1. Recent Results on the Plasma Wakefield Acceleration at FACET E 200 Collaboration 1)Beam loading due to distributed injection of charge in the wake reduces the transformer ratio 2)Local ionization injection produces monoenergetic 20+ GeV bunches containing 20-40 pC charge.

2. Why use a Rubidium source Compared to previously used Li, the Rb plasma mitigates: 1)beam head erosion problem - v etch α IP 1.73 2)emittance growth due to ion motion – use heavier atoms Singly ionized Rb plasma is created by the electric field of the beam. Periodic pinching of the drive beam can lead to ionization and injection of the second Rb electron into the wake. This distributed injection of dark current can load the wake and reduce the transformer ratio T = E+/E-

3. Experimental Set-up Foils of different thickness and composition used to increase the beam emittance

4. Unmatched Beam Undergoes Envelope Oscillations in Plasma No Dark Current Dark Current Region RbII/ ArII Ar Rb RbII ArII RbI Ar I The electric field at the tightly focused regions of the beam can further ionize Rb and Ar Distance (m ) Beam radius Neutral Rb Density

5. Emittance of the beam used to vary length of the wake and thereby vary energy loss As the length increases so does the number of envelope oscillations the beam makes. Each time the beam pinches down to a minimum it produces Rb 2+. These new electrons (excess charge) are injected into the wake

6. Beam Loading reduces Transformer ratio = E + /E - = ΔW + /ΔW -

7. PIC Simulations Confirm that Beam Loading by Distributed Injection of Rb 2+ Electrons Reduced T Peak Accelerating field decreases from 44 GeV/m to 35 GeV/m due to beam loading

8. Summary Use of Rb plasma explored for mitigating head erosion and ion motion For the beam and plasma parameters used, ionization of Rb 1+ ions leads to injection of RbII electrons in the wake in distributed fashion Beam loading of the wake reduces the average transformer ratio from 1 to 0.85 Simulations confirm the cause of beam loading as trapping of Rb II electrons. Can we use ionization injection in a controlled manner to get narrow energy spread beams??

9. . Use Ionization Injection to Generate Monoenergetic Bunches Concept of Ionization Injection Into a PWFA Use Li plasma (not Rb) to ensure no dark current. Use He:Ar mix as buffer to control ionization-injection trapped charge

10. Simulations of Ionization Injection (Li plasma -50/50% He-Ar Buffer Gas)

11. Simulations Show Monoenergetic Bunchlets with ~1% Energy spread

12. Experiments Used various Configurations 1)Li Plasma with a) Pure He buffer (He electron injection) b) He buffer with 10%, 22% and 50% Ar (Inject Ar II &/ He ) c) He buffer with 30% Ne ( He or Ne injection) 2) Pure Ar gas column (Ar II or Ar III injection) All showed monoenergetic beamlets under certain beam conditions

13. 50/50 Ar/He buffer gas, 30-cm long Li vapor flattop region 20 30 24 Energy (GeV) Mono-energietic bunchlets produced by ionization injection Initial energy Accelerated tail electrons Bunchlets 6 images with spectometer set to image at 24 GeV 1Bunchlets are from injected electrons (0 to 20+ GeV in 30 cm) 2Bunchlet energy gain ≥ gain of the FACET beam tail e-’s

14. Histogram of Trapped Charge Mean charge about 30 pC The larger charge beamlets often had two blobs. The lower charge beamlets traversed a 1mm thick W foil placed to rule out coherent Cherenkov emission.

15. Energy Spread of Monoenergetic Beamlets Energy spread of ~ 0.5 GeV (FWHM) on a 25 GeV beamlet. This corresponds to a ~ < 1% energy spread

16. Summary of Observations Ionization Injection has produced narrow energy spread beamlets with energies on the order or exceeding the beam energy 25 GeV energy gain from rest is observed in just 30 cm long Li plasma Bunchlets typically have 30 pC of charge and a 1 % energy spread Process appears to be robust with rms charge and energy spread variation of less than a factor of 2 and a success rate of ~ 80%.