UBW2012, Berlin, December 2012 Jefferson Lab Experience with Beam Halo, Beam Loss and Related beam...

UBW2012, Berlin, December 2012

Jefferson Lab Experience withBeam Halo, Beam Loss andRelated beam diagnostics

Pavel Evtushenko, Steve Benson, Dave Douglas, Kevin Jordan, Carlos Hernandez-Garcia, Dan Sexton, Jay Benesch, Arne Freyberger


Outline

1. IR/UV Upgrade (JLab FEL) overview

2. Different sources of unwanted beam

3. Beam dynamics example

4. Drive Laser related remarks

5. Setting up for high current operation

6. Dynamic range of diagnostics

7. Recent LDR measurments

8. CEBAF overview

9. RF trip rate

10. Direct measurements

11. Vacuum “events”

12. Summary


JLab IR/UV Upgrade

Ebeam 135 MeVBunch charge: 60 pC – UV FEL

135 pC – IR FEL Rep. rate up to 74.85 MHz

25 μJ/pulse in 250–700 nm UV-VIS

120 μJ/pulse in 1-10 μm IR


Four sorts of the unwanted beams

1. Fraction of the phase space distribution that is far away from the core (due to the beam dynamics)

2. Low charge due to not well attenuated Cathode Laser (ERLs) – but real bunches that have proper timing for acceleration

3. Due to the Cathode and Laser but not properly timed (scattered and reflected light on the cathode and in the DL transport)

4. Field emission: Gun (can be DC or RF), LINAC itself(is accelerated in both directions)

Flavors of Unwanted Beam

5. Actually, there is one more – ions that accumulate in are true CW electron beam, travel in both directions with thermal velocities in side the electron beam, reduce Q.E. of the cathode one really does not want this beam.


FEL Injector as an example of #1

Measured in JLab FEL injector,local intensity difference of thecore and “halo” is about 300.(500 would measure as well)10-bit frame grabber & a CCDwith 57 dB dynamic range PARMELA simulations of the same setup with 3E5 particles:

X and Y phase spaces, beam profile and its projection showthe halo around the core of about 3E-3.Even in idealized system (simulation) non-linear beamdynamics can lead to formation of halo.


Using a Log-amp is an easy way to diagnose presence of the “ghost” pulses Log-amps with dynamic range 100 dB are available

631 uA (100%)135 pC x 4.678 MHz

5.7 uA (~0.9 %)4.678 MHz “ghost”pulses

Drive Laser “ghost” pulses


DL light scattered on photo cathode

Wafer 25 mm diameter

Active area 16 mm diameter

Drive laser 8 mm diameter

Courtesy of C. Hernandez-Garcia

a view of GaAs photo cathode when running beam (probably 6 % duty cycle or 1.5 %)

measured with simple vis. CCD camera

locations of the wafer and active area are knows from the same view, HV off and white light on

we are looking in to a gap between two non-flat mirrors

with a brand new wafer (no heat cleaning) one would not see any light from the DL spot

At least two processes contribute to the generation of scattering centers

Heat cleaning of the cathode (made periodically, every 4-5 re-Cs)

HV breakdowns can result in rather large pits – scattering and field emission

Visible (green) DL preferable over UV

Preserving cathode surface will be very helpful

Get rid of heat cleaning for GaAs (H – cleaning)


Cathode Laser pulse via streak camera

appears to be close to Gaussian on linear scale; tails not so much Gaussian

the difference from Gaussian distribution is obvious on log scale

realistic (measured) distribution must be used for realistic modeling

especially is the calculations are intended for large dynamic range effects


High current operation

JLab FEL driver is setup for high current operation in three steps/phases

Most of the measurements are made with low duty cycle beam beam

(this is Step 1 that establishes best RMS setup for FEL performance)

- setting up injector (RF phases and solenoids)

- transverse match

- longitudinal match

Step 2 is to increase the duty cycle, usually to 6 %, and look at

the beam loss, small adjustments in transverse and long. match often are

required; the adjustments must preserve the high performance of the FEL

this is the reason the adjustments have to be small

When beam loss is small enough high average (9 mA) current can be

operated and the long term trends in pressure (vacuum) are used for

Step 3 of machine adjustment, also very small.


General FEL remarks

1. JLab FEL is a 9 mA average current machine, despite the fact that all four sorts of beam halo are present (9 mA and 135 MeV 1.2 MW)

2. Setting up for high current operation requires time, but has been done routinely

3. To properly (and quickly) deal with first kind of beam halo Large Dynamic Range diagnostics are needed; until then takes time and trail and error, and origins of problems are not well understood (not confirmed by measurements)

4. For Drive Laser transport Brewster angle windows (input and output); essentially light tight beam line; laser transport with spatial filter to mitigate scattering

5. Scattered DL light on the cathode is a reality one has to leave with, i.e., run beam when it is small enough and replace cathode when it is not.

6. Gradient in the LINAC is limited via requirements to keep dose rate below certain level (especially at the wiggler), but also due to other effects the same as at CEBAF (trip rate)

7. Instruments are: Beam Loss Monitors (BLM) of the MPS

Rad.Con. calibrated ionization chambers

Radiation survey just after beam operation ended (for chronic losses)

LDR diagnostics have been developed


Operation of JLab FEL with high average current requires a compromise (in terms of match) between high peak beam brightness (required by FEL) and very low beam loss

The match is iterative process and often does not converge easily (if at all…)

For the transverse beam profile measurements and transverse match JLab FEL relies heavily on beam imaging (2D distribution) large number of beam viewers

LINAC beams have neither the time nor the mechanism to come to equilibrium(unlike storage rings, which also run high current)

When setting up a high current accelerators with tune-up beam, halo is something invisible (due to the dynamic range of the measurements) during the setup, yet causing a lot of difficulties when trying to run high current

Why large dynamic range diagnostics?

Increase the DR significantly to make the halo measurable visible with tune-up

beam already; measure the phase space distribution with the LDR and use such

information for the match. When DR is large enough no need to separate what is

core and what is halo.


Imaging Sensor(s) Dynamic Range

The first issue to overcome is the DR of a single imaging sensor

The main principle is to use imaging with 2 or 3 sensors with different effective gain simultaneously and to combine data in one LDR image digitally(single sensor dynamic range 500..1000 if cost is kept reasonable)

From experience (calculations tested by experiments) we know the safe level of beam current/power for a low duty cycle (tune-up) beam

With typical beam size of few hundred μm OTR signal is attenuated by ~ 10 to keep CCD from saturation. For phosphor or YAG:Ce viewers attenuation of at least 100 is used.

Using OTR there is enough intensityto measure 4 upper decades;lower two decades need gain ofabout 100 to be measured.

The key elements: image intensifiers alignment and linearity combining algorithm(s) understanding CCD saturation

Intensity range that canbe measured without additional gain

Intensity range whereadditional gain of ~ 100is needed.(not high for an imageintensifier)

To be measured withimaging sensor #1and attenuation ~ 10

To be measured withimaging sensor #2and gain ~ 200


Raw images and combining algorithm

Two images (on the left) measured simultaneously with integration times 20 us and 400 us

Background measurements and subtraction is crucial!Made separately for two sensors and subtracted on-line.

Combining algorithm is efficient enough to provide 5 Hz rep. rate for 1024x768 images

At the time of measurements was limited by the flexibility of DLPC

Demonstrated dynamic range of ~ 5E+4 (factor of 100 increase)

Integration time is used for normalization and overlap (sufficient)

Averaging also improves SNR and therefore DR (beam stability)

Data combining algorithm


linear & log; the “trouble” with the RMS

The two images show exactly the same data (beam profile - (x,y))but in linear and log scale

Next step is to use such measurements for beam characterization, emittance and Twiss parameters measurements (add x’ and y’)

Ultimately tomographic measurements are planned; but first just quad scan

For non-Gaussian beam RMS beam width is a tricky thing!It depends on how much of tails of the distribution function f(x)is taken in to account.


Quadrupole scan raw data

Level of interest(LOI)

more tails included

less tails included


Emittance and Twiss parameters

beta function(s) alpha function(s)

RMS emittance

more tails included

less tails included


Diffraction limit and PSF

Imaging measured distribution is a convolution of source distribution and so-called Point Spread Function (PSF)

PFS determined by optical system angular acceptance but also by the source angular distribution. Different beam viewers have different PSF.

Diffraction determines rather hard limits to the DR

Ways to mitigate: increase angular acceptance, use spatial filter, coronagraph-like optics


Objective Lens Pupil Apodization

First a Lyot’s coronagraph was considered to improve the PSF, but this would not allow for simultaneous measurements of the beam core and halo, but it is a good exercise

Domain of Fourier optic, always Fresnel approximation – numerical calculations required for most of the interesting cases – becomes demanding on CPU and memory quickly due to large apertures and optical wavelength (~ 0.5 um)

Implemented and used quasi-discrete Hankel transform for optics modeling (allows to do 1D calculations vs. 2D)

Fourier optics mage plane = Fourier transform of pupil function for a point source (this is the PSF)

Then it is easy to see that the uniform pupil function, i.e., the harp lens edgeis the problem (besides the uncertainty principal, which also adds to the problem)

Apodization – modification of the pupil function; First considered Gaussian amplitude apodization

optical field propagation by means of qDHT

(false colors – intensity in log scale)

r

z

Uniform pupil function

Gaussian pupil with s=r0/3


Objective Lens Pupil Apodization

Point Spread Functions

Convolutions: PFS and 2D Gaussian

First a Lyot’s coronagraph was considered to improve the PSF, but this would not allow for simultaneous measurements of the beam core and halo, but it is a good exercise

Domain of Fourier optic, always Fresnel approximation – numerical calculations required for most of the interesting cases – becomes demanding on CPU and memory quickly due to large apertures and optical wavelength (~ 0.5 um)

Implemented and used quasi-discrete Hankel transform for optics modeling (allows to do 1D calculations vs. 2D)

Fourier optics mage plane = Fourier transform of pupil function for a point source (this is the PSF)

Then it is easy to see that the uniform pupil function, i.e., the harp lens edgeis the problem (besides the uncertainty principal, which also adds to the problem)

Apodization – modification of the pupil function; First considered Gaussian amplitude apodization


Wire scanner measurements

wire with diameter much smaller than the beam size interacts with beam as it is scanned across it

there is a number of interaction mechanisms:

beam capture secondary emission scattering

different ways to detect the signal

induced current secondary particles (counting)

Only 1D projections (no 2D distribution)

Takes time (“patience limited”)

LINAC beams are non-equilibrium(non Gaussian)

A. Freyberger, in DIPAC05 proceedings,Measurements made at CEBAF

at JLab FEL


Wire-scanner viewer combination

Designed to allow measurements of OTR photons, not Bremsstrahlung X-rays, gammas

low Z wires for OTR; still can use high Z wires if Bremsstrahlung is better

Viewer can be OTR or YAG, or other scintillator (normal to the beam)

Mirrors must be very good in terms of scratch and dig spec. – minimize scattering

Must have impedance shields (at JLab FEL)

Two diagnostics at one location


Wire-scanner electronics etc.

One key problem is the required measurements timeshould keep as short as possible to make the diagnostics practical

Maximum useful counting rate is the important parameter

The quest - counting system with the max rate of 100 MHz

PMTs themselves are fast enough for this

PMT base (HV divider) is tricky though, high rate high current loading of HV power supply changes HV changes gain (passive divider are not good for high rate)

There are a few different options for the PMT base, we plat to use so-called transistorized bases (high beta, high voltage transistors)

Preamplifiers, discriminators, logic etc. – commercially available

Need time resolution !!! Options are: gated counting, Time Correlated Single Photon Counting (TCSPC)

Counter, Multi-channel counters and TCSPC: FPGA implementation


LDR measurements state

we have demonstrated beam imaging with DR increased by ~ 100

applied the LDR imaging to beam characterization and have shown that for LINAC non-Gaussian beam the DR has strong impact on the measurements results

have modeled optics required to improve the DR range to reach 106

new diagnostic station for LDR imaging and cross-check with wire scanner was designed and built

next1 - practical implementation of the apodization optics (manufacturing, error sensitivity study, optimization)

next2 - beam measurements with new diagnostics (tomographic phase space measurements based on LDR imaging)


CEBAF: overview

Ebeam was 6 GeV is being upgrade to 12 GeV Bunch charge: 0.2 pCRepetition rate: 499 MHz (x3)Three independent beams (3 Halls)

1. Beam halo hitting beam pipe would create background in the NP detectors 2. FE in LINAC cavities affects the trip rate, which reduces up time and must be limited


CEBAF: trip rate, statistics

JLAB-TN-05-57 J. Benesch,Field Emission in CEBAF's Superconducting RF Cavities and Implications for Future Accelerators

JLAB-TN-10-008 J. Benesch,Comparison of arc models from March 2003/Nov 2004 and December 2009

JLAB-TN-12-049 J. Benesch, A. Freyberger,CEBAF Energy Reach and Gradient Maintenance Needs

Uses “accounting” and statistical analysis of the trip rate and its dependence on the cavities gradient

For 12 GeV CEBAF; 400 cavities + each cavity trips 1/(2 days) would result in on average 8 RF trips per hour

Original C25 design / unfortunate feature / RF window has a direct line of sight to the beam – charges up / eventually break down

With time performance of cavities degrades i.e. at the same gradient trip rate goes upexact mechanism is not known (speculated that # of FEs goes up)

Conclusion – gradient maintenance is needed (reprocessing cavities and refurbishing the cryo modules)


CEBAF: trip rate

Distribution of gradients in C25 cavities that resulted in 1/(2 days) trip par cavity

Initial distribution of gradients of C50 cavities

Distribution of gradients of the same C50 cavities at the end of 6 GeV operation (~ 4 years later)

Courtesy of J. Benesch


CEBAF: no Halo ?

One ways to make large dynamic range measurement is to arrange it to be frequency measurement

Then make it work for 1 Hz and for 100 MHz and this is 108 dynamic range. For instance use PMT and keep them working in counting mode

Courtesy of A. Freyberger


CEBAF: vacuum

Despite the idea/claim that CEBAF beam is quite Gaussian and has no or very little large amplitude non Gaussian tails,there are vacuum “events”

Two types of events:

1. Burn through that require a new piece of beam pipe to be fabricated as it has a hole drilled into it.2. Low current, very low intensity lose (chronic lose) that heats up a flange. This requires Rad. Con. to identify the hot spot, and then the flange is tighten up and the region recovers quickly.

Frequency of such events is 1-2 per year (35 weeks of operation)

Type2 is due to some kind of beam that is not seen (not looked for)

Type1 (some of them) related to rapid energy change due to RF changes

Fortunately it did not happen close to the SRF LINAC


Conclusion / Summary

JLab FEL (IR/UV Upgrade)

RF gradients in LINAC always require attention, set radiation background level (F.E.)

Drive Laser transport if made very carefully, seems to be not a problem

Drive Laser rep. rate control (EO cells) always need attention (extinction ration drifts)

Cathode suffers when conditioning and from breakdowns, still makes beam as needed, but scatters DL light – generates some halo

Non-linear beam dynamics is responsible for some fraction of the halo. When setting up for high current operation, a lot of effort and time goes in to “fitting” the halo through the recirculator, such that peak beam brightness does not suffer.

Radiation monitors, BLMs and vacuum are used as tuning diagnostics

CEBAF

NP detectors (background) require essentially no beam halo

Large statistics of cavity performance and its evolution (F.E.)

Direct effects of F.E. – RF trip rate, reduction of max. possible energy

Vacuum events related to beam loss (both high and very low current)


Backup slides


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

downstream ofthe gun



upstream of thebuncher cavity



downstream of thebuncher cavity



upstream of theSRF cavity 1



downstream of theSRF cavity 1



downstream of theSRF cavity 2


Beam Loss Monitors The primary BLM at the JLab is a 931B Hamamatsu photo-multiplier tube,

operated with a fixed integrator and individually variable HV power supply

The BLM electronics are 12 channel VME boards. PMTs are used in current (analog) mode There is a single FSD fiber output to the MPS for each VME board All 12 channels have analog monitors that are connected

to the Analog Monitoring System (AMS) These are used as tune-up diagnostics in the control room

Calibration procedure machine is locked into 1 uA CW operation beam is driven into chamber and detector gain is varied by changing HV the HV is adjusted until the system trips this new “gain” setting is saved in EPICS and accounts for aging of tube.

Ch3: Fault Trigger H = No Fault L = FSD Fault

CH1: Drive Laser Intensity

Courtesy of K. Jordan, D. Sexton

UBW2012, Berlin, December 2012 Jefferson Lab Experience with Beam Halo, Beam Loss and Related beam...

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