1. 1 Update on STELLA-LW Experiment Preparations W. D. Kimura Work supported by the U.S. Department of Energy, Grant Nos. DE-FG02-04ER41294, DE-AC02-98CH10886, DE-FG03-92ER40695, and DE-FG02-92ER40745 ATF Users Meeting April 4-6, 2007

2. 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. 3  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 Special Acknowledgements

4. 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. 5 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).

6. 6 Model Prediction for Seeded SM-LWFA  Predictions (1) assume: Seed pulse length = 118 fs, Focus size = 50  m (1  ), 199 pC, Witness pulse length = 1.23 ps, Focus size = 20  m (1  ), Plasma density = 0.89 x 10 17 cm -3 ; Laser power = 0.5 TW, L acc = 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).

7. 7 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).  Both may permit more controllable wakefield formation, which would be important for staging LWFA  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 [2] N. E. Andreev, et al., Phys. Rev. ST Accel. Beams 6, 041301 (2003).

8. 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  LBNL has demonstrated 1 GeV energy gain (>30 GeV/m) using LWFA -Used 3.3 cm gas-filled capillary at ~10 18 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 ~10 17 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  Case of “Too little, too late”

9. 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  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  Psuedo-resonant LWFA experiment will not be pursued

10. 10 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  m and 11.9  m

11. 11 Coherent Thomson Scattering Diagnostic

12. 12 Will Use Drive Laser Beam as CTS Probe  Pros with using drive laser beam at 10.6  m 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 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

13. 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. 14 Estimation of CTS Signal Strength  Model predicts sideband signal is 10 3 to 10 4 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  W -Even assuming worse case of I( )/I max ~ 10 -4 and 10 -3 reduction due to risetime limits, still have >10 kW available

15. 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 ~10 17 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 ~10 17 cm -3 plasma densities -Density does not appear to scale with gas pressure as expected, reason is still unclear

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

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

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

19. 19 Photo of Capillary Vacuum Chamber Vacuum Chamber

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

21. 21  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 Gas-Filled Capillary Update

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

23. 23 Gas-Filled Capillary Update (cont.)  Performed vacuum recovery tests with gas-filled capillary installed on beamline -For 10 17 cm -3 densities, vacuum-load and recovery time appear to be acceptable -Still have remotely-insertable, 1-  m-thick Ti foil pellicle available to separate linac from 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  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

24. 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 CO 2 laser beam  Different length gas-filled capillaries available -4 mm for seeded SM-LWFA -10 mm for double-bunch PWFA  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

25. 25 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!