Introduction to Plasma Physics and Plasma-based Acceleration Wakefield acceleration Various images provided by R. Bingham

Particle acceleration –Accelerate particles in an electric field –Hard to sustain constant electric field over long distance: use wave that travels with particle at (almost) the same speed, just below c

Plasma-based acceleration –Maximum field amplitude in vacuum accelerator: MV/m –Maximum field amplitude in plasma accelerator: GV/m –Scale back ultrahigh energy accelerators from kilometre to metre size !?

Conventional vs. plasma Wakefield Acceleration Experiments at the Stanford Linear Accelerator Centre 42 -> 85 GeV in 1 meter of plasma!

How to drive a plasma wave –Send driver (powerful laser pulse or high- energy particle beam) into plasma –Driver will blast plasma electrons out of the way, leading to region with net positive charge –Behind driver, electrons will rush back and “overshoot”, leading to region with net negative charge –This process is repeated a few times, leading to a plasma wave that travels at the speed of the driver

e-beam driven wakefield Laser-driven wakefield is similar

Types of plasma accelerator –Beam-driven plasma accelerators –Electron-electron accelerators –Electron-positron or positron-positron accelerators –Laser-driven plasma accelerators –Laser beat-wave accelerator –Self-modulated laser wakefield accelerator –“Forced” laser wakefield accelerator

History 1979: T. Tajima and J. Dawson, “Laser Electron Accelerator”, Phys. Rev. Lett. 43, : D. Strickland and G. Mourou develop “chirped pulse amplification” to generate intense laser pulses 1993: First observations of laser-accelerated electrons at UCLA (led by C. Joshi) : Discovery of “bubble”-shaped wakefield solutions in theoretical models : First observations of particle acceleration in beam-driven wakefields (UCLA-SLAC collaboration) 2004: First observations of laser-accelerated electron bunches with small energy spread: Dream Beam on the cover of Nature (RAL, LBNL, LOA) 2005: Beam-driven acceleration breaches 1 GeV barrier (UCLA-SLAC) 2006: Laser-driven acceleration breaches 1 GeV barrier (LBNL-Oxford) 2007: Energy doubling in beam-driven acceleration: 42 GeV -> 85 GeV (UCLA-SLAC)

Laser electron accelerator –Closely tied to development of high-power short-pulse lasers –Early schemes (when laser pulses were “long”): –beat-wave accelerator (plasma wave driven by beating of the two frequencies of a CO 2 -laser) –self-modulated laser wakefield accelerator (plasma wave “modulates” long pulse to form train of short pulses, which then drives plasma wave resonantly) –Today’s ultra-powerful ( TW), ultra-short (<50 fs) laser pulses can drive a plasma wave in one go

Two stages –Electron injection –External injection –All-optical (internal) injection –Electron acceleration –Laser pulse needs guiding –Electron bunch may diverge or perform transverse oscillations

External injection –Use photo-cathode or other method to produce short electron bunch –Inject this bunch into the plasma wave –Timing issues, bunch length issues –No longer a favoured method

All-optical injection Used in (almost) all experiments that produce high-energy low-spread electron bunches

Acceleration Create sufficiently long plasma column –Laser usually ionises background gas itself Keep laser pulse under control –Laser pulse diverges, needs to be self-guided or externally guided –Laser pulse depletes, limiting acceleration length –Laser pulse may “jump around” from shot to shot Keep electron bunch under control –Try to minimise energy spread upon injection –Acceleration stops when bunch outruns wave: dephasing –Electron bunch may oscillate transversely: betatron motion –Electron bunch may diverge

Main limits to energy gain Dephasing : Diffraction : Depletion : For small a 0 >> L dph For a 0 >~ 1L dph ~ L depl order mm! (but overcome w/ channels or relativistic self-focusing) c V gr order 10 cm x /n o

Laser pulse guiding Self-guiding: –Electrons oscillate rapidly in laser field, mass increases, plasma frequency decreases –Change in refractive index causes laser pulse to focus External guiding: –Laser pulse passes through plasma channel with depressed on-axis density –This channel acts like a glass fiber and confines the laser pulse

Bubble regime –Ultra-intense driver (laser pulse, particle beam) blasts almost all plasma electrons aside –A single “bubble” is formed behind the driver, containing only ions, with good properties for acceleration Images: L.O. Silva, IST Lisbon

Bubble regime Solves some of our problems: –Plasma electrons self-injected –Accelerating field inside bubble mostly constant -> small energy spread –Bubble provides focusing field that coincides with accelerating field -> small transverse spread –Bubble is a stable structure: acceleration over large distances possible (Virtually) All groundbreaking recent results have been obtained via such bubbles

Current status: laser wakefield 1 GeV mean energy with 5% spread (LBNL-Oxford collaboration). W. Leemans et al., Nature Physics 2, 696 (2006).

Current status: beam-driven wakefield Energy doubling of 42 GeV electrons to reach 85 GeV in 1 meter of plasma I. Blumenfeld et al., Nature 445, 741 (2007). But still need to get 42 GeV to start things up.

Near future Using the future 10 PW upgrade to the Vulcan laser at RAL, the following results may be within reach: – 40nC at 1GeV (large spread) – 14nC at 4 GeV (medium spread) – 2nC at 10GeV (small spread) Energy-wise, laser-wakefield accelerators are starting to compete

Remaining issues Energy spread: current best is 1-2 %, needed is 0.1% or less Mean energy: always need more, so we need a bigger laser Guiding: external guiding yields better results, self-guiding much easier Reproducibility: need a more stable laser that still fires at a high repetition rate and at high power…

Summary Introduction to plasma-based acceleration, wakefield generation, strengths of this scheme and remaining issues Much progress has been made, and much more to be expected in this dynamic field Many issues still to be resolved Much more ground to cover, which will happen in the coming weeks