The Feschenko Bunch Shape Monitor User experience at CERN

1. The Feschenko Bunch Shape MonitorUser experience at CERN Developed by INR, Troitsk. High resolution bunch shape monitor based on secondary emission. Various monitors in existence at CERN, SNS, DESY, Argonne…

2. Vfocus Detector (cup, electron multiplier…) Vfocus • Accelerated beam strikes a wire (at ~ -5kV) • Produce secondary electrons, which same bunch structure as primary beam. • Electrons pass through a set of slits and RF deflecting plate. • Only those electrons close to the RF zero crossing are transmitted (and measured in a cup/electron multiplier). Hence measure at a defined beam/bunch phase. • Shift the RF deflecting phase to the next pulse. • The deflecting plates are biased to make a focus onto the second slit.

3. CERN has used 3 different versions of the device: • Bunch Length and Velocity Detector – Linac 3 – Pb27+ @ 4.2 MeV/u • Bunch Shape Monitor – Linac 2 – p+ @ 10MeV and 30MeV • 3D Bunch Shape Monitor – Linac 2 – p+ @ 50MeV

4. Bunch Shape Monitor – Linac 2 – p+ @ 10MeV and 30MeV • Detector Head • SEM tube feed • RF deflection power • Lens voltage • Inter tank • Target actuator not shown 

5. Inter tank cross section (from DESY model) • Detector Head • Target actuator

6. Uses:  • Bunch shape measurement. • Limited halo investigation (wire cannot be used in beam centrethermal emission and wire damage). • RF characterisation of the RF systems.

7. Bunch Length and Velocity Detector – Linac 3 – Pb27+ @ 4.2 MeV/u • Beam sensitivity - 1uA (peak current) - using 0.1mm wire • Phase Resolution: 0.4 degrees at 101MHz - 10ps • Errors 0.3% on velocity. • Y.V. Bylinski et al, Bunch Length and Velocity Detector and its Application in the CERN Heavy Ion Linac, EPAC 94.

8. Uses:  • Measuring bunch profile and position • Measurement of beam energy, Time of Flight, by moving the detector. (Error can arise due to magnetic field variation in time, or along the movement of the BLVD).

9. Buncher cavity. Measure bunch centre as function of cavity phase and amplitude, Where position does not vary with amplitude, is the zero crossing. • With a bunch cavity, measurement of the longitudinal emittance with the bunch rotation technique (buncher effective voltage must be known). • The measurement effectively removes the need for a spectrometer magnet (must then rely on RF measurements for the cavity effective voltage). Measurement of emittance with a buncher cavity and BSM

10. 3D Bunch Shape Monitor – Linac 2 – p+ @ 50MeV • The second slit is replace with an linear array detector, with a sandwich of secondary electron emission electrodes (like an RF streak camera). • The wire can be stepped across the beam. • The slit can be stepped vertically, to provide a vertical resolution. • The array detector can be read out as a function of time during the beam pulse. S.K. Esin et al, A three dimensional bunch shape monitor for the CERN proton Linac, Linac 1996, Geneva, Switzerland,

11. Uses:  • The bunch form can be measured as a function of a H and V. The3D bunch volume density is measured (averaged over many bunches). • Projections allow any of the 3 real space measurements: • For the bunch length, use for bunch profile, and (with a cavity) a longitudinal emittance measurement. • In the H & V plane, use as a wire scanner (profile and emittance with a quadrupole). • Investigation of the halo (e.g. different phase structure).

12. 3D BSM measurement of the bunch structure, as a function of time during the Linac 2 proton beam pulse. Beam is distorted in RF phasestabilization regime.

13. 3D BSM measurement of the bunch structure (50MeV), as a function of horizontal beam position (in the stable part of the pulse). Bunch core intensity increases towards centre. Bunch tail is quite constant in intensity (suggesting tail has a differentbeam size).

14. What could be done better (@ CERN) • At CERN, we did not manage to drive them from the CERN control system. • The application also dealt with the real time measurements, conversion to CERN architecture required software and hardware modifications. • The local control system was kept, and used non-standard cards (e.g. still some Camac cards, and a unique PC->VME controller). No maintenance was followed up. • =>The BSMs are no longer operational at CERN. Renovation is not worthwhile for last years of operation.

15. Summary • BSMs are powerful bunch measurement systems, with high phase resolution. • At CERN, the BLVD was heavily used during accelerator commissioning. • Variants measure: • Bunch shape • Position • Beam velocity • Bunch 3D density • Transverse emittance (with quadrupole) • Longitudinal emittance (with buncher cavity)

16. Some useful reading: • Y.V. Bylinski et al, Bunch Length and Velocity Detector and its Application in the CERN Heavy Ion Linac, EPAC 94. • S.K. Esin et al, A three dimensional bunch shape monitor for the CERN proton Linac, Linac 1996, Geneva, Switzerland, • A.V. Feschenko et al, Study of Beam Parameters of the CERN Proton Linac Using a Three Dimensional Bunch Shape Monitor, Linac96, Geneva, Switzerland. • P.N. Ostroumov, Review of Beam Diagnostics in Ion Linacs, Linac98, Chicago, IL • N. Y. Vinogradov et al, Bunch Shape Measurement of CW Heavy Ion Beam, Linac2002, Gyeongju, Korea • A.V.Feschenko et al, Bunch Shape Monitors for the DESY H- Linac, PAC 1997 • Hurd J.W. et al. Bunch Shape Monitor for SSCL Linac. Proc of the 1993 Particle Acc. Conf., Washington, May 17-20, 1993 • S.K.Esin, A.V.Feschenko, P.N. Ostroumov. INR Activity in Development And Production of Bunch Shape Monitors, Proc. Of the 1995 PAC and Int. Conf. on High-Energy Accelerators, Dallas, 1-5 May, 1995, V.4, pp. 2408-2410 • A.V.Feschenko, V.A.Moiseev, Space Charge Effects in Bunch Shape Monitors, Linac2000, Monterey, Ca.

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