Halo, SLAC, 2014 Towards wire scanner measurements with Large Dynamic Range (> 10 6 ) Pavel Evtushenko, Jefferson Lab

Halo, SLAC, 2014 Outline  Motivation: Why large dynamic range diagnostics?  Experience with existing high average current FEL driver  Large dynamic range transverse beam profile measurements  Wire scanner measurements  experience so far (CEBAF) counting  PMT in analog mode  Signal generation

Halo, SLAC, 2014 Motivation: Why Large Dynamic Range?  there are several applications of electron LINACs under consideration / design that require average beam power of several MW  these applications also require very high peak beam brightness, comparable to the one at pulse NC LINACs  similar to low average current (NC, pulses) LINACs, with high average current LINAC a diagnostic beam mode must be used  the significant difference is the ratio of beam currents in the diagnostic mode and full current mode  for a high average current LINAC this ration can easily be tens of thousands  One example of an electron LINAC, which have operated with high average current 9 mA, while driving FEL (also high average power) JLab IR/UV Upgrade FEL.

Halo, SLAC, 2014 JLab IR/UV Upgrade: 1.2 MW beam power E beam 135 MeV average current 9 mA (135 pC at MHz) Average beam power ~ 1.2 MW ! If lost beam average =1 W  possible problem for vacuum  concern for the FEL undulator livetime 25 μJ/pulse in 250–700 nm UV 120 μJ/pulse in 1-10 μm IR

Halo, SLAC, 2014 Lessons from high current FEL operation  when setting this machine up for high current operation, at fist diagnostic beam mode is used, this gives “best” RMS setup, i.e., the setup which optimizes FEL performance and does not show any measurable beam lose (at that current level)  then as average beam current is increased we always found that there is a need to alter transverse match to further reduce beam loss to allow higher current operation  important point is that, such adjustments of the transverse match must be small  there are very small fractions of the beam, which could prevent high current operation, but are not measured when diagnostic beam mode is used  it also appears that, such small fractions of the beam have different Twiss parameters than the core of the beam, i.e., transverse phase space is not described well by a single set of Twiss parameters

Halo, SLAC, 2014 Beam dynamics driven halo generation Measured: JLab FEL injector, intensity difference of the peak and “halo” is about 300. (YAG:Ce, standard CCD - 57 dB SNR 10-bit frame grabber) Simulations: PARMELA, 3×10 5 particles; X and Y beam profile and its projection show the halo around the core of about 3× Even in idealized system non-linear beam dynamics can lead to formation of halo.

Halo, SLAC, 2014 LINAC’s non equilibrium (non Gaussian) beam Propagating in drift space … FODO matching section  This are not beam distributions from a nominal setup, but an experiment that shows complexity of the phase space distribution – no single set of Twiss parameters describes the beam  This is also not a halo. Dynamic range of this measurements is ~ 500, all of this beam later is matched to the FEL’s optical cavity and participates in the FEL interaction

Halo, SLAC, 2014 Beam viewer wire-scanner combination  Must have impedance shield, due to high average I  Two diagnostics at one location  Can use YAG:Ce or OTR viewer with easy switch  Shielded, 3 position viewer design for FEL

Halo, SLAC, 2014 Wire scanner measurements: counting  CEBAF uses wires scanners for transverse beam profile measurements  499 MHz repetition – very good for counting  One of a very few LDR beam profile measurements examples  Due to very low current (5 nA) made with CW beam  Max. counting frequency ~ 10 MHz (not a dedicated hardware)  Coincidence effective to reduce background, but at the expanse of even longer measurements time  With CW beam measurements time of about 15 min.  for non-Gaussian beams  A.Freyberger, in DIPAC05 proceedings, Measurements made at CEBAF

Halo, SLAC, 2014 PMT current range  Counting can provide LDR, but is really practical only with high (~ 100 MHz) bunch frequency  For smaller bunch frequencies alternative is analog mode - PMT current measurements  Typically average PMT current must be ≤ 100 µA  With low duty cycle beam ( Hz) PMT current within the 100 µs can be much higher  PMTs with dark current of a few nA are available (low Q.E. cathode at long wavelength)  For low duty cycle systems like diagnostic mode beam, gated integrator (GI) is a for small signal recovery  For a single GI dynamic range of 10 7 is very challenging and probably impossible (sub µV noise for 10 V signals)

Halo, SLAC, 2014 Gated Integrator (GI) Digitize Integrate Discharge Ready  PMTs with HV at the cathode and anode at the ground potential are used – this results in negative current, which needs to be inverted  A current mirror is used to 1. invert the current and 2. to make multiple “copies” of the PMT current  Two outputs of the current mirror: #1 ~ 100 % of PMT current, #2 ~ 1 % of PMT current

Halo, SLAC, 2014 GI calibration with precision source  Preparing version two with FET transistor based current mirror  The non linearity by itself is not a really a problem if the behavior is reproducible  Calibration is to be used as a look up table  Output of each GI is digitized with 16-bit ADC at 4 MS/s  Output of a GI is available for digitalization during charge integration as well – better than the gate width time resolution  Results of GI calibration with a precision DC current source (Keithley 6221) in the range from 100 pA through 10 mA are shown  RMS noise level ~ 250 µV  Non linearity of the 1 % channel (red) is du to nonlinear operation of the current mirror, too little current for bipolar transistor

Halo, SLAC, 2014 Calibration cross-check

Halo, SLAC, 2014 GI stability  the 1 % variation is attributed to the source stability  GI stability is ~ 10 times better (0.1 %)

Halo, SLAC, 2014 GI + PMT test  PMT driven by a pulsed LED, 100 us “macro pulse”  LED is driven by pulse generator at fixed micro pulse rep. rate of 100 MHz  Width of the LED pulse adjusted from 620 ps down to 380 ps to generate the plot

Halo, SLAC, 2014 PMT dark current measurements  ~ 2 nA dark current is at the level of PMT specification (3 nA typical 20 nA max)  measurements with two gates allows to subtract the dark current  then limiting factor is the GI intrinsic noise level – equivalent to ~ 100 pA RMS

Halo, SLAC, 2014 Wire Scanner: analog mode  an alternative to GIs are commercially available Logarithmic Converters  Originally designed for photo diode measurements (fiber optics communications)  Dynamic range of 160 dB and 200 dB  Bandwidth of several MHz but varies dependent of signal level  Shows more complex than GI noise behavior, which needs to be studied further  Calibration of AD8304 log-amp is shows  The calibration was made using the same setup – DC current source and ADC as used for GI evaluation and testing  4 calibration without and 4 with a CM are shown

Halo, SLAC, 2014 Wire Scanner / Cherenkov converter  one way to convert E-M shower e- and e+ to visible photons  “prompt” – much faster than a fast PMT with few ns pulse length  direction sensitive – to reduce background, i.e., insensitive to particles coming from “wrong” direction  all reflective optics – to use wavelength as short as possible (3 reflectors)  output matched to a quartz fiber to transport light to a PMT outside of the accelerator tunnel (background reduction)  thicker converter generated more photons, but limited by multiple Coulomb scattering – beam energy dependent  H 2 0 n=1.333 > sqrt(2); Cherenkov radiation is not trapped in the radiator Cylindrical reflector #1 Cone reflector #2 90˚ off axis parabolic reflector #2 Optical fiber input Cherenkov radiator

Halo, SLAC, 2014 W-S signal via Cherenkov converter How many photons would Cherenkov radiator make? - 3 mm stainless steel wall; - 50 µm W radiator; nm – 650 nm wavelength range; pC; - 50 mm diameter, 125 µm thick radiator at 0.1 rad relative to the beam direction ~ 1.1×10 5 photons

Halo, SLAC, 2014 That is all folks. Thank you.

Halo, SLAC, 2014 back up

Halo, SLAC, 2014 FEL Injector as an example of #1 (1/6) downstream of the gun

Halo, SLAC, 2014 FEL Injector as an example of #1 (2/6) upstream of the buncher cavity

Halo, SLAC, 2014 FEL Injector as an example of #1 (3/6) downstream of the buncher cavity

Halo, SLAC, 2014 FEL Injector as an example of #1 (4/6) upstream of the SRF cavity 1

Halo, SLAC, 2014 FEL Injector as an example of #1 (5/6) downstream of the SRF cavity 1

Halo, SLAC, 2014 FEL Injector as an example of #1 (6/6) downstream of the SRF cavity 2