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Update on STELLA-LW Experiment Preparations

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-FG02-04ER41294, DE-AC02-98CH10886, DE-FG03-92ER40695, and DE-FG02-92ER40745. Collaborators.

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Update on STELLA-LW Experiment Preparations

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  1. 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-FG02-04ER41294, DE-AC02-98CH10886, DE-FG03-92ER40695, and DE-FG02-92ER40745

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

  3. 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

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

  5. 1st 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). Original STELLA-LW Goals • STELLA-LW was to examine two new LWFA schemes

  6. Energy spectrum of witness Model Prediction for Seeded SM-LWFA • Predictions(1) assume: Seed pulse length = 118 fs, Focus size = 50 mm (1s), 199 pC, Witness pulse length = 1.23 ps, Focus size = 20 mm (1s), Plasma density = 0.89 x 1017 cm-3; Laser power = 0.5 TW, Lacc = 2 mm [1] N. E. Andreev, et al., “Seeded Self-Modulated Laser Wakefield Acceleration,” Phys. Rev. ST Accel. Beams 9, 031303 (2006).

  7. 1st 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 • 2nd 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 [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). Original STELLA-LW Goals • STELLA-LW was to examine two new LWFA schemes • Both may permit more controllable wakefield formation, which would be important for staging LWFA

  8. 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 GeV energy gain (>30 GeV/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 pC) - Plan to stage process in near future • STELLA-LW was to demonstrate 100 MeV gain (>1 GeV/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

  9. 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

  10. 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 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

  11. Coherent Thomson Scattering Diagnostic

  12. 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 lL2 – 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

  13. 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

  14. 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 mW - Even assuming worse case of I(l)/Imax ~ 10-4 and 10-3 reduction due to risetime limits, still have >10 kW available

  15. 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

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

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

  18. STELLA-LW Capillary Chamber Drawing CO2 laser beam CO2 laser beam Parabolic mirror w/ hole for e-beam Parabolic mirror w/ hole for e-beam E-beam Permanent magnetic focusing quadrupole Capillary discharge support system

  19. Photo of Capillary Vacuum Chamber Vacuum Chamber

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

  21. 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

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

  23. 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 CO2 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

  24. 2nd 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 CO2 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

  25. Near-Term Plans and Conclusion • Near-term emphasis will be resolving issues related to focusing TW CO2 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!

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