1. UCLA Experiments with short single e-bunch using preformed and beam ionized plasma Retain ability to run short single bunch with pre-ionized plasma Ken Marsh, Navid Vafaei, Chan Joshi

2. UCLA Optimized density W. Lu

3. UCLA Why is this important? In theory, we should be able to reach transformer ratio > 2 Experiments so far TR < 2. What limits TR? – Head erosion? – Trapped particles? Can we maximize energy gain?

4. UCLA Facet Proposal for High Energy Gain Test Head erosion is likely the most important factor limiting energy gain. Does pre-ionized plasma solve the HE problem? Experimental study: using single electron bunch in the 35 cm lithium oven Example: For 2x10 17 cm -3 E z = 110 GeV/m, total gain 40 GeV in 35 cm E loss = 45GeV/m, total loss 16 GeV in 35 cm

5. UCLA Experimental Requirements n b > n p Lithium plasma n p =2x10 17 cm -3 30 um bunch length, 10-30 um bunch radius matched radius = 2 to 4 um Charge 1.8x10 10

6. UCLA How does pre-ionization affect our most basic diagnostics? Diagnostics Energy spectrometer – Can FACET spectrometer measure 60 GeV? – Can we see drive beam energy go down to 4 GeV. Energy gain has only a weak dependence on emittance Participating charge? Should strongly increase. X-ray level should increase because more participating charge and longer propagation distance.

7. UCLA Concluding Remarks We will need – High density 30x30x30 electron beam – Pre-ionized and beam ionized lithium plasma – 200 mj to preionize

8. HIGH TR EXPERIMENT WITH LITHIUM OVEN USING MIXED GAS BUFFER

9. UCLA Example of High TR Simulation with “Ionization Trapping” Generation of fully loaded transformer ratio wakes of 3 using the FACET II beam. Drive Beam Parameters: Q = 4 nC, σ r = 5 μm, σ z = 20 μm, ε N = 5 mm mrad, E = 10.0 GeV. Trailing Beam Parameters: Q = 0.288 nC, σ r = 2 μm, σ z = 2 μm, ε N = 1.0 mm mrad, E = 100 MeV. Distance between two beams: 93.5 μm. Plasma Density n p = 2.0 x 10 17 cm -3 (the beam propagates in a pre-ionized plasma). (Courtesy Weiming An)

10. UCLA Localized Self-Trapping Experiment No Trapping with lithium oven using helium buffer gas Add argon impurity to create a large number of self trapped electrons in a controlled way What bounds the Ar impurity ratio is reduction of TR due to beam loading This experiment needs – High density electron beam to ionize Lithium – Argon or Neon impurity mixing

11. UCLA Mixed Gas Results in LWFA 4mm injector cell gas cell is compared to 8 mm two stage cell. – Two stage: Electron density of 3x10 18 cm -3, coupled laser power of 40 TW – Injector only: 3.4x10 18 cm -3, coupled laser power of 50 TW – 0.5% N2 mixed in He in both cases