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 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 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 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 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, (2006). 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 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, (2006). 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, (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, (2003). 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 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 ~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 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 Gas-filled Capillary Discharge (4 & 10 mm) 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 Photo of Capillary Vacuum Chamber Vacuum Chamber 20 Schematic of STELLA-LW Experiment Layout on ATF Beamline #1 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 Extensive Stark Broadening Measurements Performed on Gas-filled Capillary HH HH Typical spectrum from ionized gas only Typical spectrum when ablation is occurring 23 Gas-Filled Capillary Update (cont.) Performed vacuum recovery tests with gas-filled capillary installed on beamline -For 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 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 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 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 20well, maybe10 more years of working together!