Halo, SLAC, 2014

Towards wire scanner measurements with Large Dynamic Range (> 106)

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 powerof 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

Ebeam 135 MeVaverage current 9 mA(135 pC at 74.85 MHz)

Average beam power ~ 1.2 MW !

If lost beam average <P>=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 SNR10-bit frame grabber)

Simulations: PARMELA, 3×105 particles; X and Ybeam profile and its projection show the halo aroundthe core of about 3×10-3.Even in idealized system non-linear beamdynamics 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 (100 µs @ 60 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 107 is very challenging and probably impossible (sub µV noise for 10 V signals)

Halo, SLAC, 2014

Gated Integrator (GI)

Digitize

Integrate Discharge ReadyReady

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

H20 n=1.333 > sqrt(2); Cherenkov radiation is not trapped in the radiator

Cylindricalreflector #1

Conereflector #2

90˚ off axisparabolicreflector #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;- 200 nm – 650 nm wavelength range;- 200 pC;- 50 mm diameter, 125 µm thick radiator at 0.1 rad relative to the beam direction

~ 1.1×105 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 ofthe gun

Halo, SLAC, 2014

FEL Injector as an example of #1 (2/6)

upstream of thebuncher cavity

Halo, SLAC, 2014

FEL Injector as an example of #1 (3/6)

downstream of thebuncher cavity

Halo, SLAC, 2014

FEL Injector as an example of #1 (4/6)

upstream of theSRF cavity 1

Halo, SLAC, 2014

FEL Injector as an example of #1 (5/6)

downstream of theSRF cavity 1

Halo, SLAC, 2014

FEL Injector as an example of #1 (6/6)

downstream of theSRF cavity 2