1. A Fast Chopping System for High Intensity Linac Beams F. Caspers, T. Kroyer CERN-AB-RF E. Mahner CERN-AT-VAC CARE’06 Frascati, November 15 – 17, 2006

2. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 2   Introduction CERN chopping scheme Layout Technical requirements   Evolution of the SPL chopper Modifications in 2005 Status by September 2006   Measurements & tests Electrical properties Vacuum & leak test Heat transfer test   Coverage factor Measurement Simulation   Remaining jobs Contents

3. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 3   Superconducting Proton Linac (SPL)   Fast beam chopping at E kin = 3 MeV ; thus  is about 8% Fast chopper required to establish desired beam pattern SPL Layout   RFQ: Radio frequency quadrupole   DTL: Drift tube linac   CCDTL: Coupled cavity DTL   SCL: Side coupled linac    = 0.65,  = 1.0: superconducting cavities

4. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 4 8*2.84 ns ( 44 MHz)   Most demanding scheme for SPL operation: cutting out three bunches out of eight repetition rate 44 MHz bunch spacing 2.84 ns ; 10 to 90% rise and fall time required < 2 ns CERN Chopping Scheme

5. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 5 Chopper off   Chopper line lattice designed such as to magnify the kick from the chopper; this reduces required kick field   The chopper plates have to be installed in the quads; this saves length and reduces space-charge related emittance growth Chopper Line (1) Chopper on Kick-magnifying quad

6. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 6 Chopper Line (2) chopper beam dump bunching cavities

7. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 7   The chopper plates are no longer DC-wise floating (no more triaxial mode of operation)   Now we have a coaxial instead of a triaxial chopper structure   The triaxial version was meant for simultaneous dual mode of operation, i.e. 0 to 10 MHz: electrostatic deflector, above 10 MHz travelling wave mode   Removal of isolating units both in water cooling circuits and coaxial driving lines Modifications in 2005

8. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 8   Initially samples of the meander lines were produced at CERN   While all parameters were basically ok (electrical, vacuum), reproducibility of certain electrical parameters (electrical length, match) was not always perfectly satisfactory   After the accomplished proof of principle with CERN technology, a supplier that can well control all the process parameters was needed; a possible change in technology is not a problem as long as the key properties are preserved   Kyocera was eager to enter in a cooperation with CERN and willing to adapt their technology to our needs   The plate that was recently furnished compared well in all aspects with the best CERN samples and we hope that the promised good reproducibility will show up in reality Meander lines

9. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 9   Technological differences between the CERN and Kyocera meander structures: Meander line: Technology CERNKyocera CERN orders from Wesgo (D) ceramic plates with fixation holes and 10 to 15  m thick homogeneous MoMn layer (fired at 1400 C in hydrogen atmosphere Kyocera produces the alumina plates in-house. Then a meander pattern is created in thick film silver paste of about 10 to 15  m thickness. This thick film silver paste has after firing a much higher resistivity than bulk silver (factor 5 to 10) The meander structure is etched into the MoMn layer. Afterwards a 1 to 2  m thin-film layer of Ag is attached by sputtering. Onto the Ag thick-film layer 30 to 40  m copper are deposited electrochemically In a final step this silver-coated MoMn layer gets another 30  m silver by electrochemical deposition Finally 1 to 2  m of Au are applied for good contacts and protection against oxidation

10. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 10 Metallize printing (Ag thick-film) Firing Cu Plating Resistance Testing Final inspection Packing Machining Ceramic incoming inspection Au Plating CERAMIC PLATE Process Flow RF Property Testing Machining Electrode for electrochemical deposition removed by grinding Grinding Area Courtesy: Kyocera

11. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 11   First meander line plate from Kyocera were received in June 2006 but it turned out that the attenuation was too high   In the second iteration the technological parameters were properly adjusted and the last sample was very satisfactory   After extensive electrical tests this single plate was installed in the chopper tank   Vacuum, leak and heat tests performed successfully Status by September 2006

12. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 12 Electric Measurements: Transmission Attenuation   Measurements performed on single chopper plate with an image plane 10 mm above the line’s surface to simulate the presence of a second plate   Frequency domain transmission   A DC resistance of 1.1  was measured, which agrees very well with the low-frequency limit of the measured attenuation   3 dB bandwidth 940 MHz. If there was no phase distortion the rise time would be   All rise times quoted are 10 to 90% values Attenuation over one chopper plate Generator bandwidth

13. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 13 Transmission Step Response   Response for 0 to 700 MHz low pass step function (Kaiser Bessel weighting function with  = 6)   Comparison between a measurement with and without the image plane. Due to the high electric field energy in the alumina the kick field does not change much when the symmetry is broken   Measured rise time t rm = 1.771 ns, to be compared with t ri = 1.407 ns of input pulse; structure rise time   This is a conservative estimate of t r since the t ti is rather short and we get into the highly dispersive region of the response

14. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 14 Phase Distortion   From the measured phase without image plate the electrical delay of 16.73 ns (linear term) was removed   With image plane (realistic configuration) the electrical length was 16.83 ns, within 0.1 ns of the required value   The remaining phase is not flat as for a dispersion-free line; thus we have phase distortion   The phase distortion is due to coupling between adjacent lines in the meander structure. This coupling increases quickly with frequency like in a microstrip directional coupler   In an ordinary first-order low-pass the 45 degree points coincide with the 3 dB points. Here they are at 375 MHz, i.e. much lower than the 3 dB points Generator bandwidth Phase without image plane

15. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 15 Reflection   Very good impedance match of meander line to 50  : reflection in frequency domain of the order of -30 dB below 500 MHz   These data were measured on a test jig consisting of a single plate with SMA connectors fixed on either side Generator BW

16. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 16 Reflection Step Response   Response for 0 to 700 MHz low pass step function   S 11 very small, of the order of 0.02 which is another indication of good match   The line impedance is not perfectly constant over the meander length as can be seen from the bump at t = 10 ns.   Towards the end of the line an apparent increase in line impedance can be seen. This is an artifact caused by the lossy line; could be corrected numerically Twice the line length of ¼17 ns

17. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 17 Tuning of Electrical Length   It was tried to adjust the electrical delay of a chopper plate by modifying the metal ground plane   Cutting a longitudinal groove into the ground plane reduces the effective  and thus increases the group velocity on the line. Since the variation in line impedance is over distances much shorter than the wavelength, the other electrical properties should not be affected much   For two 5 mm wide and 3 mm deep grooves a 5% decrease in the electrical length was found on a CERN plate Generator bandwidth

18. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 18 Vacuum & Leak Testing   Initially the obtained vacuum pressure was about 1.5e-7 mbar   Leak rate smaller than 2e-10 mbar.l/s   The ceramic plates were not baked before installation and the entire tank cannot be baked due to the presence of the integrated quadrupole magnet   However, there is a rather larger thermal resistance between the ceramic plate and its aluminium support structure and the rest of the tank   Therefore an “in-situ bake-out” at about 150 degrees C was possible by passing current through the meander line with the water- cooling off   Now a vacuum pressure of about 3.5e-8 mbar is reached, which is within specs for the chopper line water cooling

19. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 19 Heat Transfer Test (1)   In order to settle question about the heat contact between the ceramic plates and the water-cooled aluminium support structure a heat transfer test was carried out   The temperature of the ceramic plate was monitored in two ways, by Observation with an infrared camera through a window at one end of the tank measuring the resistance of the meander line, knowing that the resistivities of the Cu, Ag and Au all have about the same temperature coefficient of 4e-3 K -1   In two runs the heating with and without water cooling was measured   The vacuum pressure was monitored, as well water-cooling off water- cooling on

20. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 20 Heat Transfer Test (2)   The device under test…

21. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 21 Heat Transfer Test (3)   The pictures on the right show a heating cycle without water cooling (Note the change in scale for the last picture!)   Without cooling the steady state conditions were reached at a temperature difference  T = 128 K. The heating power was P = 28.4 W returning a thermal resistance R th = 4.5 K/W.   Radiative heat losses were neglected here. This can be justified by the fact that inner surface of the chopper tank has very high reflectivity, thus reducing considerably the effective power flux. The measured temperature curves follow very closely the exponential curves predicted for heat conduction, indicating that this is the main process involved.   With the cooling of the aluminium support plate switched on, the steady state  T decreases to about 18.2 K for P = 20.4 W giving a R th = 0.89 K/W   Thus more than 100 W of heat power can be dissipated for an operation temperature 100 K above the cooling water temperature t = 0 min,  T = 0 K t = 35 min,  T = 20 K t = 500 min,  T = 120 K

22. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 22   The chopper must be able to stand the 500 V pulses on the plates   High voltage testing showed that a single chopper plate can stand at least 2 kV to ground which is largely sufficient for the present requirements High Voltage Testing

23. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 23   The field coverage factor is defined as the ratio of the deflecting electric field on the beam path to that for the case when a continuous metal plate replaces the meander pattern   Time domain simulations presented at EPAC02 predict a coverage factor of 0.8 in the center of the structure [1]   In the electrostatic approximation it follows from Gauss’s law that the vertical electric field integrated over any horizontal plane above the chopper plate is constant. However, in dependence of the transverse position the field strength may change.   Simulations by M. A. Clarke-Gayther predict a coverage factor of 0.78 in the center of the chopper aperture; 10 mm to the side the coverage factor decreases to about 0.73   Since the beam dump is round, the maximum kick field is needed in the center of the structure Coverage factor [1] Caspers, F; Mostacci, A; Kurennoy, S; Fast Chopper Structure for the CERN Superconducting Proton Linac, EPAC02, Paris, 2002 Time domain simulation (MAFIA)

24. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 24   In terms of field homogeneity the CERN structure compares favorably to the SNS meander, since the meander period to aperture ratio is considerably smaller Field homogeneity (1) +25mm -25mm +66mm -66mm SNS structure image: S.S. Kurennoy and J.F. Power, Development of a fast traveling-wave beam chopper for the SNS project, LINAC’98, Chicago, 1998 CERN meander line SNS meander line

25. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 25   An electrostatic simulation with Ansoft Maxwell was run to determine the field homogeneity of the structure in the beam aperture   Very close to the conductor surfaces the fields peak, while the smoothen out as one approaches the beam axis Field homogeneity (2) Beam goes into plane of the “paper”

26. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 26 Coverage Factor Measurements (1)   Alumina plate with meander structure compared with full metal plate   Metal plate made twice as wide to assure that its fringe field can be neglected   Second metal plate used at the position of the beam (10 mm above ceramic surface) as image plane   In the center of the structure field measured using probe with 10 mm diameter using a button pick-up-like set-up Port 2 Port 1 Port 3

27. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 27 Coverage Factor Measurement (2) Button as probe at Port 3 image plane reference line meander line

28. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 28 Coverage Factor Measurement (3)   We have a 3-port, with ports 1 and port 2 being the input and output of the slow-wave structure and the probe at port 3   Full two-port calibration used, since for the reference measurement we have a 4  line thus strong mismatch   Measured quantities S 21 : Transmission along the plate with port 3 matched Reflection on ports 1 and 2 S 31 : Transmission from the plate to the probe with port 2 matched   These quantities were measured for the slow-wave structure (subscript DUT) and for the full metal plate as reference (index R)   From these quantities the coverage factor CF in dB can be calculated from CF = S 31DUT - S 21DUT /2 - (S 31R - S 21R ); the S parameters have to be plugged in in dB   This formula was derived under the assumptions negligible losses for the reference measurement but heavy mismatch mismatch of the slow-wave structure small but certain losses

29. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 29 Coverage Factor Measurement (4)   CF = 0.78 at low frequencies   Ripple with ¼25 MHz due to small mismatch of slow-wave structure   For high frequencies (>50 MHz) standing waves strongly impact reference measurement   Mechanical uncertainty of half beam gap (chopper plate to image plane) estimated as ¼0.1 mm; effect on results determined by introducing a 0.2 mm offset ( cyan traces)   At about 1 MHz comparison with modified technique: port 2 left open in reference measurement. At low frequency S 21R is then expected to double. The corrected result is plotted at one frequency (black star)

30. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 30 Remaining Jobs   Final assembly of both chopper tanks including outgassing, leakage and electrical tests   Power test with new pulser   Test with beam Static: A DC voltage applied at the plates, 50  termination removed Dynamic test with pulser   Whatever else you can suggest or dream up...

31. CARE06 Frascati, November 15-17, 2006 A Fast Chopping System for High Intensity Linac Beams 31 Acknowledgements   We would like to thank the AB-RF workshops for assembling the tank, F. Wurster and M. Nagata from Kyocera for fruitful cooperation in development and implementation of the technologies for printing the meander structure and J. Borburgh for assistance with the heat transfer measurements   Thanks to R. Garoby and T. Linnecar for support