Instrumentation needs for ERLs (selected topics)

1. Instrumentation needs for ERLs(selected topics) Pavel Evtushenko, JLab • 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

2. 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

3. Injector / Transverse Phase Space The multislit or a single slit scanning through the beam (or a beam scanning across the 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)

4. 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.

5. 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

6. 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 1000 Courtesy of S. Zhang Drive Laser transverse profile dynamic range ~500: 10-bit frame grabber, 60 dB SNR CCD

7. 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. QE scanner data Laser profile on cathode Photoemission profile

8. 2-pass BPM (ideas) • 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 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

9. 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 thin • 2. Diamond-like carbon foils ~ 1 micron thin

10. Large dynamic range beam profile measurements Measured in JLab FEL injector, local intensity difference of the core and halo is about 300. (500 would measure as well) 10-bit frame grabber & a CCD with 57 dB dynamic range PARMELA simulations of the same setup with 3e5 particles: X and Y phase spaces, beam profile and its projection show the halo around the core of about 3e-3. Even in idealized system (simulation) beam dynamics can lead to formation of halo.

11. 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)

12. 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

13. 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?

14. 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 (???)

15. 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 • Problem: • this does not work for CW beam current measurements with a linear circuit current measurements with A Log-amp circuit (60 dB)

16. Bright and small beams / COTR mitigation longitudinal BFF TR spectral power density of a bunch TR spectral power density of one electron transverse BFF BFF of a transversally Gaussian beam, when beam size σ>γλ/2π Q: How well will it work for planned ERLs? σ=200 micron Eb=250 MeV (LCLS) Courtesy of R. Fiorito (see PAC09 proceedings)

18. Fig. 1 Fig. 3 Fig. 4 Fig. 5

19. Pulsed and CW beam measurements / transition • 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 interferogram Power spectrum

20. 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

21. 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

22. 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

23. The END (not really)

24. 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 51010 e-/train; rep. rate of the bunch trains 1.5 Hz for 5 minutes OTR radiator (initially) optically polished 500 m Be With ~ 10 time less charge per train for 30 min no degradation Suggested explanation – radiator deformation beyond elastic limit 51010e– 10077 pC bunches

25. unpolarized vertically polarized horizontally polarized Optical Diffraction Radiation amplitude of a Fourier component of transversal Coulomb field of an electron - transverse beam distribution intensity of the ODR from the beam Is 2D convolution of the fb and Er2 Example assuming 4.597 GeV; x=215 m; y=110 m; =550 nm; h=1.1 mm

26. 5A tune beam; OTR

27. 10A CW beam; ODR V. polarized

28. 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  K1 – modified Bessel function