Demonstration of EnergyChirp Control in Relativistic Electron Bunches

Demonstration of Energy-Chirp Control in Relativistic Electron Bunches at LCLS Using a Corrugated Structure Karl Bane, 28 March 2016, HBB 2016 Workshop, Havana, Cuba

Introduction At the end of acceleration in an X-ray FEL, the beam may be left with an longitudinal position/energy correlation. The metallic beam pipe with small corrugations—a “dechirper”--was proposed as a passive device to “dechirp” the beam The Radia. Beam/LCLS dechirper was recently installed in the LCLS, to give added flexibility to operations. Device commissioning was performed over the period Oct 2015—Feb 2016 I will present (i) basic wakefield measurements—energy loss, induced chirp, transverse kick, …—, compare to calculations, and (ii) measurements of their effect on the lasing process These are the first measurements of a dechirper at high energies (multi-Ge. V), short bunch lengths (10’s of um’s), and in a functioning FEL 2

Outline Description of Radia. Beam/LCLS dechirper Brief review of wakefield theory of corrugated structures Basic wakefield measurements—average energy loss, induced chirp, transverse kick Dechirper/FEL interaction Conclusions Will not discuss transverse kick for bunch length measurements—A. Novokhatski et al See also C. Emma’s talk for using the dechirper in two color, self-seeding, etc. applications Selected references on wake theory of corrugated dechirpers: K. Bane and G. Stupakov, Nucl Inst Meth A 690, 106 (2012) A. Novokhatski, Phys. Rev. ST Accel. Beams 18, 104402 (2015) K. Bane, G. Stupakov, Nucl Inst Meth A 820, 156 (2016) K. Bane, G. Stupakov, I. Zagorodnov, “Analytical formulas of short bunch wakes in a flat dechirper, ” SLAC-PUB-16497, March 2016 3

Contributors to Dechirper Commissioning Success FEL Physics Radiabeam Systems METS Controls T. Maxwell M. Ruelas A. Cedillos A. Babbitt M. Harrison M. A. Carrasco Z. Oven J. Mc. Nevin G. Gassner M. Petree Z. Huang D. Martin E. Reese J. Bong K. Bane A. Murokh K. Caban M. D’Ewart G. Stupakov P. Frigola S. Jansson S. Hoobler P. Emma T. Montagne S. Alverson J. Frisch J. Garcia L. Piccoli P. Krejcik M. Guetg J. Zemella H. Loos A. Lutman A. Fisher A. Novokhatski R. Atkinson We especially thank Radia. Beam for building the dechirper and collaborating on the commissioning R. Iverson I want to thank T. Maxwell for helping me with this talk, and others who contributed slides (will be noted at bottom right of slides) Work supported in part by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC 02 -76 SF 00515 4

Radia. Beam/LCLS Dechirper Installed in the LCLS 25 um precision over 2 m (P. Krejcik) 5

6 Vertical Dechirper Module - Actuation Y Z BEAM Page 6 (A. Cedillos)

6 Vertical Dechirper Module - Actuation Y Z BEAM Page 7 (A. Cedillos)

6 Vertical Dechirper Module – Insertion/Retraction Y Z BEAM • • Carrier position repeatability 25 um Carrier linear speed 2. 5 mm/s Gear reducer Large safety factor for motor Page 8 *Will have a manual method to retract the jaws along with E-stop. (A. Cedillos)

6 Vertical Dechirper Module – Trim Actuation Y Z BEAM Approximate pivot points • • • Carrier tip trim adjustment range • +/- 1 mm Gear reducer Large safety factor for motor Page 9 LTU Stand (A. Cedillos)

(P. Krejcik) 10

(P. Krejcik) 11

Dechirper at LCLS Head Note: on dump screen x is proportional to time and y to energy Simulation Head by Z. Zhang (R. Iverson) 12

Theory of Corrugated Plate Dechirper Flat geometry was chosen to allow for adjustability of strength When h/p~> 1, and in perturbative regime (h, p<< a), wake given by a single mode with (ah/2)1/2/2. For short bunch, point charge wake w(s) w(0+)= Z 0 c/(16 a 2) Three periods of a vertical dechirper For short, uniform bunch with current I, induced voltage Vw(s) w(0+)ILs/c (L is structure length), a linear chirp, the maximum chirp for aperture a For beam offset by y, transverse wake wy(s) wy’(0+)s= 3 Z 0 cs/(64 a 4) For short, uniform bunch induced voltage Vwy(s) wy’(0+)y. ILs 2/2 c, a quadratic variation along bunch; for small a => strong kick to bunch tail Even for beam on axis, quad wake is excited, with wyq(s) wyq’(0+)s a 4 s 13

Radia. Beam/LCLS Dechirper The dechirper unit consists of 2 m of a vertical dechirper followed by 2 m of a horizontal one This configuration was chosen to partially cancel the unavoidable quad wake mismatch at the tail of the bunch h, p not << a => not in perturbation regime. Wakes have a droop, and dechirp in a uniform bunch is not quite as strong, not completely linear Note: a dechirper based on dielectric-lined, metallic plates will behave similarly Three periods of the vertical dechirper 14

Single X-band deflector measurement: @ 4. 4 Ge. V / 180 p. C / 1 k. A Data from dump screen analytical measured head tail Clear, ~linear additional chirp observed Using both dechirpers (L= 4 m) Obtained from data from dump screen 15 (T. Maxwell)

Measurements @ 4. 4 Ge. V / 180 p. C / 1 k. A for uniform bunch, average loss As gap becomes smaller here EFWHM increases again E loss Projected E spread Chirp 16 (T. Maxwell)

Average Eloss vs. Bunch Offset g= 2 a= 2. 2 mm I= 1 k. A Q= 190 p. C E= 4. 425 Ge. V Single dechiper L= 2 m Measured using BPMs in dispersive region, averaged over many shots; dashes show analytical function To obtain this agreement, a slight adjustment, g -> 2. 1 mm, was made 17

Transverse Kick vs Bunch Offset from Axis For uniform bunch: near axis 2 a= 2. 0 mm away from axis I- current g- gap ell- bunch length L- structure length y- bunch offset 2 a= 3. 1 mm (J. Zemello) 18

Translates directly to measured X-ray spectra From SXR spectrometer @ 870 e. V Near nominal setting (g= 1. 4 mm) does not 19 degrade FEL performance (T. Maxwell)

Adding Chirped Hard X-ray Bandwidth Just as effective at high energy: Observe center downshift / BW increase on FEE HXRS Can increase chirp for over-compressed bunch—desirable for some experiments (T. Maxwell) 20

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First evidence of lasing suppression with the dechirper 10 October 2015 x [mm] y [mm] x [mm] Time [uncalibrated] No kick Bunch head y [mm] Bunch head Moderate transverse kick x [mm] Time [uncalibrated] Large kick v Larger kick was given by closing more the gap (instead of changing the structure offset), evidently the beam was travelling slightly off-axis from the structure v Trajectory feedbacks keep the center of mass of the electron beam on the straight trajectory v Larger kicks yield a shorter lasing slice (A. Lutman) 22

Fresh-slice double-pulses: Two color scheme, with color separation and tail lasing first Dechirper Configuration Gap Offset Vertical 3. 5 mm 0. 8 mm Horizontal OUT / Undulator Configuration Status K value 1 -8 IN K~3. 455, Strong Saturation taper from Und #6 10 -25 OUT / 26 -33 IN K~3. 505 Variable Taper (Regular/Reverse) Tail Lasing Head Lasing (A. Lutman)

(A. Lutman) Fresh-slice double-pulses: Two color scheme, with color separation and tail lasing first 1. 03 m. J No lasing FEL Statistics: 746 +/- 125 m. J Regular Taper Delta in circular polarization Tail Lasing first color: ~250 m. J second color: ~ 500 m. J Head Lasing Single-Shot orbit first color: ~8 fs second color: ~ 15 fs

Conclusions Large-scale dechirper system has been realized for high-energy (Ge. V) short bunch (10 s of um) bandwidth control at the LCLS It is a precision instrument fully integrated into the LCLS. The vanes are straight and settings are reproducible to 25 um over 2 m Wake measurements—energy loss, chirp, transverse kick—agree well with theory Still working on other measurements, e. g. emittance vs gap The fast kicker capability of the dechirper is being studied for twocolor and self-seeding applications; already delivering improved two -color radiation to users Vital path for bandwidth control at present and planned X-ray free electron laser facilities (e. g. LCLS-II, PAL) 25

Some of the Contributors 26