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Update on STELLA-LW Experiment Preparations W. D. Kimura ATF Users Meeting April 4 -6, Update on STELLA-LW Experiment Preparations W. D. Kimura ATF Users Meeting April 4 -6, 2007 Work supported by the U. S. Department of Energy, Grant Nos. DE-FG 02 -04 ER 41294, DE-AC 02 -98 CH 10886, DE-FG 03 -92 ER 40695, and DE-FG 02 -92 ER 40745 1

Collaborators - Nikolai Andreev (RAS) Ilan Ben-Zvi (BNL) Simon Hooker (Oxford) Sergei Kuznetsov (RAS) Collaborators - Nikolai Andreev (RAS) Ilan Ben-Zvi (BNL) Simon Hooker (Oxford) Sergei Kuznetsov (RAS) Igor Pogorelsky (BNL) Loren Steinhauer (UW) Antonio Ting (NRL) Xiaoping Ding (UCLA) - 2 Marcus Babzien (BNL) David Cline (UCLA) Karl Kusche (BNL) Igor Pavlishin (BNL) Alla Pogosova (RAS) Daniil Stolyarov Vitaly Yakimenko (BNL) Arie Zigler (Hebrew University)

Special Acknowledgements · Synergism between STELLA-LW experiment and other experiments at BNL ATF has Special Acknowledgements · Synergism between STELLA-LW experiment and other experiments at BNL ATF has greatly benefited all experiments Wish to acknowledge contributions by: · Resonant PWFA Experiment (University of Southern California) - Efthymios (Themos) Kallos - Patric Muggli - Tom Katsouleas · PASER Experiment (Technion - Israel Institute of Technology) - Samer Banna 3

Outline · Background - Experiment goals as result of redirection - Modified experiment approach Outline · Background - Experiment goals as result of redirection - Modified experiment approach · Update on experiment preparations - Gas-filled capillary - Coherent Thomson scattering diagnostic · Future plans 4

Original STELLA-LW Goals · STELLA-LW was to examine two new LWFA schemes · 1 Original STELLA-LW Goals · STELLA-LW was to examine two new LWFA schemes · 1 st Method: Seeded self-modulated LWFA(1) (seeded SM-LWFA) - Use seed e-beam bunch to generate wakefield - Laser pulse immediately follows to amplify wakefield - Probe amplified wakefield using second witness e-beam bunch [1] N. E. Andreev, et al. , Phys. Rev. ST Accel. Beams 9, 031303 (2006). 5

Model Prediction for Seeded SM-LWFA · Predictions(1) assume: Seed pulse length = 118 fs, Model Prediction for Seeded SM-LWFA · Predictions(1) assume: Seed pulse length = 118 fs, Focus size = 50 mm (1 s), 199 p. C, Witness pulse length = 1. 23 ps, Focus size = 20 mm (1 s), Plasma density = 0. 89 x 1017 cm-3; Laser power = 0. 5 TW, Lacc = 2 mm Energy spectrum of witness [1] N. E. Andreev, et al. , “Seeded Self-Modulated Laser Wakefield Acceleration, ” Phys. Rev. ST Accel. Beams 9, 031303 (2006). 6

Original STELLA-LW Goals · STELLA-LW was to examine two new LWFA schemes · 1 Original STELLA-LW Goals · STELLA-LW was to examine two new LWFA schemes · 1 st Method: Seeded self-modulated LWFA(1) (seeded SM-LWFA) - Use seed e-beam bunch to generate wakefield - Laser pulse immediately follows to amplify wakefield - Probe amplified wakefield using second witness e-beam bunch · 2 nd Method: Pseudo-resonant LWFA(2) (PR-LWFA) - Use nonlinear plasma interaction to steepen tail of laser pulse - Causes laser pulse to generate wakefield like shorter pulse · Both may permit more controllable wakefield formation, which would be important for staging LWFA [1] N. E. Andreev, et al. , Phys. Rev. ST Accel. Beams 9, 031303 (2006). [2] N. E. Andreev, et al. , Phys. Rev. ST Accel. Beams 6, 041301 (2003). 7

STELLA-LW Experiment Forced to Conclude at End of 2007 · DOE decided not renew STELLA-LW Experiment Forced to Conclude at End of 2007 · DOE decided not renew STELLA-LW grant - Reviewers’ primary criticisms: not compelling work, progress too slow · Case of “Too little, too late” · LBNL has demonstrated 1 Ge. V energy gain (>30 Ge. V/m) using LWFA - Used 3. 3 cm gas-filled capillary at ~1018 cm-3 to guide 40 TW laser beam - Small energy spread (2. 5% rms), good charge (~30 p. C) - Plan to stage process in near future · STELLA-LW was to demonstrate 100 Me. V gain (>1 Ge. V/m) using LWFA - Planned to use <1 cm gas-filled capillary at ~1017 cm-3, where guiding is more difficult, and 1 TW laser beam - Would modulate e-beam energy, no trapping of electrons - Next phase would involve staging to create monoenergetic electrons - Would not happen for at least another 3 years 8

STELLA-LW Will End by Performing Seeded SM-LWFA Experiment Only · Would be first to STELLA-LW Will End by Performing Seeded SM-LWFA Experiment Only · Would be first to demonstrate this new hybrid process - Must complete experiment by end of 2007 - No plans for follow-on experiments to seeded SM-LWFA · Psuedo-resonant LWFA experiment will not be pursued · STELLA team together with new collaborators are proposing entirely new experiments - Generation of Tunable Microbunch Train (presented tomorrow) - Advanced PASER Development (presented tomorrow) - Advanced Capillary Discharge Development (would involve ATF three years from now) · Proposals are currently under review by DOE 9

Experimental Approach for Seeded SM-LWFA Must be Modified · Seeded SM-LWFA experiment needs single Experimental Approach for Seeded SM-LWFA Must be Modified · Seeded SM-LWFA experiment needs single ~100 fs seed bunch · ATF chicane/dogleg system produces a pair of 100 -fs bunches – no easy way to prevent this from occurring · Can deliver single bunch by blocking second bunch, but this also blocks any witness bunch · Will indirectly observe wakefield formation and amplification by using coherent Thomson scattering (CTS) diagnostic to detect anti-Stokes/Stokes sidebands on either side of fundamental laser line - Standard technique used by others (e. g. , UCLA) - Wakefields act like traveling grating that scatters and shifts laser radiation - Sidebands at , corresponding to 9. 6 mm and 11. 9 mm 10

Coherent Thomson Scattering Diagnostic 11 Coherent Thomson Scattering Diagnostic 11

Will Use Drive Laser Beam as CTS Probe · Pros with using drive laser Will Use Drive Laser Beam as CTS Probe · Pros with using drive laser beam at 10. 6 mm as CTS probe: - Convenient because does not require second laser source and dichroic optics to permit using two different wavelengths - Sidebands automatically generated with no separate alignment needed of probe beam - CTS signal scales as l. L 2 – favors long wavelength - Have copious amounts of probe power available (~1 TW) · Cons with using drive laser beam as probe: - Must be careful to completely filter-out fundamental signal - CTS signal strength and wakefield amplification both scale with drive laser beam power, but amplification should scale nonlinearly · Will use N. Andreev’s model to help interpret CTS data 12

Model Prediction(3) for CTS Signal Using parameters from: N. E. Andreev, et al. , Model Prediction(3) for CTS Signal Using parameters from: N. E. Andreev, et al. , “Seeded Self-Modulated Laser Wakefield Acceleration, ” Phys. Rev. ST Accel. Beams 9, 031303 (2006). [3] Courtesy: N. Andreev, RAS 13

Estimation of CTS Signal Strength · Model predicts sideband signal is 103 to 104 Estimation of CTS Signal Strength · Model predicts sideband signal is 103 to 104 times smaller than fundamental - Note, model assumed 0. 5 TW and 2 mm long plasma length - Actual experiment will be delivering ~1 TW and using 4 mm long capillary - CTS signal strength should be stronger, but highly nonlinear so scaling may be faster than linear · Sensitivity of IR detector limited by risetime and oscilloscope bandwidth (both ~1 ns) - Photovoltaic detectors have good detectivities D* with Noise-Equivalent. Power (NEP) of ~40 m. W - Even assuming worse case of I(l)/Imax ~ 10 -4 and 10 -3 reduction due to risetime limits, still have >10 k. W available 14

Either Polypropylene or Gas-Filled Capillary Discharges Can be Used · Earlier performed Stark broadening Either Polypropylene or Gas-Filled Capillary Discharges Can be Used · Earlier performed Stark broadening measurements on ablative polypropylene capillary - Trigger section = 2. 3 mm, main discharge = 3. 8 mm, ID = 1 mm - Measured ~1017 cm-3 plasma densities · Recently, performed Stark broadening measurements on gas-filled capillary - ID = 0. 5 mm, length = 4 mm - Determined conditions necessary to achieve ~1017 cm-3 plasma densities - Density does not appear to scale with gas pressure as expected, reason is still unclear 15

Polypropylene Capillary Discharge 6 mm for STELLA-LW Basic Zigler design [4] Entrance to capillary Polypropylene Capillary Discharge 6 mm for STELLA-LW Basic Zigler design [4] Entrance to capillary [4] D. Kaganovich, et al. , Appl. Phys. Lett. 71, 2925 (1997). 16

Gas-filled Capillary Discharge (4 & 10 mm) 17 Gas-filled Capillary Discharge (4 & 10 mm) 17

STELLA-LW Capillary Chamber Drawing CO 2 laser beam Parabolic mirror w/ hole for e-beam STELLA-LW Capillary Chamber Drawing CO 2 laser beam Parabolic mirror w/ hole for e-beam E-beam Capillary discharge support system 18 Permanent magnetic focusing quadrupole

Photo of Capillary Vacuum Chamber 19 Photo of Capillary Vacuum Chamber 19

Schematic of STELLA-LW Experiment Layout on ATF Beamline #1 20 Schematic of STELLA-LW Experiment Layout on ATF Beamline #1 20

Gas-Filled Capillary Update · Performed DC breakdown measurements and compared with Paschen curve predictions Gas-Filled Capillary Update · Performed DC breakdown measurements and compared with Paschen curve predictions for hydrogen gas - Used data to determine time required for gas to reach quasi-static condition inside capillary - Operate at minimum charge voltage to avoid operating in ablation mode 21

Extensive Stark Broadening Measurements Performed on Gas-filled Capillary Typical spectrum from ionized gas only Extensive Stark Broadening Measurements Performed on Gas-filled Capillary Typical spectrum from ionized gas only Hb Ha Typical spectrum when ablation is occurring 22

Gas-Filled Capillary Update (cont. ) · Performed vacuum recovery tests with gas-filled capillary installed Gas-Filled Capillary Update (cont. ) · Performed vacuum recovery tests with gas-filled capillary installed on beamline - For 1017 cm-3 densities, vacuum-load and recovery time appear to be acceptable - Still have remotely-insertable, 1 -mm-thick Ti foil pellicle available to separate linac from capillary · Initial tests of focusing TW CO 2 laser beam into 0. 5 -mm diameter capillary indicate excessive scraping of laser beam on capillary walls - Easiest solution is to use 1 -mm diameter capillary - Less laser guiding expected, but less critical for short (4 mm) capillary · Still need to determine whether laser beam further ionizes plasma - May be minor issue if discharge is near 100% ionized - Should be able to compensate for additional ionization by reducing gas reservoir pressure 23

2 nd Gas-Filled Capillary System Installed · Second gas-filled capillary system fabricated and assembled 2 nd Gas-Filled Capillary System Installed · Second gas-filled capillary system fabricated and assembled - One system installed on beamline for interaction with e-beam - Second one installed in “FEL” room for interaction with TW CO 2 laser beam · Arrangement permits performing capillary-based experiments in parallel - Capillary in FEL room will be used to test effects of laser beam on plasma and for preliminary testing of CTS diagnostic - Capillary on beamline will be used for double-bunch PWFA experiments, which are precursor to seeded SM-LWFA experiment - Once laser tests are completed in FEL room, can move CTS diagnostic to Experimental Hall and perform seeded SM-LWFA experiment · Different length gas-filled capillaries available - 4 mm for seeded SM-LWFA - 10 mm for double-bunch PWFA 24

Near-Term Plans and Conclusion · Near-term emphasis will be resolving issues related to focusing Near-Term Plans and Conclusion · Near-term emphasis will be resolving issues related to focusing TW CO 2 laser beam into capillary and setting up CTS diagnostic - These are still nontrivial challenges - Actual seeded SM-LWFA experiment should begin in next few months · Assuming one or more of new proposals are approved by ATF Review Committee and funded by DOE, then hope to transition from STELLA-LW to new experiment(s) in last quarter of 2007 - Same team members involved, including ATF staff - Much overlap with existing hardware and equipment · Wish to thank ATF staff for outstanding support during past 20 years! - Experiment evolved from inverse Cerenkov acceleration to IFELs to laser wakefield acceleration - Looking forward to another 20…well, maybe… 10 more years of working together! 25