Advanced Beam Instrumentation CASA Retreat, 26-27 September 2001

1. Advanced Beam InstrumentationCASA Retreat, 26-27 September 2001 Jean-Claude Denard

2. Outline • Types of machine considered • Bunch length monitoring (100 fs) • High resolution beam profile monitors • Halo monitors for high power beams • Machine protection • Diagnostics that would be nice • Summary CASA Retreat 2001/J-C. Denard

3. Types of Machines Considered • Energy Recovery Linacs (ERLs) for colliders • FEL driver accelerators • Linac based 4th generation light sources • CEBAF and Jlab FEL as test benches CASA Retreat 2001/J-C. Denard

4. Bunch LengthRequirements • Measure short bunches’ length down to 100 fs • Quickly tune the machine (injector) • Non-invasive continuous monitoring is better • Measuring actual time structure is better than retrieving bunch length from spectrum CASA Retreat 2001/J-C. Denard

5. Pros Demonstrated on several machines (JLab IR FEL with transition radiation) Shortest bunch length measured: 50 fs bunches of Stanford’s sunshine facility 1994 Non-invasive with diffraction radiation => beam through slit Non-invasive with synchrotron radiation Cons Invasive with transition radiation => beam through foil (Jlab IR FEL) Cannot reconstruct actual bunch longitudinal distribution from autocorrelation function Slow device (a problem for tuning, not for monitoring) Bunch Length Monitors (1):Autocorrelation Technique With CSR, CTR, or CDR CASA Retreat 2001/J-C. Denard

6. pros Non-invasive w/ CSR and CDR Prototype tested at CEBAF (D.X. Wang, G. Krafft) Very useful to quickly adjust bunch length: min. length => max. power THz detectors do not need to be as sensitive for higher current machines Cons Does not measure actual bunch length Problem with reliability of high sensitivity shottky THz detector Bunch Length Monitors (2): Direct Power Spectrum Measurements CASA Retreat 2001/J-C. Denard

7. Pros Non-invasive w/ CSR or DR Measures actual time distribution Cons Sub-picosecond resolution difficult to achieve Small dynamic range Expensive Not easy to operate Bunch Length Monitors (3):Streak Camera Types CASA Retreat 2001/J-C. Denard

8. Pros Non-invasive (but crystal must come close to the beam) Successfully tested at FELIX (X. Yan et al, phys. Rev. Letters V85 no16 p 3404) Yields time distribution Short bunch measurements should be achievable with fs laser Cons Risk of damaging the crystal with the beam Resolution limit not known yet. Need testing with short bunches 100 to 200 ps Bunch Length Monitors (4):Electro-optic Sampling CASA Retreat 2001/J-C. Denard

9. Bunch Length Monitors (5):Zero-phasing Technique Best time domain measurement is the deflecting cavity method also called zero-phasing • Measured 150 fs CEBAF beam over large current range • 72 fs measured at SLAC • Invasive unless used with pulsed magnet to make it mildly invasive (H. Areti’s suggestion for CEBAF) • At SLAC, 1 pulse out of 10 can be used for bunch length measurement, the 9 others still delivered to physics • Diagnostic to study before the machine design phase CASA Retreat 2001/J-C. Denard

10. Beam Size Requirements • Measure 50 mm beam size • Continuous monitoring => non-invasive • High current beams ≥ 10 ma • Measurement rate ≥ 1 Hz CASA Retreat 2001/J-C. Denard

11. Beam Size: Instruments • Diffraction radiation monitors • Only computer simulations for profile • Resolution < 100 mm seems difficult to achieve with reasonable size holes or slits (≥ 2 mm) • Residual gas monitors • Demonstrated technique with fat beams (a few 100 mm rms ?) • Small beam size resolution measured with actual instrument using fake beam (light shining on 10 mm wire, W. Sellyey PAC2001) • Possible problem: micro-channel plates have to be close to the beam pipe; They are expensive, sensitive to radiation, and difficult to shield CASA Retreat 2001/J-C. Denard

12. Beam Size: Instruments (2) • Most promising technique is synchrotron radiation interferometer (T. Mitsuhashi, PAC1999, NY) • Demonstrated on several machines: KEK and DELTA • Prototype in development on CEBAF hall A line for energy spread monitoring CASA Retreat 2001/J-C. Denard

13. Synchrotron Radiation Interferometer (1) CASA Retreat 2001/J-C. Denard

14. Synchrotron Radiation Interferometer (2) CASA Retreat 2001/J-C. Denard

15. Halo Monitor • Continuous losses due to halo at the 100 nA level may be a problem for electron machines: • Flange heating can open vacuum leaks • Aperture restrictions may become high radiation areas • High radiation doses damage equipment (BPM electronics, CCD cameras, cooling water hoses, etc…) • With ERL’s beam of 100 ma, a 100 nA halo is 106 times weaker than the main beam; It is difficult to measure CASA Retreat 2001/J-C. Denard

16. Halo Monitors • Tentative requirements • Detect 10 nA halo => 1E-6 sensitivity for 10 ma beam 1E-7 sensitivity for 100 ma beam • Non invasive continuous monitoring • No need for actual profile of the whole beam • State of the art • Hall B harp (Jlab): 1E-5 to 1E-6 dynamic range with 1 to 10 nA beams. A PMT and associated electronics count the particles scattered on the harp wire • OTR imaging (Haouat et al, CEA, BIW1994): 1E-4 with beams < 1 ma peak current at ~0.1% duty factor • Harp wire + plate scanned into low duty factor 100 ma proton beam (D. Gilpatrick, LANL, PAC2001). Achieves 1E-4 dynamic range profiles CASA Retreat 2001/J-C. Denard

17. Halo Monitors • All those high performance monitors are invasive; Wire or foil is in the beam path • None of them can measure a 1E-7 halo • Ideas to be tested soon in CEBAF hall A and C: • A plate is very sensitive to the halo around the beam • The monitor is not invasive if there is a hole in the plate at the location of the beam core • Count high energy photons with pmts • Use coincidence on several pmts to eliminate events that are not originated on the plate (as in hall B halo harp) CASA Retreat 2001/J-C. Denard

18. Machine Protection • CEBAF beam loss accounting system • Coming upgrade is to detect ~ 0.1% of loss out of a 200 ma beam • 10 ma and 100 ma machines would need 1E-5 and 1E-6 current monitor accuracy in order to detect the same 200 nA of loss ! • It seems difficult to achieve. A squid-based current monitor may be a solution • Advanced concepts for machine protection • CEBAF beam scraping monitor (V. Lebedev) CASA Retreat 2001/J-C. Denard

19. Diagnostics That Would Be Nice • 6-D electron beam characterization • Michele Castellano, (DIPAC2001) proposed diffraction radiation and transition radiation • Needs a significant development effort • CEBAF beam divergence or beam size seem out of reach • Multi-pass BPMs • Another significant development effort to plan ahead • CEBAF has multi-pass BPMs in tune up mode only (250 ms pulses at 60 Hz) CASA Retreat 2001/J-C. Denard

20. SLAC Linac Coherent Light Source (LCLS) Need same path length for electrons and photons within 0.1 nm • Longitudinal requirement sets stringent steering tolerances • Beam based alignment scheme included in machine design • Sets tight BPM requirements (sub-micron resolution) • Beam and X-ray viewers to compare both beam orbits CASA Retreat 2001/J-C. Denard

21. Summary • Non invasive bunch length and beam halo measurements as well as machine protection are important instruments we do not know how to build yet • Machine tuning procedures specific to each machine need to be developed. They will require new diagnostics that take time to develop Developing beam instrumentation takes time. Better start now using CEBAF and Jlab FEL as test benches CASA Retreat 2001/J-C. Denard