1. Hollow Channel Plasma Wakefield Acceleration Spencer Gessner 5 th SAREC Review September 15 th, 2014

2. 2 Collaboration N. Vafaei-Najafabadi, C.E. Clayton, K.A. Marsh, W. An, W. Lu, W.B. Mori, C.Joshi E. Adli J. Allen, C.I. Clarke, S. Corde, J. Frederico, S. Gessner, S.Z. Green, M. Litos, D. Walz, M.J. Hogan, V. Yakimenko P. Muggli

3. 3 Goal Our aim is to demonstrate a method for accelerating positron beams in a plasma using extremely large gradients while preserving the beam emittance.

4. 4 Why Explore Hollow Channel Plasmas?

5. 5 Accelerating but defocusing for positrons.

6. 6 Why Explore Hollow Channel Plasmas? Where can we load a positron beam in an electron beam- driven blowout wake? Wrong slope for positron beam loading.

7. ... Hollow channel plasmas were created in the laboratory in 2000... 7 The Hollow Channel Solution First proposed and studied theoretically in the 90’s...... And we can actually test this technique with positron beams at FACET!

8. 8 What is a Hollow Channel Plasma? Osiris2D simulations performed on Hoffman2 cluster at UCLA. Hollow channel plasmas provides a scenario where we can access GeV/m scale accelerating gradients without focusing (or defocusing) forces from background ions.

9. 9 How do you make a Hollow Channel Plasma? Phase profile: Intensity profile: The high-order Bessel profile has zero on-axis intensity and transverse shape does not depend on z. Mask PatternIntensity Pattern Vertical Lineout

10. 10 T504 Hollow Channel Test Beam Experiment e+e+ e+e+ Laser in Laser out OTR light Spiral Optic Ionized Region The goal of the T504 Test Beam Experiment was to demonstrate the production of a hollow channel plasma.

11. 11 T504 Raster Scan Channel position relative to positron beam We use the positron beam to probe the structure of the channel.

12. 12 T504 Results Beam Area [μm 2 ] The beam area growth is smallest when the beam is near the center of the channel.

13. 13 T504 Results The beam is minimally deflected near the center of the channel.

14. 14 Looking Forward: Improved Optics The new optics double the on-target energy.

15. 15 Looking Forward: Improved Diagnostics The hollow channel experiment requires laser diagnostics that are only possible with a gas beamline.

16. 16 Goals and Timeline Final Goal: Demonstrate GeV scale acceleration of a positron and show that the beam’s emittance is preserved. Spring 2014 Fall 2014 Spring 2015 Fall 2015 Spring 2015 Develop and Improve techniques for creating hollow channel plasmas Develop laser-plasma techniques for diagnosing hollow channel plasmas Characterize hollow channel wakefields with a low-intensity beam (linear regime) Characterize hollow channel wakefields with a high-intensity beam (nonlinear regime) Establish tolerances for beam offset and angular misalignment Two-bunch acceleration with positron driver and positron witness

17. 17 Conclusion The T504 Test Beam Experiment was successful; we clearly demonstrated the formation of a hollow channel plasma. We have identified critical areas where the experiment can be improved and are actively working on these items. The hollow channel experiment should be preformed in gas as opposed to lithium vapor. There is a limited window for positron experiments at FACET. Let’s make the most of the time we have left!