Electron beam diagnostic methods William S. Graves MIT-Bates Laboratory Presented at 2003 ICFA S 2 E Workshop DESY-Zeuthen August, 2003 W. S. Graves 1
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 2
DUVFEL Facility at BNL 50 m Coherent IR diagnostics NISUS 10 m undulator Bend Undulators Bunch compressor with post accel. RF zero phase screen 70 Me. V Bend 70 -200 Me. V Dump 1. 6 cell gun with copper cathode Triplet Linac tanks Triplet 5 Me. V Dump 30 m. J, 100 fs Ti: Sapphire laser Photoinjector: 1. 6 cell BNL/SLAC/UCLA with copper cathode 4 SLAC s-band 3 m linac sections Bunch compressor between L 2 and L 3 Approximately 60 CCD cameras on YAG screens. August, 2003 W. S. Graves 3
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 4
Thermal Emittance (1) (2) (3) August, 2003 W. S. Graves 5
Projected emittance vs charge and FWHM HOMDYN simulations estimate limits on maximum bunch length and charge. Choose working parameters of 2 p. C, 2 ps FWHM. 0. 67 0. 68 Emittance (mm-mrad) 0. 68 0. 72 Emittance (mm-mrad) 0. 74 Simulation 0. 66 0. 64 0. 62 Simulation 0. 65 0. 64 0. 63 0. 62 0. 6 0. 58 0 0. 66 2 4 6 FWHM (ps) 8 Charge 2 p. C Energy 3. 7 Me. V Laser spot 0. 5 mm RMS 10 0. 61 0 5 10 Charge (p. C) 15 20 FWHM 3 ps Energy 3. 7 Me. V Laser spot 0. 5 mm RMS 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 6
Can measure • charge • energy • x and y centroid • x and y beamsize • px and py Solenoid Scan Layout 65 cm 33 cm YAG screen Mirror 12 cm 1 cm Telecentric lens magnif. = 1 Dipole trim 1. 6 cell photoinjector Solenoid CCD Camera • 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 7
Low Energy Beam Measurements pop 02 a. bmp 8 5 x 10 -9 File: a 10 pc. txt a = -30. 8 ± 1. 7 b = 8. 11 ± 0. 44 m e = 0. 605 ± 0. 025 mm-mrad N s = 0. 773 ± 0. 0177 mm ' s = 2. 94 ± 0. 0658 mrad E = 3. 69 Me. V 7 10 6 15 s 11 5 20 25 4 3 pixels 30 5 10 15 20 25 2 30 1 Hor. RMS width = 39. 5 um Intensity (A. U. ) 2000 0 102 1500 104 105 106 Solenoid Current (Amp) 107 Video processing Monte Carlo method using measured beam size jitter. • Dark current image subtracted. 500 50 100 150 200 Beam size (um) August, 2003 W. S. Graves 250 300 108 Error estimates • 3 x 3 median filter applied. 1000 0 0 103 • Pixels < few % of peak are zeroed. 8
Emittance vs laser size Horizontal Vertical 1 e (mm-mrad) N 1. 2 0. 8 0. 6 0. 4 0. 2 0 0 0. 2 0. 4 0. 6 0. 8 1 1. 2 0 0 Horizontal RMS laser size (mm) 0. 2 0. 4 0. 6 0. 8 1 1. 2 Horizontal RMS laser size (mm) Emittance shows expected linear dependence on spot size. FWHM 2. 6 ps Small asymmetry is always present. Charge 2. 0 p. C Gradient 85 MV/m RF phase 30 degrees August, 2003 W. S. Graves 9
Beam size and divergence vs laser spot size RMS e-beam hor. size (mm) Size Low 1. 4 1. 2 1 0. 8 0. 6 0. 4 0. 2 0 0 0. 2 0. 4 0. 6 0. 8 RMS laser hor. size (mm) 1 1. 2 RMS ebeam hor. divergence (mrad) High 1. 6 9 Divergence 8 High 7 Low 6 5 4 3 2 1 0 0 0. 2 0. 4 0. 6 0. 8 RMS laser hor. size (mm) 1 1. 2 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. August, 2003 W. S. Graves 10
Emittance vs RF phase Error bars are measured data points. Curve is nonlinear least squares fit with βrf and Φcu as parameters: βrf = 3. 10 +/- 0. 49 and Φcu = 4. 73 +/- 0. 04 e. V. The fit provides a second estimate of the electron kinetic energy Ek = 0. 40 e. V, in close agreement with the estimate from the radial dependence of emittance. August, 2003 W. S. Graves 11
Time profile of UV laser pulse 100 fs IR Cross-correlaton difference frequency generation – experiment by B. Sheehy and H. Loos 200 fs blue Power meter 5 ps UV BBO crystal Signa l (V) 2 1. 5 Phase matching angle of harmonic generation crystals used to produce UV affects time and spatial modulations. 1 0. 5 0 40 30 20 10 Phase matching angle (mrad) 0 -10 -20 -30 -40 -10 August, 2003 W. S. Graves -8 -6 -4 -2 Time (ps) 0 2 Note: “Sub-ps” streak camera is inadequate for this measurement 12
Laser masking of cathode image Above: Laser cathode image of air force mask in laser room. Below: Resulting electron beam at pop 2. August, 2003 W. S. Graves Above: Laser cathode image with mask removed showing smooth profile. Below: Resulting electron beam showing hot spot of emission. 13
Streak Camera • Hammamatsu FESCA 500 50 100 • 765 fs FWHM measured resolution 150 • Reflective input optics (200 -1600 nm) 200 Time 250 • Wide response cathode (200 -900 nm) • Optical trigger (<500 fs jitter) 300 400 • Designed for synchroscan use. Also good single-shot resolution. 450 • 6 time ranges: 50 ps - 6 ns 350 500 100 200 300 400 50 ps window Streak image Very helpful for commissioning activies and for timing several optical signals. Limited time resolution below 1 ps. August, 2003 W. S. Graves 14
234 232 230 228 226 224 222 220 218 216 214 212 210 208 206 204 202 200 198 196 360 220 340 215 320 210 signal (arb units) Streak camera time profiles of laser pulses 205 200 195 190 300 280 260 240 220 200 185 180 6 8 10 streak delay (picoseconds) 160 32 34 36 14 16 Amplified IR FWHM 18 20 22 24 26 streak delay (picoseconds) Oscillator 796 nm 765 fs 38 796 nm 1. 01 ps single shot UV 266 nm FWHM 2. 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 15
RF zero-phase time profile L 4 phase = +/-90, amp. set to add known chirp L 2 phase varies, amp. fixed L 3 phase = +90, amp. set to remove chirp L 1 phase = 0, amp. fixed Chicane varies from 0 cm < R 56 < 10. 5 cm L 4 Pop 14 YAG screen L 3 L 2 L 1 YAG images at pop 14 L 4 phase = -90 degrees August, 2003 W. S. Graves L 3 corrects residual chirp, L 4 is off L 4 phase = +90 degrees 16
Quad scan during RF zero phase Movie of slice emittance measurement. August, 2003 W. S. Graves 17
Left side Beam size squared vs quadrupole strength. Each plot is a different time slice of beam. 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. Collaboration with Dowell, Emma, Limborg, Piot Software is used to time-slice beam. August, 2003 W. S. Graves 18
Slice emittance and Twiss parameters z is parameter that characterizes mismatch between target and each slice. z = ½ (b 0 g – 2 a 0 a + b g 0) a 0, b 0, g 0 are target Twiss param. a, b, g are slice Twiss param. Beam Parameters: 200 p. C, 75 Me. V, 400 fs slice width 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. August, 2003 W. S. Graves 19
Different slices require different solenoid strength Current projection Tail Time Tail Head Tail Head 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. Increasing solenoid current Head has higher current and so reaches waist at higher solenoid setting. August, 2003 W. S. Graves 20
Slice emittance vs solenoid strength. Charge = 200 p. C. Data Solenoid = 98 A Parmela Projected Values (parmela in parentheses) Solenoid 98 A 104 A 108 A Eyn 3. 7 um (3. 2) 2. 1 um (2. 8) 2. 7 um (2. 7) Alpha 0. 4 -6. 9 -9. 0 (-9. 6) Beta 1. 3 m (1. 3) 45 m (36) (1. 0) (-3. 6) 9. 8 m (6. 8) Solenoid = 104 A August, 2003 W. S. Graves Solenoid = 108 A 21
Slice parameters vs charge 10 p. C 50 p. C Low charge cases show low slice emittance and little phase space twist. 100 p. C 200 p. C High charge cases demonstrate both slice emittance growth and phase space distortion. August, 2003 W. S. Graves 22
Longitudinal structure 50 100 150 200 250 300 File: csr 01, FWHM = 2. 2 ps 200 Analysis of RF zero phasing data can be complicated by modulations in energy plane. Current (A) See contribution from T. Shaftan for detailed description. 150 100 50 0 August, 2003 W. S. Graves 0 2 4 Time (ps) 6 23
RF zero phasing vs RF deflectors RF zero phasing • Method uses accelerating mode to “streak” the beam by increasing the energy spread (chirping). • Uncorrelated energy spread is ~5 ke. V and coherent modulations can be ~20 ke. V. Streak chirp must be much larger than this. • Time and energy axes are difficult to disentangle. RF deflector cavity • Transverse momentum is ~ 1 e. V/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. August, 2003 W. S. Graves 24
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. August, 2003 W. S. Graves 25
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