August, 2003 W.S. Graves 1 Electron beam diagnostic methods William S. Graves MIT-Bates Laboratory Presented at 2003 ICFA S2E Workshop DESY-Zeuthen August, 2003

August, 2003 W.S. Graves 2 Experimental Methods Hardware/software controls. Thermal emittance measurement using solenoid scan. Cross-correlation of UV photoinjector drive laser with 100 fs IR oscillator. Streak camera time resolution Electron beam longitudinal distribution measured using RF zero phasing method. Slice transverse parameters are measured by combining RF zero phase with quadrupole scan. Will not address undulator diagnostics. See Shaftan, Loos, Doyuran. Enables injector optimization and code benchmarking.

August, 2003 W.S. Graves 3 DUVFEL Facility at BNL 1.6 cell gun with copper cathode 70 MeV Bend 50 m 5 MeV Bend Dump Coherent IR diagnostics RF zero phase screen UndulatorsLinac tanks Bunch compressor with post accel. 30 mJ, 100 fs Ti:Sapphire laser NISUS 10m undulator Photoinjector: 1.6 cell BNL/SLAC/UCLA with copper cathode 4 SLAC s-band 3 m linac sections Bunch compressor between L2 and L3 Approximately 60 CCD cameras on YAG screens. Triplet MeV

August, 2003 W.S. Graves 4 Control system Automated measurements very important for gathering large amounts of data and repeating studies. All sophisticated control is done in the MATLAB environment on a PC. Physicists quickly integrate hardware control with data analysis. Low level control is EPICS on a SUN and VME crates.

August, 2003 W.S. Graves 5 Thermal Emittance (1) (2) (3)

August, 2003 W.S. Graves 6 Projected emittance vs charge and FWHM FWHM (ps) Emittance (mm-mrad) Charge (pC) Emittance (mm-mrad) Charge 2 pC Energy 3.7 MeV Laser spot 0.5 mm RMS FWHM 3 ps Energy 3.7 MeV Laser spot 0.5 mm RMS HOMDYN simulations estimate limits on maximum bunch length and charge. Choose working parameters of 2 pC, 2 ps FWHM. Simulation YAG:Ce screen very useful for low charge, high resolution profiles. Screen thickness, surface quality, multiple reflections, and camera lens depth-of-focus and resolution are all important issues.

August, 2003 W.S. Graves 7 Solenoid Scan Layout CCD Camera YAG screen Mirror Solenoid 1.6 cell photoinjector Dipole trim 1cm 65 cm 12 cm33 cm Can measure charge energy x and y centroid x and y beamsize px and py Telecentric lens magnif. = 1 YAG:Ce screen very useful for low charge, high resolution profiles. Screen thickness, surface quality, multiple reflections, and camera lens depth-of-focus and resolution are all important issues. See SLAC GTF data.

August, 2003 W.S. Graves 8 Low Energy Beam Measurements x Solenoid Current (Amp)  11 File: a10pc.txt  =  1.7  = 8.11  0.44 m  N =  mm-mrad  =  mm  ' = 2.94  mrad E = 3.69 MeV pop02a.bmp Hor. RMS width = 39.5 um Beam size (um) Intensity (A.U.) pixels Video processing 3x3 median filter applied. Dark current image subtracted. Pixels < few % of peak are zeroed. Error estimates Monte Carlo method using measured beam size jitter.

August, 2003 W.S. Graves 9 Emittance vs laser size Horizontal RMS laser size (mm)  N (mm-mrad) Horizontal  N (mm-mrad) Vertical Horizontal RMS laser size (mm) FWHM2.6 ps Charge2.0 pC Gradient85 MV/m RF phase30 degrees Emittance shows expected linear dependence on spot size. Small asymmetry is always present.

August, 2003 W.S. Graves 10 Beam size and divergence vs laser spot size Error bars are measured data. Blue lines are from HOMDYN simulation using RF fields from SUPERFISH model and measured solenoid B-field. Upper blue line has 1/2 cell field 10% higher than full cell. Lower blue line has 1/2 cell field 10% lower than full cell RMS laser hor. size (mm) RMS e-beam hor. size (mm) RMS laser hor. size (mm) RMS ebeam hor. divergence (mrad) High Low SizeDivergence

August, 2003 W.S. Graves 11 Error bars are measured data points. Curve is nonlinear least squares fit with β rf and Φ cu as parameters: β rf = / and Φ cu = / eV. The fit provides a second estimate of the electron kinetic energy E k = 0.40 eV, in close agreement with the estimate from the radial dependence of emittance. Emittance vs RF phase

August, 2003 W.S. Graves Time (ps) Phase matching angle (mrad) Signa l (V) Time profile of UV laser pulse Phase matching angle of harmonic generation crystals used to produce UV affects time and spatial modulations. Cross-correlaton difference frequency generation – experiment by B. Sheehy and H. Loos 100 fs IR 5 ps UV BBO crystal 200 fs blue Power meter Note: “Sub-ps” streak camera is inadequate for this measurement

August, 2003 W.S. Graves 13 Above: Laser cathode image of air force mask in laser room. Below: Resulting electron beam at pop 2. Above: Laser cathode image with mask removed showing smooth profile. Below: Resulting electron beam showing hot spot of emission. Laser masking of cathode image

August, 2003 W.S. Graves 14 Streak Camera Streak image Time Hammamatsu FESCA fs FWHM measured resolution Reflective input optics ( nm) Wide response cathode ( nm) Optical trigger (<500 fs jitter) Designed for synchroscan use. Also good single-shot resolution. 6 time ranges: 50 ps - 6 ns 50 ps window Very helpful for commissioning activies and for timing several optical signals. Limited time resolution below 1 ps.

August, 2003 W.S. Graves 15 Streak camera time profiles of laser pulses signal (arb units) streak delay (picoseconds) signal (arb units) streak delay (picoseconds) signal (arb units) streak delay (picoseconds) Oscillator796 nm FWHM765 fs Amplified IR 796 nm FWHM1.01 ps single shot UV266 nm FWHM2.40 ps singleshot Time resolution depends on photon energy: energetic UV photons create photoelectrons with energy spread that degrades time resolution

August, 2003 W.S. Graves 16 L1L2L3L4 RF zero-phase time profile L3 phase = +90, amp. set to remove chirp L4 phase = +/-90, amp. set to add known chirp L2 phase varies, amp. fixed L1 phase = 0, amp. fixed Chicane varies from 0 cm < R56 < 10.5 cm L4 phase = -90 degreesL4 phase = +90 degrees L3 corrects residual chirp, L4 is off Pop 14 YAG screen YAG images at pop 14

August, 2003 W.S. Graves 17 Quad scan during RF zero phase Movie of slice emittance measurement.

August, 2003 W.S. Graves 18 Right side Circle is matched, normalized phase space area at upstream location. Ellipse is phase space area of slice at same location. Straight lines are error bars of data points projected to same location. Left side Beam size squared vs quadrupole strength. Each plot is a different time slice of beam. Software is used to time-slice beam. Collaboration with Dowell, Emma, Limborg, Piot

August, 2003 W.S. Graves 19 Slice emittance and Twiss parameters Note strongly divergent beam due to solenoid overfocusing at tail, where current is low. Space charge forces near cathode caused very different betatron phase advances for different parts of beam.  is parameter that characterizes mismatch between target and each slice.  = ½ (  0  – 2  0  +   0 )  0,  0,  0 are target Twiss param. , ,  are slice Twiss param. Beam Parameters: 200 pC, 75 MeV, 400 fs slice width

August, 2003 W.S. Graves 20 Different slices require different solenoid strength HeadTailHeadTailHeadTail HeadTailHeadTailHeadTail Increasing solenoid current Vertical dynamics Lattice is set to image end of Tank 2 to RF-zero phasing YAG. Particles in tail of beam are diverging, and in head converging. Head has higher current and so reaches waist at higher solenoid setting. Current projection Time

August, 2003 W.S. Graves 21 Solenoid = 98 A Solenoid = 108 A Solenoid = 104 A Slice emittance vs solenoid strength. Charge = 200 pC. Solenoid Eyn Alpha Beta 98 A 3.7 um (3.2) 0.4 (1.0) 1.3 m (1.3) 104 A 2.1 um (2.8) -6.9 (-3.6) 9.8 m (6.8) 108 A 2.7 um (2.7) -9.0 (-9.6) 45 m (36) Projected Values (parmela in parentheses) Data Parmela

August, 2003 W.S. Graves 22 Slice parameters vs charge 10 pC 100 pC 200 pC 50 pC Low charge cases show low slice emittance and little phase space twist. High charge cases demonstrate both slice emittance growth and phase space distortion.

August, 2003 W.S. Graves 23 Longitudinal structure Time (ps) Current (A) File: csr01, FWHM = 2.2 ps Analysis of RF zero phasing data can be complicated by modulations in energy plane. See contribution from T. Shaftan for detailed description.

August, 2003 W.S. Graves 24 RF zero phasing Method uses accelerating mode to “streak” the beam by increasing the energy spread (chirping). Uncorrelated energy spread is ~5 keV and coherent modulations can be ~20 keV. Streak chirp must be much larger than this. Time and energy axes are difficult to disentangle. RF deflector cavity Transverse momentum is ~ 1 eV/c. Relatively small deflecting field gives excellent time resolution. Less sensitive to coherent energy modulations. Can obtain simultaneous time/energy/transverse beam properties when combined with dipole in other plane. RF zero phasing vs RF deflectors

August, 2003 W.S. Graves 25 Concluding Remarks Experience seems to indicate that most differences between experiment and simulation are due to experimental inaccuracies. Beam can be used to diagnose many hardware/applied field difficulties. “Easy to use” control system integrating realtime analysis and hardware/beam control very important. With adequate diagnostics, meeting beam quality and FEL performance goals is straightforward. See work by Shaftan, Loos, Doyuran of BNL on undulator diagnostics and trajectory analysis.