Direct Observation of the Injection Dynamics of a Laser Wakefield Accelerator Using Few-Femtosecond Shadowgraphy.

We present few-femtosecond shadowgraphic snapshots taken during the nonlinear evolution of the plasma wave in a laser wakefield accelerator with transverse synchronized few-cycle probe pulses. These snapshots can be directly associated with the electron density distribution within the plasma wave and give quantitative information about its size and shape. Our results show that self-injection of electrons into the first plasma-wave period is induced by a lengthening of the first plasma period. Three-dimensional particle-in-cell simulations support our observations.

K-shell spectroscopy of silicon ions as diagnostic for high electric fields.

We developed a detection scheme, capable of measuring X-ray line shape of tracer ions in µm thick layers at the rear side of a target foil irradiated by ultra intense laser pulses. We performed simulations of the effect of strong electric fields on the K-shell emission of silicon and developed a spectrometer dedicated to record this emission. The combination of a cylindrically bent crystal in von Hámos geometry and a CCD camera with its single photon counting capability allows for a high dynamic range of the instrument and background free spectra. This approach will be used in future experiments to study electric fields of the order of TV/m at high density plasmas close to solid density.

Harmonic generation from relativistic plasma surfaces in ultrasteep plasma density gradients.

Harmonic generation in the limit of ultrasteep density gradients is studied experimentally. Observations reveal that, while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale lengths (L(p)/λ < 1), the absolute efficiency of the harmonics declines for the steepest plasma density scale length L(p)→0, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the relativistic oscillating mirror was estimated to be in the range of 10(-4)-10(-6) of the laser pulse energy for photon energies ranging from 20-40 eV, with the best results being obtained for an intermediate density scale length.

Note: A large aperture four-mirror reflective wave-plate for high-intensity short-pulse laser experiments.

We report on a four-mirror reflective wave-plate system based on a phase-shifting mirror (PSM) for a continuous variation of elliptical polarization without changing the beam position and direction. The system presented and characterized here can replace a conventional retardation plate providing all advantages of a PSM, such as high damage-threshold, large scalability, and low dispersion. This makes reflective wave-plates an ideal tool for ultra-high power laser applications.

Controlling the spacing of attosecond pulse trains from relativistic surface plasmas.

When a laser pulse hits a solid surface with relativistic intensities, XUV attosecond pulses are generated in the reflected light. We present an experimental and theoretical study of the temporal properties of attosecond pulse trains in this regime. The recorded harmonic spectra show distinct fine structures which can be explained by a varying temporal pulse spacing that can be controlled by the laser contrast. The pulse spacing is directly related to the cycle-averaged motion of the reflecting surface. Thus the harmonic spectrum contains information on the relativistic plasma dynamics.

Laser-plasma acceleration of quasi-monoenergetic protons from microstructured targets.

Particle acceleration based on high intensity laser systems (a process known as laser-plasma acceleration) has achieved high quality particle beams that compare favourably with conventional acceleration techniques in terms of emittance, brightness and pulse duration. A long-term difficulty associated with laser-plasma acceleration--the very broad, exponential energy spectrum of the emitted particles--has been overcome recently for electron beams. Here we report analogous results for ions, specifically the production of quasi-monoenergetic proton beams using laser-plasma accelerators. Reliable and reproducible laser-accelerated ion beams were achieved by intense laser irradiation of solid microstructured targets. This proof-of-principle experiment serves to illuminate the role of laser-generated plasmas as feasible particle sources. Scalability studies show that, owing to their compact size and reasonable cost, such table-top laser systems with high repetition rates could contribute to the development of new generations of particle injectors that may be suitable for medical proton therapy.

Is there a role for heavy ion beam therapy?

The aim of this contribution is to review the radio-oncological rationale of heavy ion beam radiotherapy in the management of cancer. Protons and helium ions are being investigated because of the improved dose distributions, perhaps superior in many clinical situations to those obtainable with photons or electrons. Heavy ions also bear the advantage of superior dose distribution and may additionally provide higher biological effectivity. A substantial database of historical results supports the hypothesis that conformal radiotherapy is superior to conventional radiotherapy. For the selection of patients for ion beam therapy, the following questions have to be evaluated: Can any significant radiation morbidity by conventional treatment expected? Is the radiation morbidity caused by unintended irradiation of non-target tissue outside the planning target volume? Can the tumor control be improved due to dose escalation with less radiation morbidity? A substantial number of patients have been treated by light ion radiotherapy. There are only a few clinical trials attempting to compare conventional photon radiotherapy with light ion radiotherapy. Clinical results with heavy ions such as carbon ions seem very promising in certain tumors. However results of randomized trials are still to be seen. Clearly the control arm has to be designed to be as close as possible to the optimal photon/electron treatment methods in use. The feasibility of heavy ion treatment has been demonstrated worldwide, with more than 10,000 patients. In the near future more centers worldwide will start patient treatments. The wider availability of light ion beams for clinical use will enable the establishment of a database of clinical results and the elucidation of the role of heavy ion beams in the treatment of cancer.