Instrumentation needs for ERLs(selected topics)

Light source applications of ERLs under consideration

High power long wavelength FEL oscillators High energy hard X-ray sources (like Cornell ERL) Soft X-ray source (like JLamp)

In this talk (some of the important issues)

Injector diagnostics: e- beam, drive laser, cathode 2-beam problems: BPMs, Viewers Large dynamic range measurements Problems with OTR Outlook/Discussion topics

Pavel Evtushenko, JLab

Gun / Injector diagnostics

There is a good overlap between the needs of different ERLs.Such for any ERL we need to:

know the transverse phase space distribution

know the longitudinal phase space distribution

be able to measure and control halo

make sure that the beam parameters measured with pulsed beam do not change when going to CW beams and changing the beam average current

know phase of the beam in RF cavities (1. setup 2. monitor)

have drive laser diagnostics

cathode (Q.E.) diagnostics

Injector / Transverse Phase SpaceThe multislit or a single slit scanning through the beam (or a beam scanning acrossthe slit) does the job very well (pulsed beam only).

well established technique works for space charge dominated beam beam profile is measured with YAG, phosphor or ceramic viewer measures not only the emittance but the Twiss parameters as well enough information to reconstruct the phase space has been implemented as on-line diagnostics works with diagnostics mode only (low duty cycle, average current)

Injector / Longitudinal Phase Space Single cell cavity or multi-cell structure with TM dipole mode impose on the bunch time dependent transverse kick.

The dipole creates dispersion in the transverse direction perpendicular to the cavity kick

Problems:

the same as multislit – pulsed beam only

resolution limited by transverse emittance; solution – put a small hole in front of the cavity and do the measurements for small beamlet AND as f(x,y) - 3D charge distribution

usually a special setup placed at the beam line for injector study and removed when it is done.

Injector / Drive Laser Diagnostics Transversal profile of the D.L. on the cathode (standard technique) imaging reflection from the input window to a CCD placed on the same distance from the window as the cathode

Longitudinal profile: auto-correlation works well for Gaussian pulses for non Gaussian pulses streak camera (dynamic range ~103)

Amplitude stability: photo diode (at JLab FEL is monitored all the time in control room)

Transversal stability: long term slow drifts or misalignment – the same CCD as for profile meas. fast transverse jitter 4-quadrant position sensitive photodiode

Phase stability (phase noise) all the way down to DC fast (few GHz) photodiode to look at FM at very high harmonic number BUT also must do measurements at DC to separate AM from FM + Signal Source Analyzer (SSA) expansive but saves a lots of time

Injector / Drive Laser Diagnostics

Typical auto-correlator signal JLab FEL D.L.(observed in the control room permanently)now the dynamic range can be few 1000Courtesy of S. Zhang

Drive Laser transverse profiledynamic range ~500:10-bit frame grabber, 60 dB SNR CCD

Injector / Cathode Q.E. For accurate modeling of the injector beam dynamics the input in to a code must be accurate. This includes the emission transversal profile, which is a product of the DL transverse distribution and QE transverse distribution

One technique to measure the Q.E. profile is scanner with a laser (spot on the cathode is much smaller than cathode dimensions) and two scanning mirrors. Usually automated and gives 2D distribution of the Q.E. Can be used only with beam and gun voltage off.

Another technique: run low current emittance dominated beam, measure the laser trans. profile and image the cathode to a view screen. The ration of the beam profile and the laser profile gives Q.E. profile. If used with the real drive laser (not always possible) gives the emission profile.

Laser profile on cathode Photoemission profile

QE scanner data

2-pass BPM (ideas)

Stripline BPM signal measured with scope, the picture limited by the scope BW

V. Lebedev proposed at ERL05 to make a system based on small AM of the beam current: switch the mod. frequency every turn, measure sidebands Time domain approach might be very different for different machines ✔ long recirculation time vs. short; ✔ every bucket filled vs. not The phase difference is not always 180 deg, especially when tuning machine In JLab FEL the ΔT between two beam depends on rep. rate.; smallest is 6 ns; Time domain approach: (will not measure every bunch)

1. make BPM pulses broader to 1.5 ns (LPF or dispersion)2. grab peak value with S&H (GHz BW)3. digitize S&H output with

Motivation: To do differential orbit measurements (measure transport matrix) with both beams in the LINAC

The decelerating beam gets adiabatically anti-dumped – small errors corrections in the beginning leads to big orbit change at the end

Orbit stabilization and feedback

2-pass viewers there are two beams in the LINAC when trying to measure decelerating beam with a viewer the accelerating beam gets also intercepted ultimately the measurements should be done with non intercepting technique (or intercepting but negligibly ) JLab FEL uses OTR viewers with 5 mm hole (first beam goes in to the hole) SRF cavities see the radiation due to the intercepted beam (can trip cavity)

JLab FEL LINAC OTR viewer

With the ultra bright beam OTR might be useless (OTR COTR like @ LCLS) The best solution might be to use wire scanners (intercepts small part of the beam). If the scanner measures radiation created by the wire, must take care of the background. Laser wire scanner:

Will the measurements time and cost be acceptable?What is the lowest energy when the technique is practical?

JLab FEL will test with beam:1. Electroformed mesh: 44% transparent ~5 micron thin2. Diamond-like carbon foils ~ 1 micron thin

Large dynamic range beam profile measurements

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) beam dynamics canlead to formation of halo.

Large dynamic range measurements (example)

Main principal of one of the ways to make large dynamic range measurements is to reduce a measurement to frequency measurements.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. Can be applied to e- beam measurements, laser, (light), X-rays. Example: wire scanner measurements:

Courtesy of A. Freyberger (measured at CEBAF)

LINAC beams are different from ring beams

JLab FEL transversal beam profile:

• Obtained in a specially setup measurements to show how much beam is non Gaussian• It in not how we have it during standard operation• There is no Halo shown in this measurements in sense that all of it participates in FEL interaction• The techniques we can borrow from rings assume Gaussian beam and therefore are concentrating on beam size (RMS) measurements

Drive Laser large dynamic range measurements

typically in simulation the drive laser pulse is assumed “ideal” i.e. as we want it

to find out how far is it true and if concerned about details of the phase distribution at 1e-6 level, there is the need to make D.L. pulse measurements with 1e6 dynamic range

transverse and longitudinal profile of the D.L. needs to be measured

measure and control reflections/scattering from D.L. transport; these might be far from D.L. spot on the cathode both transversally and longitudinally

for transversal profile measurements:➟ could use 120 dB dynamic range CCD (2 CCD in one camera each 60 dB)➟ use a usual ~ 60 dB CCD with an ND filter and accurate cross-calibration

for longitudinal profile measurements:➟ auto-correlator with dynamic range 120 dB can be built using PMT➟ auto-correlator works fine for Gaussian pulses➟ WHAT to do for non Gaussian? Will cross-correlation work with 120 dB?

Drive Laser “ghost” pulses

For machine tune up, beam studies, intercepting diagnostics a “diagnostic beam” with very low average current but nominal bunch charge is used (all beam can be lost without damaging machine)

For example, JLab FEL: max rep. rate 74.85 MHz (CW)

diagnostic mode: rep. rate 4.678125 MHz (÷16), 250 μs / 2 Hz (÷2000)average current ~300 nA

Most of the laser pulses are “stopped” by EO cell(s), but the extinction ratio of the an EO cell is about 200 (typical), two in series ~ 4×104

Another example: want to reduce 1300 MHz (100 mA) to 300 nA (218) than for every bunch Qb we want we also get 6.55×Qb of “ghost” pulses (655 % !!!) we do not want

“ghost” pulses overall intensity must be kept much lower than real pulses!!! for “usual” measurements ~ 1% might be fine much bigger problem if want to study halo, let’s say 10-6 effects, than “ghost” pulses should be kept at 10-8 (???)

Drive Laser “ghost” pulses

Using a Log-amp is an easy way to diagnose presence of the “ghost” pulses

Log-amps with dynamic range 100 dB are available

current measurements witha linear circuit

current measurements withA Log-amp circuit (60 dB)

Problem: this does not work for CW beam

Bright and small beams / COTR mitigation

TR spectral powerdensity of a bunch

TR spectral powerdensity of one electron

longitudinal BFFtransverse BFF

σ=200 micronEb=250 MeV(LCLS)

BFF of a transversally Gaussian beam,when beam size σ>γλ/2π

Courtesy of R. Fiorito(see PAC09 proceedings)

Q: How well will it work for planned ERLs?

Fig. 1

Fig. 3Fig. 4

Fig. 5

Pulsed and CW beam measurements / transition

interferogram

Power spectrum

Most affordable way to measure ps and sub-ps bunches

Works with pulsed beam (tune up) and CW beam, essentially at any average current

Used at JLab FEL to ensure that the bunch length does not change for pulsed or CW and when the average current is increased

Ultimately needs to be setup in vacuum (or N2 purge) due to atmosphere absorption of THz

Phase information is lost – no direct bunch profile reconstruction

A detector measuring total CTR (CSR) power – a bunch length monitor

ERL Instrumentation Discussions

• Diagnostics of high current CW beams especially at low energies in the injector region is an area where we have least of experience:

- mostly beam profile measurements- BPMs for beam position work- bunch length (longitudinal profile)

• At high energy outside of the LINAC can used SR the same way at the presently operated high energy rings

- the BIG difference is that LINAC beams are not Gaussians, since they are not in equilibrium- pin hole cameras- zone optics systems- two slit interferometer with visible SR

• Can not do that in the LINAC- Can Optical Diffraction Radiation (ODR) be used (impedance) ?- Laser wire (measurements time) ?

• Make use of the fact that the beam is CW – very small modulation + lock-in amp

Conclusion and Outlook

For an injector transverse and longitudinal phase spaces can be measured very well with pulsed (diagnostics) beams.

One direction to improve that is to increase the dynamic range of such measurements to (106 – 108 ?)

In the injector area: how to deal with CW beam and transition from pulsed to CW beam? What exactly will we need to deal with?

- drifts in RF phase and amplitude – for sure!!! - beam loading ? Do we need beam based measurements for that?- wake fields?

measurements of D.L. with big dynamic range to understand beam dynamics with the same big dynamic range

LINAC (part of the machine with 2 beams) needs specific solutions

reducing the CW beam to diagnostic beam needs careful consideration this (via “ghost” pulses) affects all diagnostics

Conclusion and Outlook (2)

OTR (the working horse) will not be as easy to use as it used to be need to mitigate COTR (experimental demonstration)

At the present light sources there is a lot of experience with SR diagnostics for non distracting beam measurements. A lot of that can be applied to ERLs with emittance ~ 10 times better.

Beam loss, fast shutdown systems

More topics for discussions

will the COTR mitigation work for ERL parameters? for light sources: beam stability, orbit feedbacks – extend experience of 3rd

generation light sources and large scale LINACS CW beam monitoring Optical Diffraction Radiation (ODR) applicability Transfer function (transverse and longitudinal) measurements and monitoring

The END(not really)

Again from SLAC-PUB-9280, courtesy of M. Ross

“Foil” deformation – experiment

OTR image of a beam ~ 10 m 10 m before (left) after (right)the OTR radiator was exposed to 51010 e-/train;rep. rate of the bunch trains 1.5 Hz for 5 minutes OTR radiator (initially) optically polished 500 m BeWith ~ 10 time less charge per train for 30 min no degradation

Suggested explanation – radiator deformation beyond elastic limit 51010 e– 10077 pC bunches

Optical Diffraction Radiation

horizontally polarized

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amplitude of a Fourier componentof transversal Coulomb field of anelectron

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intensity of the ODR from the beamIs 2D convolution of the fb and Er

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Example assuming 4.597 GeV; x=215 m; y=110 m; =550 nm; h=1.1 mm

unpolarized

vertically polarized

5A tune beam; OTR

10A CW beam; ODR V. polarized

It sounds simple, but it was debated till ~ 1996

The reason for the debate was: transversal size of a Fourier component of a transversal component of Coulomb field of relativistic charged particle is:

OTR resolution is diffraction limited

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~ γλ /2π

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