Laser-plasma accelerators (LPAs) are capable of accelerating charged particles to very high energies in very compact structures. In theory, therefore, they offer advantages over conventional, large-scale particle accelerators. However, the energy gain in a single-stage LPA can be limited by laser diffraction, dephasing, electron-beam loading and laser-energy depletion. The problem of laser diffraction can be addressed by using laser-pulse guiding and preformed plasma waveguides to maintain the required laser intensity over distances of many Rayleigh lengths; dephasing can be mitigated by longitudinal tailoring of the plasma density; and beam loading can be controlled by proper shaping of the electron beam. To increase the beam energy further, it is necessary to tackle the problem of the depletion of laser energy, by sequencing the accelerator into stages, each powered by a separate laser pulse. Here, we present results from an experiment that demonstrates such staging. Two LPA stages were coupled over a short distance (as is needed to preserve the average acceleration gradient) by a plasma mirror. Stable electron beams from a first LPA were focused to a twenty-micrometre radius--by a discharge capillary-based active plasma lens--into a second LPA, such that the beams interacted with the wakefield excited by a separate laser. Staged acceleration by the wakefield of the second stage is detected via an energy gain of 100 megaelectronvolts for a subset of the electron beam. Changing the arrival time of the electron beam with respect to the second-stage laser pulse allowed us to reconstruct the temporal wakefield structure and to determine the plasma density. Our results indicate that the fundamental limitation to energy gain presented by laser depletion can be overcome by using staged acceleration, suggesting a way of reaching the electron energies required for collider applications.

Compact, tunable, radially symmetric focusing of electrons is critical to laser-plasma accelerator (LPA) applications. Experiments are presented demonstrating the use of a discharge-capillary active plasma lens to focus 100-MeV-level LPA beams. The lens can provide tunable field gradients in excess of 3000 T/m, enabling cm-scale focal lengths for GeV-level beam energies and allowing LPA-based electron beams and light sources to maintain their compact footprint. For a range of lens strengths, excellent agreement with simulation was obtained.

During the past decade, anterior approaches to the spine have been shown to be much more effective than laminectomy for the relief of pain and neurological deficits due to vertebral metastases. Laminectomy has failed because it does not allow adequate decompression of epidural lesions anterior to the thecal sac. In an effort to combine the advantages of the posterior approach with an adequate decompression, a one-stage posterolateral decompression-stabilization procedure was performed on nine patients with thoracolumbar spine tumors. The approach has been used for decompression and stabilization after thoracolumbar burst fractures. Marked lasting improvement was seen in all six patients with preoperative neurological deficits and in four patients with severe back pain and/or radiculopathy. Three nonambulators and two marginal ambulators could walk postoperatively without assistance. Of five patients who were working preoperatively, four returned full-time to their prior occupations. Three patients had serious complications, including one early postoperative death. No patient deteriorated neurologically due to the procedure. Although the series is small, it demonstrates that adequate one-stage decompression-stabilization of spinal epidural lesions is possible via the posterolateral approach and should be considered in certain cases as an alternative to the anterior approach.