Two-Color Ionization Injection Experiment Using CO2 Laser-Plasma Accelerator

This experiment conducted at ATF II focused on generating beams with ultra-low transverse emittance utilizing two-color ionization injection with a CO2 laser-plasma accelerator. The technique involves using a pump laser pulse in circular polarization and an injection laser pulse in linear polarization to achieve remarkable results. Technical considerations, parameter sensitivity, and prospects for a compact sub-100 nm emittance diagnostic were explored in detail.

• Ionization Injection
• CO2 Laser
• Laser-Plasma Accelerator
• Ultra-Low Emittance
• Experimental Physics

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1. Two-color ionization injection experiment using a CO2 laser-plasma accelerator at ATF II C. Schroeder, C. Benedetti, J. van Tilborg, C. Geddes, E. Esarey, W. Leemans Lawrence Berkeley National Laboratory ATF Users Meeting, BNL, Oct 1-2, 2015 Office of Science UNIVERSITY OF CALIFORNIA 1 1

2. Outline Outline Concept of two-color ionization injection: <100 nm normalized transverse emittance Example of two-color ionization injection using CO2drive laser and Ti:Al2O3ionization laser Technical/practical considerations: parameter sensitivity Prospects for a compact sub-100 nm emittance diagnostic Office of Science UNIVERSITY OF CALIFORNIA 2 2

3. Laser field: Ponderomotive force: Ionization rate: L.-L. Yu, E. Esarey, et al., SPIE Conf. Proc. (2013) L.-L.Yu et al., PRL (2014) Xu et al., PRST-AB (2014) C. Schroeder et al., PRST-AB (2014) C. Schroeder et al., SPIE Conf. Proc. (2015) Office of Science UNIVERSITY OF CALIFORNIA 3 3

4. 2-color ionization injection generates beams with ultra-low (tens of nm) transverse emittance Pump laser pulse (circular-polarization): a=1.2, 5 um wavelength 92 fs (rms), 36 um spot Injection laser pulse (linear-polarization): a=0.1, 0.4 um wavelength 16 fs (rms), 5 um spot plasma: Krypton gas, 2x1017 cm-3 Modeled using 3D PIC (WARP) 20 nm emittance C. Schroederet al., Phys. Rev. ST Accel. Beams. 17, 101301(2014) transverse momentum (p/mc) = 20 nm Ultra-low (tens of nm) emittance achievable using two-color ionization injection Office of Science UNIVERSITY OF CALIFORNIA 4 4

5. 2-color ionization injection: CO2 drive laser ng = 1.25x1015cm-3 Gas jet: (L~0.5--1 cm) Krypton ne = 1016 cm-3 Ti:Al2O3 Ionization laser a0 Hs-Hi >0 Ez/E0 CO2Drive laser a1 Soon to be available at BNL - ATF-II [I Pogorelsky and I. Ben-Zvi, PPCF (2014)] CO2pump laser pulse: a0=2 10 um wavelength 470 fs (FWHM intensity) 155 um spot (ZR = 7.5 mm) P=20 TW (P/Pc=1) (linear polarization) 10 J Kr9+ Kr8+ E1 E0 Ti:Al2O3(frequency-doubled )injection laser pulse: a1=0.13 0.4 um wavelength 118 fs (FWHM intensity) 20 um spot (ZR = 3 mm) 114 mJ delay= 1.6 ps kp (z-ct) Office of Science UNIVERSITY OF CALIFORNIA 5 5

6. 2-color ionization injection: CO2 drive laser Gas jet: (L~1 cm) Kr CO2pump laser pulse: a0=2 10 um wavelength 470 fs (FWHM intensity) 155 um spot (ZR = 7.5 mm) 10 J frequency-doubled Ti:Al2O3injection pulse: a1=0.13 0.4 um wavelength 118 fs (FWHM intensity) 20 um spot (ZR = 3 mm) 114 mJ Ti:Al2O3 Ionization laser CO2Drive laser ne = 1016 cm-3 final emittance in laser polarization plane (~ thermal emittance): Normalized emittance (nm) PIC = 80 nm PIC final emittance out of laser polarization plane: = 42 nm z (mm) Office of Science UNIVERSITY OF CALIFORNIA 6 6

7. 2-color ionization injection: CO2 drive laser Gas jet: (L~1 cm) Kr PIC simulation Normalized emittance (nm) Ti:Al2O3 Ionization laser CO2Drive laser ne = 1016 cm-3 CO2pump laser pulse: a0=2 10 um wavelength 470 fs (FWHM intensity) 155 um spot (ZR = 7.5 mm) P=20 TW (P/Pc=1) (linear polarization) 10 J Propagation distance, z (mm) RMS energy spread Energy [MeV] Ti:Al2O3(frequency-doubled )injection laser pulse: a1=0.13 0.4 um wavelength 118 fs (FWHM intensity) 20 um spot (ZR = 3 mm) 114 mJ delay= 1.6 ps Charge [pC] pC 62 MeV 5% (rms) z = m Propagation distance, z (mm) Office of Science UNIVERSITY OF CALIFORNIA 7 7

8. Optimization: charge vs emittance C. Schroeder et al., PRST-AB (2014) C. Schroeder et al., SPIE Conf. Proc. (2015) charge: points = PIC Charge Q[pC] wi (mm) ionization laser spot size (um) Office of Science UNIVERSITY OF CALIFORNIA 8 8

9. Technical/practical considerations for 2-color ionization injection experiment Sensitivity to plasma wakefield amplitude - CO2laser driver amplitude and duration Sensitivity to timing/jitter between CO2 and Ti:Al2O3lasers - Increased ionization laser duration (energy) relaxes timing requirements Performance with ionization laser with 0.8 micron wavelength Interaction geometry: - co-linearity of lasers - required pointing - required alignment Measurement of <100 nm emittance: single-shot, energy-resolved emittance diagnostic Office of Science UNIVERSITY OF CALIFORNIA 9 9

10. Trapping requires weakly-relativistic wakefields Wakefield amplitude required to trap: Normalized laser intensity, a trapping trapping laser energy (J) Normalized laser duration FWHM laser duration (fs) Office of Science UNIVERSITY OF CALIFORNIA 10 10

11. Synchronization and jitter: ~100 fs timing desired 1.6 ps a0 Hs-Hi >0 Ez/E0 Operate at 1016 cm-3 with ~500 fs (FWHM) CO2 pulse ~1.6 ps delay between CO2 and Ti:Al2O3 a1 Trapping wake phases: t~500 fs ~0.5 ps kp (z-ct) Optical synchronization achieved by seeding Ti:Al2O3 using solid-state front-end of CO2 amplification chain Longer (more energetic) ionization laser can relax timing/delay sensitivity Office of Science UNIVERSITY OF CALIFORNIA 11 11

12. Performance using 0.8-micron ionization pulse: larger emittance in laser-polarization plane Efficiency of 2- generation (~30-50%) may limit energy in ionization pulse For fixed gas: CO2pump laser pulse: a0=2 10 um wavelength 470 fs (FWHM intensity) 155 um spot (ZR = 7.5 mm) P=20 TW (P/Pc=1) (linear polarization) 10 J Ti:Al2O3injection laser pulse: a1=0.26 0.8 um wavelength 118 fs (FWHM intensity) 20 um spot 114 mJ delay= 1.6 ps Charge (pC) ionization laser spot size (um) Office of Science UNIVERSITY OF CALIFORNIA 12 12

13. Interaction geometry: pointing and alignment tolerances Ti:Al203 C02 Gas jet 90-degree interaction reduces bunch charge: charge = (interaction volume) x (gas density) - Co-propagating: (volume) ~ w2 ZR - 90-deg: (volume) ~ w2 (TLc) charge reduced by ~(cTL)/ZR To avoid injection over larger transverse area (proportionally increasing emittance): - Pointing tolerance: mradfor 20 micron spot, 0.4 micron wavelength Alignment tolerances: - xm< 4 um Office of Science UNIVERSITY OF CALIFORNIA 13 13

14. Emittance diagnostic: single-shot, energy-resolved beam size measurement Geometric emittance: ~10-10 m Consider lens scan for emittance measurement: m few um Plasma-based discharge-capillary to provide symmetric, variable, high-field gradient (solenoid) lens J. van Tilborg et al., PRL (submitted, 2015) Weingartner et al., PRST-AB (2012) x m] Dispersed beam at screen x m] y m] Screen y m] dipole z[m] Energy [MeV] (transverse position on screen) end of LWFA: 27 pC, 62 MeV, 5% (rms) energy Discharge-cap lens: 1 cm length, 200 micron radius, 46 A, ~230 T/m measurement requires ~micron resolution at screen -wave-plate for Ti:Al2O3 to vary laser polarization and measure x, y-planes Office of Science UNIVERSITY OF CALIFORNIA 14 14

15. Summary Summary Two-color ionization injection using CO2 drive laser and Ti:Al2O3 ionization laser: produces e-beam with tens of nm normalized transverse emittance, tens of pC charge Emittance diagnostic: single-shot, energy-resolved measurement of beam size using plasma-based discharge-capillary lens ATF-II ideal facility to test concept Office of Science UNIVERSITY OF CALIFORNIA 15 15