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

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

Ref: http://accelconf.web.cern.ch/AccelConf/DIPAC2011/papers/weoc03.pdf

Dirk Lipka | 3rd oPAC Topical Workshop on Beam Diagnostics | May 9, 2014 |

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


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: 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


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


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

Dirk Lipka | 3rd oPAC Topical Workshop on Beam Diagnostics | May 9, 2014 | Page 7

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)

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

Base line

Voltage amplitude

Signal from electronics


Measurement of bunch charge at PITZ

> 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

Signal without electronics Signal with electronics

Response function of electronics calibrated

Due to logarithmicdetection loweramplitudes amplifiedresults in highdynamic range: 70 dB


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

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.

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

Ref.: http://accelconf.web.cern.ch/AccelConf/ibic2013/papers/wepf25.pdf


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


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


Measurement of dark current at FLASH

> Measured behind second accelerator module

> Gun and module are producing dark currents


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


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


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


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

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

Documents

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beam q

page nr

vogel3rd opac topical

measurement resonator

bunch distance

bunch chargelength

bunch compressor