DaMon : a resonator to observe bunch charge/length and dark current .

1. DaMon: a resonator to observe bunch charge/length and dark current. Principle of detecting weakly charged bunches Setup of resonator and electronics Measurement of bunch charge at PITZ and REGAE Principle of detecting dark current Measurement of dark current at PITZ and FLASH Principle of detecting bunch length Measurement of bunch length at PITZ Summary and Outlook Dirk Lipka, W. Kleen, J. Lund-Nielsen, D. Nölle, S. Vilcins, V. Vogel 3rdoPAC Topical Workshop on Beam Diagnostics Vienna, Austria, May 8 – 9, 2014 Ref: http://accelconf.web.cern.ch/AccelConf/DIPAC2011/papers/weoc03.pdf

2. Principle of detecting weakly charged bunches with a resonator • A passive resonator is used because the induced field strength due to electrons has the potential to detect very low beam charges Induced voltage in a resonator from a beam oscillates with resonance frequency f and decays with decay time t. QL: loaded quality factor. Single bunch detection possible with bunch distance >200 ns By measuring U0 the charge of the beam qis determined. for monopole modes Field distribution of 1. monopole mode Simulation view Sensitivity S can be determined by resonance frequency f, line impedance Z=50Ω, external quality factor Qext and normalized shunt impedance (R/Q).

3. Setup at the Photo Injector Test Facility at DESY Zeuthen (PITZ) • PITZ: characterize, optimize and prepare electron source for FEL • Dark current Monitor (DaMon) situated 2.36 m behind cathode followed by booster 0.68 m • Measurement: fl=1299.3±0.1 MHz, QL=193±5, Qext=252±4 • Expectation agree with measurement (resonator without tuner) • Shunt impedance from simulation, results in sensitivity of 11.83 V/nC NWA measurement result in tunnel to detect resonator properties Photo: J. Lund-Nielsen

4. Setup of the monitor in FLASH • Free-electron Laser in Hamburg (FLASH) • A 34 mm type is installed at the first bunch compressor like in PITZ • Measurement: fl=1299.0±0.1 MHz, QL=192±5, Qext=248±4 • Shunt impedance from simulation, results in sensitivity of 11.88 V/nC

5. Electronics • Two inputs according of two outputs of DaMon: • Beam charge • Dark current • Includes circulator, band-pass filters, limiter, pre-amplifier, down conversion to IF, logarithmic detector, offset and gain control • Two outputs for ADC

6. Measurement of bunch charge at PITZ • smaller fluctuation compared to Faraday Cup for low charges • Sub-Pico-Coulomb resolution with DaMon visible • Still 20 dB attenuation used • DaMon 2% higher charge compared to FC (loss of charge at FC) Voltage amplitude Signal from electronics Base line Voltages calibrated with electronics response function, attenuation of cables and attenuators. Good agreement between laboratory calibration measurement including simulated shunt impedance and measured charge with FC

7. Measurement of bunch charge at PITZ Signal without electronics Signal with electronics • The bunch spacing at PITZ is 1 ms, decay time withouth and with electronics measurement sufficient • Bunch spacing for European XFEL is 222 ns, decay time is sufficient for single bunch measurements q≈ 0.34 nC Response function of electronics calibrated Due to logarithmic detection lower amplitudes amplified results in high dynamic range: 70 dB

8. Measurement of bunch charge at REGAE • Relativistic Electron Gun for Atomic Exploration (REGAE) produces bunches with few fC charges for femtosecond electron diffraction • Necessary to measure these charges in a non-destructive way • DaMon system is installed because it has to potential to resolve the requirement Both DaMon output difference for intrinsic resolution One input for higher charge with 10 dB attenuation, second for best resolution, results in 2.3 fC for 200 fC Resolution for each channel as a function of charge used at REGAE. The resolution is corrected by 10 dB difference between channel 1 and 2. Photo of the DaMon installed at REGAE. Ref.: http://accelconf.web.cern.ch/AccelConf/ibic2013/papers/wepf25.pdf

9. Principle of detecting dark current with resonator • Charge of one dark current bunch too weak to be detected: superimposing of induced fields from the dark current bunches when resonance frequency of resonator harmonic of accelerator, therefore the notation Dark current Monitor (DaMon) Dt = 1/(1.3 GHz) = 1/f I I=q/Dt=q*fmean current q=U0/S S resonator sensitivity of monopole mode UDaMonis envelope voltage after transient oscillation when amplitude of N dark current bunches are added after another This results in I=UDaMonf (1-e-p/QL) / S Dark current bunches within 2 RF oscillations t Expected/simulated voltage f=1.3 GHz, QL=205 for I=1 mA and 1000 dark current bunches Transient oscillation finished after 150 ns

10. Measurement of dark current at PITZ • Strong dark current in backward direction from booster, on calibration limit • Gun dark current much lower, can be separated by different RF pulse duration • Observation limit at 40 nA Gun and booster RF on • A vacuum event in the gun produced an additional charge spike in the dark current output • This can be seen in the time domain compared to the RF pulse Only gun RF on

11. Measurement of dark current at FLASH • Measured behind second accelerator module • Gun and module are producing dark currents

12. Measurement of dark current at FLASH • At the history of the dark current measurement a gap was visible • This was verified by the beam loss system (with lower resolution) Beam loss output, z=177 m z=32m A RF event stops RF in the gun and restarted after several µs, therefore such gap can be produced

13. TM01 TM02 TM11 TM12 TM21 TM03 TM22 Principle of detecting bunch length Complete measured spectrum up to 6 GHz • Amplitude from monopole mode is corrected by form factor • Ratio of amplitude for different monopole modes should dependent on bunch length First three monopole modes frequencies: 1.299; 3.236; 5.074 GHz • Form factor as a function of expected bunch length after injector • After compressor bunch length < 1 ps: form factor tends to be unity; therefore this method applicable only for bunches longer than < 1 ps: after the photo injector at PITZ and FLASH • Best resolution by using largest frequency difference

14. Beam direction Streak Station DaMon 4 m Measurement of bunch length • Amplitude at different frequencies taken with spectrum analyzer • Measurement as a function of injector acceleration phase; highest energy gain at phase 0 • Compare DaMon results (combination TM01 and TM03, because best resolution) with aerogel radiator and streak camera method, detector positions differs by 4 m • Streak measurement differs from simulation for phases < 0, same as it is for DaMon • Both show same behavior (maybe simulation parameters not perfect) • Result: agreement of bunch length taken with DaMon to the streak camera results DaMon Streakcamera

15. Summary and Outlook • A passive resonator used to detect bunch charge with fC resolution at PITZ, FLASH and REGAE • Same resonator used to measure dark current at PITZ and FLASH (not at REGAE because acceleration frequency is 3 GHz) • Single electron events cause signals at DaMon system to be able to detect those • Higher order modes can be used to detect bunch length longer than 1 ps Outlook: • electronics for the bunch length measurement is developed and will be tested next at PITZ • Several of this monitor system for bunch charge and dark current will be installed at the European XFEL