Applications of transverse deflecting cavities in x-ray free-electron lasers Yuantao Ding SLAC National Accelerator Laboratory7/18/2012

Outline Background of XFEL TCAV: a diagnostic tool in XFEL facilities :  e-beam bunch length  slice emittance  slice energy spread and longitudinal phase space  e-beam timing  x-ray pulse temporal profile Other applications:  e-beam energy spread control  emittance exchange (skip) Page 2

XFELs: the 4 th generation light source X-ray FELs: probe of the ultra-small and ulstra-fast worlds; XFELs in operation and to be online: FLASH, LCLS, SACLA, Euro-XFEL, Swiss- FEL, PAL-XFEL… Page 3 H.-D. Nuhn, H. Winick

X-ray FEL requires extremely bright e  beam Power grows exponentially with undulator distance, z Power grows exponentially with undulator distance, z …but only if time-sliced energy spread <<10  3 and the transverse emittance is ~ 1 /4  FEL power reaches saturation at ~ 18L G SASE performance depends exponentially on e  beam quality (emittance & peak current!) local peak current …power gain length: time-sliced emittance Z. Huang Page 4

Slippage and ‘Slice’ Emittance For an FEL… e  slips back in phase w.r.t. photons by r per period, u + +   + +   + +     + +   + +     z 1  1  eeee eeeex-raysx-rays ~2 µm slippage length, l s zzzz Slippage length: l s = 1 ×  undulator periods At 1 Å: l s ≈ 1 fs << 100-fs typical pulse length Each part of x-ray pulse is amplified by those electrons within a slippage length (an FEL slice  beam quality in slice)  Motivation to measure beam sliced parameters, how ? Page 5 P. Emma

TCAV: an RF “streak” camera for e-beam* Page 6 eeee zzzz  y ~  z RF ‘streak’ V0(t)V0(t)V0(t)V0(t) S-band (2856 MHz) transverse RF deflector beam screen single-shot, absolute bunch length measurement Higher frequency and voltage, lower energy and emittance, larger beta function at deflector are preferred. Calibration is done by varying rf phase (near zero-crossing) and recording the beam centroids. *: Emma, Frisch, Krejcik, LCLS-TN-00-12, 2000.

Measure beam temporal profile with TCAV  S-band ( slide from D. Xiang IPAC12 )  C-band  X-band UCLA NLCTA, SLAC SACLA LCLS Tsinghua UniversityFLASH  L-band Cornell University SPARC PSI

Bunch length measurements at LCLS (0.25 nC) BC2 (4.3 GeV) BSY (14 GeV) TCAV3 (5.0 GeV) 550 m  z > 25  m  z < 5  m Stronger BC2 L2 4 wires Bane et al, PRSTAB time time TCAV3: S-band, 8-foot long, ~20MV. Page 8

Slice emittance measurements at LCLS deflector OFF deflector ON time ( ~ 3 ps) ~0.05 mm ~1 mm time-sliced emittance 20pC* *: Ding et al., PRL Central slice

YAGS2 Laser: 40 µ J σ E  45 keV YAGS2 Laser OFF σ E < 12 keV RF deflector ON time energy TCAV for e-beam long. phase space measurements Page 10  TCAV streaks beam vertically  time;  Spectrometer bends beam horizontally  energy. Measure time-sliced energy spread and check laser-heater timing. LCLS injector Z. Huang, PRSATAB 2010

Page 11 (slide from C. Behrens)

Now measure BPM jitter both with transverse RF OFF, and then ON (at constant phase)  t  ±0.6 ps slope =  2.34 mm/deg 9  m rms 110  m rms TCAV ON TCAV OFF BPM Y Position (mm) Measuring Bunch Arrival Time Jitter (LCLS) eeee V(t)V(t)V(t)V(t) S-band (2856 MHz) BPM Timing Jitter (w.r.t. RF) = (110  m)/(2.34 mm/deg) = deg  46 fsec rms ( slide from P. Emma )

Improve time resolution for TCAV ? LCLS TCAV3 runs out of resolutions at about ~3 m (10 fs) bunch length level. LCLS TCAV3 runs out of resolutions at about ~3  m (10 fs) bunch length level. A longitudinal mapping method has been used in this study, with a resolution about 1 fs rms. (Huang et al, PRSTAB 2010, PAC11.) A longitudinal mapping method has been used in this study, with a resolution about 1 fs rms. (Huang et al, PRSTAB 2010, PAC11.)  To overcome emittance effect, we proposed to do exact mapping (to the first order) with a chicane and a deflector. (Xiang and Ding, PRSTAB 2010.)  Another natural way, to go to higher frequency.  from S-band to X-band  we are installing two 1-m X-band deflecting structures at LCLS for both e-beam and x-ray pulse temporal diagnostics, to be online in Page 13

The principle to retrieve x-ray temporal profile The E-loss scan for measuring x-ray pulse energy: Horizontal BPMs Dump BPMs  undulator ~100 meters  initial  E i final  E f  E =  E f   E i vary FEL power with oscillations & record e  energy loss time-resolved We propose to streak the beam in time to measure the time-resolved energy loss, or energy spread, where the x-ray profile has been imprinted in the e-beam time-energy phase space. 10MeV/e- Page 14

XTCAV + e-beam energy spectrometer at LCLS Page 15 X-band TCAV (XTCAV): GHz. High resolution, ~ few fs; Applicable in all FEL wavelength; Beam profiles, single shot; No interruption with operation; Both e-beam and x-ray profiles.  Maximum deflecting voltage: 48 MV;  Required input power: 40MW. project started in summer 2011 commissioning in spring 2013 Ding et al., PRSTAB 2011.

LCLS XTCAV simulations: fs e - and x-ray diagnostics FEL ON FEL OFF Page 16 Ding et al., PRSTAB 2011.

Just that simple to make streaking?  have to follow Panofsky-Wentzel theorem: Page 17  This introduces additional (correlated) energy spread and energy chirp while streaking.

Page 18 TCAV “heater” for energy spread control In the Echo-enabled harmonic generation (EEHG) free electron lasers experiment at SLAC, an X-band TCAV has been used to increase the beam energy spread, to verify EEHG still works. D. Xiang et al., PRL 2012.

Page 19 (Slide from C. Behrens)

Other challenges Jitter issues small rf phase jitter is expected during calibration and measurement Initial correlations some initial correlation can be corrected by both measuring at 0deg and 180deg. Coherent OTR problem this is a headache. Using YAG type screens, at dispersive location. Page 20

Summary The key point is to make a time-resolved streaking, then you get the bunch length, slice parameters…. Also can cause energy spread growth, but it is also possible to use it. Some other applications…. Page 21 Thanks C. Behrens, P. Emma, Z. Huang and D. Xiang for providing slides. Thanks the workshop program committee for the invitation. Thanks for your attention!