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 3 rd oPAC Topical Workshop on Beam Diagnostics Vienna, Austria, May 8 – 9, 2014 Ref: http://accelconf.web.cern.ch/AccelConf/DIPAC2011/papers/weoc03.pdf
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DaMon: a resonator to observe bunch charge/length and dark current. Principle of detecting weakly charged bunches Setup of resonator and electronics.
Dirk Lipka | 3 rd oPAC Topical Workshop on Beam Diagnostics | May 9, 2014 | Page 3 Setup at the Photo Injector Test Facility at DESY Zeuthen (PITZ) Photo: J. Lund-Nielsen NWA measurement result in tunnel to detect resonator properties > 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: f l =1299.3±0.1 MHz, Q L =193±5, Q ext =252±4 > Expectation agree with measurement (resonator without tuner) > Shunt impedance from simulation, results in sensitivity of V/nC
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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. Vogel3rd oPAC Topical Workshop on Beam DiagnosticsVienna, Austria, May 8 – 9, 2014
Dirk Lipka | 3rd oPAC Topical Workshop on Beam Diagnostics | May 9, 2014 | Page 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
fQf
ttUU
L
2
expsin0Induced voltage in a resonator from a beam oscillates with resonance frequency f and decays with decay time . QL: loaded quality factor.
By measuring U0 the charge of the beam q is determined.
Field distribution of 1. monopole modeSimulation view
SQR
QZf
qU
ext
0
Sensitivity S can be determined by resonance frequency f, line impedance Z=50Ω, external quality factor Qext and normalized shunt impedance (R/Q).
for monopole modes
Single bunch detection possible with bunch
distance >200 ns
Dirk Lipka | 3rd oPAC Topical Workshop on Beam Diagnostics | May 9, 2014 | Page 3
Setup at the Photo Injector Test Facility at DESY Zeuthen (PITZ)
Photo: J. Lund-Nielsen
NWA measurement result in tunnel to detect resonator properties
> 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
Dirk Lipka | 3rd oPAC Topical Workshop on Beam Diagnostics | May 9, 2014 | Page 10
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)
t
I Dt = 1/(1.3 GHz) = 1/fI=q/Dt=q*f mean current
q=U0/SS resonator sensitivity of monopole modeUDaMon is envelope voltage after transient
oscillation when amplitude of N dark current bunches are added after another
This results inI=UDaMon f (1-e-/QL) / S
Expected/simulated voltage f=1.3 GHz, QL=205 for I=1 mA and 1000 dark current bunches
Dark current buncheswithin 2 RF oscillations
Transient oscillation finished after 150 ns
Dirk Lipka | 3rd oPAC Topical Workshop on Beam Diagnostics | May 9, 2014 | Page 12
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
Only gun RF on
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
Dirk Lipka | 3rd oPAC Topical Workshop on Beam Diagnostics | May 9, 2014 | Page 13
Measurement of dark current at FLASH
> Measured behind second accelerator module
> Gun and module are producing dark currents
Dirk Lipka | 3rd oPAC Topical Workshop on Beam Diagnostics | May 9, 2014 | Page 14
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
A RF event stops RF in the gun and restarted after several µs, therefore such gap can be produced
z=32m
Dirk Lipka | 3rd oPAC Topical Workshop on Beam Diagnostics | May 9, 2014 | Page 15
Principle of detecting bunch length
> Amplitude from monopole mode is corrected by form factor
> Ratio of amplitude for different monopole modes should dependent on bunch length
2/exp,
,22
0
ziGausszi
ziii
F
FSqU
j
iz U
Ujig0
0,,
TM01
TM11
TM21
TM02
TM12
TM22
TM03
First three monopolemodes frequencies:1.299; 3.236; 5.074 GHz
Complete measured spectrum up to 6 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
Dirk Lipka | 3rd oPAC Topical Workshop on Beam Diagnostics | May 9, 2014 | Page 16
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
DaMonStreakStation
4 m
Beamdirection
DaMon
Streak camera
Dirk Lipka | 3rd oPAC Topical Workshop on Beam Diagnostics | May 9, 2014 | Page 17
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