Injector Experimental Progress

Transcription

1 Injector Experimental Progress LCLS TAC Meeting December John Schmerge for the GTF Team

2 GTF Group John Schmerge Paul Bolton Steve Gierman Cecile Limborg Brendan Murphy Dave Dowell Leader Laser Accelerator Simulations Laser Accelerator

3 Outline Review 2000 results and 2001 goals 2001 transverse measurements BNL transverse measurements 2001 longitudinal measurements Laser Upgrade Control System Upgrade 2002 Plans Conclusions

4 Emittance Measurement 2000 (presented at last TAC meeting)

5 2001 Plans (presented at last TAC meeting) Optimize the laser spatial profile Measure QE as a function of position on the cathode Demonstrate low emittance using temporally shaped laser pulses Measure emittance versus charge Measure the emittance as a function of longitudinal position at the gun exit

6 Laser Transverse Profile

7 QE vs Position QE = 3.6 ± June Yposition mm Xposition mm

8 Emittance Measurement Technique Quadrupole Scan Technique Measure beam size vs quadrupole current or strength. Background subtracted image rms spot size calculation using projection cut off at 5% of maximum

9 Quad Scan Fit ± 31 pc 2.18 kg 1.8 ps uv pulse rms beamwidth (mm) Best Fit: α = 14.6 β = 5.98 m ε n = 1.8 mm mrad k (1/m)

10 Emittance vs Charge ps data 4.3 ps data 2 ps parmela 4 ps parmela Normalized rms emittance ε (mm mrad) Parameters E gun = 110MV/m φ gun = 40 R cat = 1mm B sol 2.0kG E linac = 8.3 MV/m Charge (pc)

11 Laser Pulse Length Measurement Normalized Counts ps FWHM 4.3 ps FWHM Time (ps) Streak camera resolution at 263 nm 1 ps

12 Analysis with Space Charge σ '' x ε 2 = g + σ 3 I γ 3 x A I p ( ) σ + σ x y Assumptions: ε = ε, α = α, β = x y x y At the entrance to the first quadrupole. No significant space charge effects until the end of the second quadrupole. x β y

13 Effects of Space Charge RMS spot size (µm) εn = 1.69 µm fit no space charge measured RMS spot size (µm) εn = 1.45 µm measured fit with space charge k (m -1 ) k (m -1 ) Q = 280 pc t laser = 4 ps FWHM t ebeam = 3 ps FWHM Typically 20-25% lower emittance when including space charge effects.

14 Effects of YAG screen resolution Emittance as a function of measurement system resolution εnrms (µm) emittance spot size σ rms (µm) resolution (µm) 188 σ beam = σ 2 measured σ 2 resolution

15 Beam spot exiting gun as a function of solenoid current due to 15 m focal length quad field in solenoid. Solenoid field imperfections 143 A 148 A 145 A 149 A 146 A 151 A 147 A 95 MV/m, 40, 300 pc.

16 BNL Measurements 0.8 mm-mrad at 0.5 nc reported at FEL 2001, Darmstadt Yakimenko and Wang et al at ATF Multiple screen emittance measurement intentionally avoiding measurements at waist positions to prevent screen saturation. 0.6 mm-mrad/mm reported at PAC 2001 Graves at SDL Thermal emittance for flat-top distribution Emittance measured at very low charge (2 pc)

17 Longitudinal Measurement Technique similar to quadrupole scan of transverse emittance Gun Booster Spectrometer Determine Longitudinal φ-space at Exit of Gun φ booster Energy Screen Longitudinal : Measure Energy Spectra vs booster phase Transverse: Measure Beam Size vs quad strength

18 Longitudinal Data Analysis 2 2 γ t + 2α t E + β E = E Longitudinal beam ellipse εl π ε β = Uncorrelated l ε γ = Uncorrelated Include distortions by adding quadratic and cubic terms = α t β ± α t β 2 2 εl γ t π β l + a t 2 Bunch Length Energy Spread + b t 3 E Ray trace to fit 5 parameters [ cos( φ ) ( )] RF + t0 cos φrf t1= t0 1 = E0 + ERF ;

19 Measurements Booster Phase = -13 degrees Booster Phase = Minimum Energy Spread Maximum Energy Correlated Energy Spread E Total = -E RF cos(φ RF ) φ = 400 kev or 8 % FWHM PARMELA predicts 2% correlated energy spread Energy (MeV) Longitudinal Distribution After Gun Time (ps)

20 Proposed Explanations Wakefield in the gun Wakefield in gun to linac drift region Ratio of half cell to full cell gun field < 1

21 Proposed Measurements Repeat longitudinal measurement with different Q and t Repeat gun energy measurement Measure space charge limit to determine E field at cathode (Q vs laser energy) Simulate beam with large correlated energy spread Electro-optic pulse length measurement

22 Laser Upgrade Installing a dual pass amplifier after the regen amplifier to boost IR energy > 10 mj. Installing an optical apodizer which produces a flat-top transverse profile without wasting energy. Improving transport losses and clipping to preserve optical image produced in the laser room. Complete Spring 2002

23 Control System Upgrade New Labview control system utilizing a single PC. Multiple frame grabbers allows simultaneous acquisition of laser and e-beam images. All hardware will be computer controlled for fast data acquisition and analysis. Complete Spring 2002

24 FY 2002 Plans Test laser uniformity/energy up to 1 nc Compare YAG and OTR resolution Remeasure longitudinal emittance Install new solenoid Measure gun field balance Winter Winter Winter Spring Spring Measure emittance with temporally shaped laser pulses - expect 1 mm-mrad at 1 nc Summer Measure transverse and longitudinal emittance directly exiting gun Fall Klystron/modulator upgrade Fall

25 Conclusions Measured 1.5 mm-mrad at 100 A (200 pc) Measured 8 % correlated energy spread exiting gun Laser upgrade to generate 1 nc in progress Control system upgrade to speed data acquisition in progress Install new solenoid and improve laser profile to decrease emittance

26 Drift with Space Charge Spot Size vs drift distance Distance (m) x - no space charge x - with space charge y - no space charge y - with space charge RMS spot size (µm) Spot Size vs drift distance Distance (m) x - no space charge x - with space charge y - no space charge y - with space charge RMS spot size (µm) Q = 0 Q = 280 pc with t = 4 ps FWHM