High resolution energy-angle correlation measurement of hard x rays from laser-Thomson backscattering.

Thomson backscattering of intense laser pulses from relativistic electrons not only allows for the generation of bright x-ray pulses but also for the investigation of the complex particle dynamics at the interaction point. For this purpose a complete spectral characterization of a Thomson source powered by a compact linear electron accelerator is performed with unprecedented angular and energy resolution. A rigorous statistical analysis comparing experimental data to 3D simulations enables, e.g., the extraction of the angular distribution of electrons with 1.5% accuracy and, in total, provides predictive capability for the future high brightness hard x-ray source PHOENIX (photon electron collider for narrow bandwidth intense x rays) and potential gamma-ray sources.

Enhanced proton acceleration by an ultrashort laser interaction with structured dynamic plasma targets.

We experimentally demonstrate a notably enhanced acceleration of protons to high energy by relatively modest ultrashort laser pulses and structured dynamical plasma targets. Realized by special deposition of snow targets on sapphire substrates and using carefully planned prepulses, high proton yields emitted in a narrow solid angle with energy above 21 MeV were detected from a 5 TW laser. Our simulations predict that using the proposed scheme protons can be accelerated to energies above 150 MeV by 100 TW laser systems.

Laser-driven proton oncology--a unique new cancer therapy?

In 2000, the University of Strathclyde, collaborating with the Rutherford Appleton Laboratory, organized the first workshop dealing with the potential of high-power laser technology in medicine. Two areas of potential were identified: firstly the production of positron emission tomography (PET) isotopes; and secondly, the potential for laser-accelerated proton and heavy ion beams for therapy. The attendees, mainly clinicians and radiation physicists, emphasised that the laser community should concentrate on developing laser and target technology for therapy rather than isotope production because of the potential advantages over conventional accelerator technology for that purpose. On the 30 March 2007, the universities of Strathclyde and Paisley organized a follow-up meeting to identify the progress made in laser-driven proton and ion beam technology with applications leading to proton and ion beam therapy for deep-seated tumours. The meeting was supported by the Scottish Universities Physics Alliance (SUPA)--an organization set up in Scotland to bring together all of the physics departments collaborating with life scientists to work on ground-breaking new science which no single university could attempt. This is a summary of the meeting.

Lateral electron transport in high-intensity laser-irradiated foils diagnosed by ion emission.

An experimental investigation of lateral electron transport in thin metallic foil targets irradiated by ultraintense (>or=10(19) W/cm2) laser pulses is reported. Two-dimensional spatially resolved ion emission measurements are used to quantify electric-field generation resulting from electron transport. The measurement of large electric fields ( approximately 0.1 TV/m) millimeters from the laser focus reveals that lateral energy transport continues long after the laser pulse has decayed. Numerical simulations confirm a very strong enhancement of electron density and electric field at the edges of the target.

Radiological characterisation of photon radiation from ultra-high-intensity laser-plasma and nuclear interactions.

With the increasing number of multi-terawatt (10(12) W) and petawatt (10(15) W) laser interaction facilities being built, the need for a detailed understanding of the potential radiological hazards is required and their impact on personnel is of major concern. Experiments at a number of facilities are being undertaken to achieve this aim. This paper describes the recent work completed on the Vulcan petawatt laser system at the CCLRC Rutherford Appleton Laboratory, where photon doses of up to 43 mSv at 1 m per shot have been measured during commissioning studies. It also overviews the shielding in place on the facility in order to comply with the Ionising Radiation Regulations 1999 (IRR99), maintaining a dose to personnel of less than 1 mSv yr(-1) and as low as reasonably practicable (ALARP).

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.

Broad energy spectrum of laser-accelerated protons for spallation-related physics.

A beam of MeV protons, accelerated by ultraintense laser-pulse interactions with a thin target foil, is used to investigate nuclear reactions of interest for spallation physics. The laser-generated proton beam is shown (protons were measured) to have a broad energy distribution, which closely resembles the expected energy spectrum of evaporative protons (below 50 MeV) produced in GeV-proton-induced spallation reactions. The protons are used to quantify the distribution of residual radioisotopes produced in a representative spallation target (Pb), and the results are compared with calculated predictions based on spectra modeled with nuclear Monte Carlo codes. Laser-plasma particle accelerators are shown to provide data relevant to the design and development of accelerator driven systems.

Interaction mechanism of some alkyl iodides with femtosecond laser pulses.

The interaction of 1-iodopropane, 2-iodopropane, 1-iodobutane, 2-iodobutane, and 1-iodopentane with (5 x 10(13-)5 x 10(15) W/cm2) femtosecond laser fields is studied by means of a time-of-flight mass spectrometer. It is found that multiphoton ionization (MPI) and field ionization (FI) processes are involved in the molecular ionization. The contribution of these processes can be distinguished using the peak profile of the ions in the mass spectra. Thus, from the mass spectra of 2-iodoropane and 2-iodobutane, it is concluded that MPI processes are taking place even for Keldysh parameter values gamma approximately 0.3. The field ionization process depends on the characteristics of the molecular binding potential well and leads to an asymmetric charge distribution of the transient multiply charged parent ions. In the case of 1-iodobutane, the MPI processes lead to a stable doubly charged parent ion production with a laser intensity threshold higher than that found for I2+ ions. In addition, the isomers studied exhibit distinct differences in their mass spectra and their origin is discussed in detail.

Observations of the filamentation of high-intensity laser-produced electron beams.

Filamented electron beams have been observed to be emitted from the rear of thin solid targets irradiated by a high-intensity short-pulse laser when there is low-density plasma present at the back of the target. These observations are consistent with a laser-generated beam of relativistic electrons propagating through the target, which is subsequently fragmented by a Weibel-like instability in the low-density plasma at the rear. These measurements are in agreement with particle-in-cell simulations and theory, since the filamentation instability is predicted to be dramatically enhanced when the electron beam density approaches that of the background plasma.

Characterization of proton and heavier ion acceleration in ultrahigh-intensity laser interactions with heated target foils.

Proton and heavy ion acceleration in ultrahigh intensity ( approximately 2 x 10(20) W cm(-2) ) laser plasma interactions has been investigated using the new petawatt arm of the VULCAN laser. Nuclear activation techniques have been applied to make the first spatially integrated measurements of both proton and heavy ion acceleration from the same laser shots with heated and unheated Fe foil targets. Fe ions with energies greater than 10 MeV per nucleon have been observed. Effects of target heating on the accelerated ion energy spectra and the laser-to-ion energy conversion efficiencies are discussed. The laser-driven production of the long-lived isotope (57 )Co (271 days) via a heavy ion induced reaction is demonstrated.

High-intensity-laser-driven Z pinches.

Experiments were performed in which ultrahigh intensity laser pulses (I>5 x 10(19) W cm(-2)) were used to irradiate thin wire targets. It was observed that such interactions generate a large number of relativistic electrons which escape the target and induce multimega ampere return currents within the wire. MHD instabilities can subsequently be observed in the pinching plasma along with field emission of electrons from nearby objects. Coherent optical transition radiation from adjacent objects was also observed.

Demonstration of fusion-evaporation and direct-interaction nuclear reactions using high-intensity laser-plasma-accelerated ion beams.

Heavy-ion induced nuclear reactions in materials exposed to energetic ions produced from high-intensity (approximately 5 x 10(19) W/cm(2)) laser-solid interactions have been experimentally investigated for the first time. Many of the radionuclides produced result from the creation of "compound nuclei" with the subsequent evaporation of proton, neutron, and alpha particles. Results are compared with previous measurements with monochromatic ion beams from a conventional accelerator. Measured nuclide yields are used to diagnose the acceleration of ions from laser-ablated plasma to energies greater than 100 MeV.

Propagation instabilities of high-intensity laser-produced electron beams.

Measurements of energetic electron beams generated from ultrahigh intensity laser interactions (I>10(19) W/cm(2)) with dense plasmas are discussed. These interactions have been shown to produce very directional beams, although with a broad energy spectrum. In the regime where the beam density approaches the density of the background plasma, we show that these beams are unstable to filamentation and "hosing" instabilities. Particle-in-cell simulations also indicate the development of such instabilities. This is a regime of particular interest for inertial confinement fusion applications of these beams (i.e., "fast ignition").

Experimental study of proton emission from 60-fs, 200-mJ high-repetition-rate tabletop-laser pulses interacting with solid targets.

Measurements of proton emission have been made from a variety of solid targets irradiated by a 60-fs, 200-mJ, 7 x 10(18)-W cm(-2) laser system operating at 2 Hz. Optimum target conditions were found in terms of target material and thickness. For Mylar targets of thickness 20-40 microm, a maximum proton energy of 1.5 MeV was measured. For aluminum targets, a maximum energy of 950 keV was measured for 12 microm, and for copper, 850 keV for 12.5 microm.

Applications for nuclear phenomena generated by ultra-intense lasers.

The amplification of laser light to generate powers large enough to affect the nucleus has been the desire of scientists since the invention of the laser 40 years ago. Many lasers, including tabletop varieties, now have pulse powers greater than the electrical power generated by all the world's power plants combined. When this power is focused to dimensions of a few microns, laser-driven nuclear phenomena can occur. Here we review the developments in this research field and describe the potential of laser produced proton, neutron, and heavy ion beams, together with isotope and isomer production.

Femtosecond laser time-of-flight mass spectrometry of labile molecular analytes: laser-desorbed nitro-aromatic molecules.

Femtosecond laser time-of-flight mass spectra of solid samples of trinitrobenzene (TNB), trinitrotoluene (TNT) and trinitrophenol (TNP) have been recorded. Desorption of the solid samples was enacted by the fourth harmonic output (266 nm) of a 5 ns Nd:YAG laser. Subsequent femtosecond post-ionisation of the plume of neutral molecules was achieved using 800 nm laser pulses of 80 fs duration. Mass spectra have been recorded for desorption laser intensities from 2-6 x 10(9) W cm(-2) with ionisation laser intensities between 2 x 10(14) and 6 x 10(15) W cm(-2). Femtosecond laser ionisation has been shown to be capable of generating precursor and characteristic high-mass fragment ions for labile nitro-aromatic molecules commonly used in high-explosive materials. This feature is critical in the future development of femtosecond laser-based analytical instruments that can be used for complex molecular identification and quantitative analysis of environmentally important labile molecules. Furthermore, a comparison of femtosecond post-ionisation mass spectra with standard 70 eV electron impact data has revealed similarities in the spectra and hence the fragmentation processes.