No abstract

Each successive generation of X-ray machines has opened up new frontiers in science, such as the first radiographs and the determination of the structure of DNA. State-of-the-art X-ray sources can now produce coherent high-brightness Xrays of greater than kiloelectronvolt energy and promise a new revolution in imaging complex systems on nanometre and femtosecond scales. Despite the demand, only a few dedicated synchrotron facilities exist worldwide, in part because of the size and cost of conventional (accelerator) technology 1 . Here we demonstrate the use of a new generation of laserdriven plasma accelerators 2 , which accelerate high-charge electron beams to high energy in short distances 3-5 , to produce directional, spatially coherent, intrinsically ultrafast beams of hard X-rays. This reduces the size of the synchrotron source from the tens of metres to the centimetre scale, simultaneously accelerating and wiggling the electron beam. The resulting X-ray source is 1,000 times brighter than previously reported plasma wigglers 6,7 and thus has the potential to facilitate a myriad of uses across the whole spectrum of light-source applications.There are a number of proposals to use extreme nonlinear interactions of the latest generation of high-power ultrashort-pulse laser systems to produce beams of high-energy photons with high brightness and short pulse duration. For example, high-order harmonic generation promises trains of coherent pulselets 8 and Compton scattering could extend energies into the γ -regime 9,10 . An alternative proposal has been the use of compact laser-plasma accelerators to drive sources of undulating/wiggling radiation 11 .These accelerators use the plasma wakefield generated by the passage of an intense laser pulse through an underdense plasma 12 . Such wakefields can have intrinsic fields of more than 1,000 times greater than the best achievable by conventional accelerator technology, and thus can accelerate particles to high energies in a fraction of the distance. Recently, it has been demonstrated that at high laser power, the wakefield can be driven to sufficient amplitude to be able to trap large numbers of particles (>100 pC) from the background plasma and accelerate them in a narrow energy spread beam 3-5 , now producing beams of electrons of gigaelectronvoltscale energy of the order of 1 cm (refs 13,14).Such electron sources are of interest to replace the accelerators that drive current synchrotron sources, and typically use multiple periods of alternately poled magnets (undulators or wigglers) to reinforce the synchrotron emission over a length of a few metres. The first demonstrations of wakefield-driven radiation using external wigglers have also been reported, though still being limited to optical or near-optical wavelengths and modest peak brightness 15,16 .However, the particles being accelerated in the plasma accelerator also undergo transverse (betatron) oscillations when subject to the focusing fields of the plasma wave. These oscillations occur at the betatron frequen...

The problem of the 'hole-boring' (HB)-type of radiation pressure acceleration of ions by circularly polarized laser pulses interacting with overdense plasmas is considered in the regime where the dimensionless scaling parameter I/ρc 3 becomes large. In this regime a non-relativistic treatment of the 'HB' problem is no longer adequate. A new set of fully relativistic formulae for the mean ion energy and 'HB' velocity is derived and validated against one-dimensional particle-in-cell simulations. It is also found that the finite acceleration time of the ions results in large energy spreads in the accelerated ion beam even under the highly idealized conditions of constant laser intensity and uniform mass density.

The acceleration of electrons to approximately 0.8 GeV has been observed in a self-injecting laser wakefield accelerator driven at a plasma density of 5.5x10(18) cm(-3) by a 10 J, 55 fs, 800 nm laser pulse in the blowout regime. The laser pulse is found to be self-guided for 1 cm (>10zR), by measurement of a single filament containing >30% of the initial laser energy at this distance. Three-dimensional particle in cell simulations show that the intensity within the guided filament is amplified beyond its initial focused value to a normalized vector potential of a0>6, thus driving a highly nonlinear plasma wave.

Transport modeling of idealized, cone-guided fast ignition targets indicates the severe challenge posed by fast-electron source divergence. The hybrid particle-in-cell [PIC] code Zuma is run in tandem with the radiation-hydrodynamics code Hydra to model fast-electron propagation, fuel heating, and thermonuclear burn. The fast electron source is based on a 3D explicit-PIC laser-plasma simulation with the PSC code. This shows a quasi two-temperature energy spectrum, and a divergent angle spectrum (average velocityspace polar angle of 52 • ). Transport simulations with the PIC-based divergence do not ignite for > 1 MJ of fast-electron energy, for a modest (70 µm) standoff distance from fast-electron injection to the dense fuel. However, artificially collimating the source gives an ignition energy of 132 kJ. To mitigate the divergence, we consider imposed axial magnetic fields. Uniform fields ∼50 MG are sufficient to recover the artificially collimated ignition energy. Experiments at the Omega laser facility have generated fields of this magnitude by imploding a capsule in seed fields of 50-100 kG. Such imploded fields are however more compressed in the transport region than in the laser absorption region. When fast electrons encounter increasing field strength, magnetic mirroring can reflect a substantial fraction of them and reduce coupling to the fuel. A hollow magnetic pipe, which peaks at a finite radius, is presented as one field configuration which circumvents mirroring.