Progress on Beam Loading Studies Veronica Berglyd Olsen University of Oslo AWAKE 17th Physics Board CERN, May 3rd 2016

Introduction & Outline Case: Parameter scans for optimal beam position, charge and length. Evaluated in terms of energy gain and energy spread. Using AWAKE nominal plasma density, N0 = 7 × 1014 cm-3. Results presented in Lisbon: Broad parameter scan with 5 current steps from 425 A to 2125 A, and Gaussian beams with σ ranging from 60 µm to 120 µm. A phase scan for 60 µm beams with low charge and current. A charge scan with low to medium charge beams for a few selected phases. Updates since Lisbon: Extended current scan down to 100 A, with a 1 A reference. Added 40 µm electron beams, and dropped 120 µm electron beams. 03.05.2016 17th AWAKE Physics Board, CERN

Reminder: Small Scale Simulation Setup In order to reduce simulation time, these simulations are set up using a short, pre- modulated proton beam with 26 micro-bunches. The electron beam is injected between bunch 20 and 21. Approximately at ξ = 12 mm. Density profile is given as: where A is a charge scaling factor, and µ1 is the center of the first bunch. ρ 𝑠 =𝐴 1 2 2 +cos μ 1 − 𝑘 𝑝 ξ exp − 𝑟 2 2 σ 𝑟 03.05.2016 17th AWAKE Physics Board, CERN

Parameter Scan Presented in Lisbon 03.05.2016 17th AWAKE Physics Board, CERN

Updated Results from Wider Scan 03.05.2016 17th AWAKE Physics Board, CERN

Closer Look: IEB = 200A, σz = 60 µm 03.05.2016 17th AWAKE Physics Board, CERN

Closer Look: IEB = 400A, σz = 60 µm 03.05.2016 17th AWAKE Physics Board, CERN

Closer Look: IEB = 800A, σz = 60 µm 03.05.2016 17th AWAKE Physics Board, CERN

Observations & Conclusions As expected, an optimal loading (flattening) of the field gives a good energy gain and a relatively low energy spread. A shorter beam performs better as the bulk of the electrons sit closer to the peak of the e-field. The radial size of the electron beam drops from 100µm to about 20–25µm when it enters the plasma. There are two consequences of this: The witness beam charge density increases by a factor of about 16–20 times, which significantly increases the loading of the field near the axis. The witness beam extracts energy in a much narrower region along the axis due to its radial extent. Next step will be to try to counteract this radial compression of the electron beam by increasing its emittance. Thanks! 03.05.2016 17th AWAKE Physics Board, CERN

Upcoming 3D Simulations! 03.05.2016 17th AWAKE Physics Board, CERN