. (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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

2. 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
SPARC Review Committee Meeting M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

3. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

4. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

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

6. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

7. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

8. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

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

10. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

11. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

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

13. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

14. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

15. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

16. 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 2 3 4 5 6 7 8 9 10
(1) About +1.1 MHz, evaluated with FSCALE from LLNL
SPARC Review Committee Meeting M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

17. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

18. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

19. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

20. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams

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

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

23. 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 M.Mauri - CORA: Design and Construction of a RF compressor for High Brightness Electron Beams