UCLA RAL Beam characteristics • What is a “perfect” beam? • It comes from the Injector. • It is affected by many factors • A few highlights from contributed talks… • Beam Environment • Capillary discharge (S. Hooker); scaling matched wo; wall ultimately comes into play at small radii or low ne. • Hollow channel (N. Andreev); multimode not detrimental to wakefield; phase relation between Ez and Ey changes in a channel. • Few-cycle driver (M. Geissler); very rapid evolution of wake; effects of density ramps; sharp edge needed for external injection, Nbunch > Nion. • “Non-linear” effects; Beam Loading • Beam loading (A. Reitsma); h <-> dE/E tradeoff; f slippage to flatten dE/E; function of Lbunch/lp. • Beam loading in PWFA (K. Lotov); minimize residual energy flux --> optimal witness pulse shape, linear to blowout regimes. • Dark current (T. Katsouleas); Seen in SLAC PWFA experiment; Ecrit ~ SQRT(k’) = Dawson cold wavebreaking for blowout conditions.
UCLA RAL Beam characteristics (cont.) • What is a “perfect” beam? • It comes from the Injector. • It is affected by many factors • A few highlights from contributed talks (cont.)… • Beam Environment • “Non-linear” effects; Beam Loading • Transport and Staging • Transport and focusing (Y. Saveliev); finite path length differences in divergent beam -> temporal stretch; curved photocathode. General issue for all transport optics. • Head-tail coupling; (part of T. Katsouleas’ talk); for long bunchs in plasma, head defines a “structure” and head-tail coupling is similar to RF structures. For short bunch (blowout/bubble), equivalent “structure” is time-dependent -> head-tail coupling is damped. • Transverse dynamics (A. Reitsma); strong beam loading <-> transverse field modification -> damping of head-tail breakup; helps slice-dependent ‘beta matching’.
UCLA RAL Related topics (Participant input)… • Is it possible to marry the bubble/blowout structure with external injection? • Questions are; Can a witness beam “load” enough to distort the bubble and prevent self-injection? How to precisely place the bunch initially or at the next stage? (see W. Leemans, session1; W. Lu & M. Tzoufras, session2; M. Geissler, session3, A. Pukov, session2) • Beam breakup instability (BBI) in a linac -> betatron motion couples head-to-tail. • Is this the same as hosing? Is there a damping mechanism (e.g., BNS)? • Yes, similar. Yes, ideas for damping (see T. Katsouleas session3; P. Muggli, session1&2). • Electromagnetic Magnus Effect -> non-ideal driver -> meandering of wake vector. • Seen in self-sidescatter (RAL, LBNL). Due to asymmetries in transverse ponderomotive force. Related to laser hosing, but not an instability. Stabilized in plasma channel(?) (B. Bingham, session3; W. Leemans comment). • Synchotron-damping is larger off axis -> halo reduction? Emittance damping? • Effective damping for E ~ TeV. Synchrotron radiation is getting into the codes. For positron emittance damping? (see P. Muggli, ibid).
UCLA RAL Related topics (Participant input, cont.)… • Axicon channel between acceleration stages -> minimize temporal dispersion. • = temporal distortion -> minimize q (see Y. Saveliev session3; N. Lopes, slide 5). • Laser shaping: • Plasma mirror to setup a “matched beam” (pre-erode the head) • Works in simulation. (see W. Lu, session2) Need “FROG” measurements from experiments. • Short length of plasma to increase a0 via photon deceleration. • Seen in simulations; responsible for the “Dream Beams” (L. Silva, slide 6). • Diagnostics and feedback • sub-micron BPM -> Thomson scattering off wake; collection of expelled e- • Technologies could be developed/tested in near-term experiments. • coherent THz -> current profile • Multi-shot autocorrelator (see W. Leemans, session1), single shot electrooptic (see D. Jaroszynski, session4).
Prevent e- bunch expansion…ion-channel guiding Maryland, Texas, Oxford, IST, UCLA etc. technologies Nelson Lopes
UCLA Nonlinear evolution of laser - a0 amplifier Conservation of number of photons classical wave action Initially, no wave breaking |a|/a0 1/ a0 = 3 ctL/lp0 = 1/2 Photon deceleration/frequency downshift Higher a0 leads to wave breaking Nonlinear evolution of laser pulse for long propagation distances leads to single cycle laser pulse with amplified a0 F. Tsung et al, Proc. Natl. Acad. Sci. v. 99, 29 (2002)