What is High Field Science?
Modern laser technology can now consistently produce conditions of incredibly high energy density, which is providing anumber of exciting new regimes of physics to research. Studies of these includeareas of research identified as some of five grand challenges by the recent DOE BESAC report “Directing Matter and Energy: Five Challenges for Science and the Imagination” [BESAC Report]. The report highlights the understanding of non-equilibrium systems, the development of fundamentally new methods for the complete quantum control of electron dynamics, understanding emergent phenomena, better design and tailoring of materials, and nanotechnology. Highintensity laser-matter interactions are naturally almost always far from equilibrium, and hence research in this area will lead to better understanding of non-equilibrium systems. The technologies that develop from high-power lasers, including particle acceleration and radiation generation will ultimately result in new methods for control of electron dynamics, and will result in advanced diagnostics for materials, such as temporal resolution of atomic processes.
Chirped pulse amplification (CPA), firstconceived in the mid-1980s [Strickland OC 1985], now routinely allows incredibly short laser pulses to be amplified to petawatt powers. This method stretches the laser pulse in time, using diffraction gratings to ‘chirp’ the pulse, so that amplification to high energy does not damage the gain medium. The pulse is then recompressed, which further increases the power dramatically. Over the past ten years this advancement in optical engineering has driven the development of ultra-high intensity, ultra-fast laser systems atnational laboratories and universities around the world. Terawatt facilities are now common in universities, petawatt class lasers are now found at a number of government laboratories, and10-100 PW lasers are being proposed for the near future [ELI]. Oneparticularly exciting prospect, enabled by this technology, is the production of compact sources of high quality energetic particle and radiation beams having ultra-short durations. Recent advances are projected to have a major impact on many diverse and inter-related technologies, some with direct relevance to areas of important national need, including homeland security, renewable energy, and advanced medical treatment and diagnosis.
For presently operating laser systems,the focused intensity can reach over 1022 Wcm-2 [LaserFocus World], allowing ultra-relativistic plasma physics to be accomplished.The parameter that parameterizes the physics of high-field matter interactionsis the normalized vector potential, a0 = eE/mecω0,where E is the laser electric field strength, me is the electronmass and ω0 is the laser frequency. The magnitude of the momentum ofan electron due to the accelerating field, normalized to mec, isapproximately given by a0. This means that interactions can beclassed as non-relativistic for a0 <1, for a0 = 1the kinetic energy of an electron is equal to its rest mass, and for a0> 1, the interaction can be considered to be relativistic. For laser intensities exceeding 1022 Wcm-2, and with 1 µmwavelength, which is typical of current technologies, the vector potential approaches a0 = 100, and the interaction can therefore beconsidered ultra-relativistic.
Under these conditions, the physics of matter interactionsis not well understood, as exotic effects such as radiation damping [Dirac Proc. Roy. Soc. 1938, Spohn EPL 2000, Medina JPA 2006, Rohrlich PRD 1999] startto become important, and the propagation of radiation is strongly affected bythe change in the optical properties induced by the strong field. The mostsignificant change, compared with lower intensity interactions, is that of thecomplete expulsion of the electrons, and eventually ions, from the focal region[Sun POF 1987, Pukhov PPCF 2004, Lu POP 2006] due to the ponderomotive force ofthe laser [P Brown PR 1964, Quesnel PRE 1998]. The electromagnetic fields found in plasmas, due tothese density perturbations, can be extremely high and are promising for use asa next generation of particle accelerators [Mangles PT Roy. Soc. 2006]. Tunable, short duration radiation sources based onfree electron lasers or harmonic generation will provide useful diagnostics formany areas of scientific research. In addition, the intensities of the nextgeneration of lasers [ELI] would reach a regime where the energy densities areso high that production of exotic matter could test quantum electrodynamics(QED) theory [Piazza EPJ-ST2008]. Notably, at I~ 1029 Wcm-2, the electric field is sufficient to separate electron-positron pairs from the zero-point energy vacuum fluctuations [Schwinger PR 1951].