Matthew Zepf

Observation of a highly directional γ-ray beam from ultrashort, ultraintense laser pulse interactions with solids

Novel measurements of electromagnetic radiation above 10 MeV are presented for ultra intense laser pulse interactions with solids. A bright, highly directional source of γ rays was observed directly behind the target. The γ rays were produced by bremsstrahlung radiation from energetic electrons generated during the interaction. They were measured using the photoneutron reaction [63Cu(γ,n)62Cu] in copper. The resulting activity was measured by coincidence counting the positron annihilation γ rays which were produced from the decay of 62Cu. New measurements of the bremsstrahlung radiation at 1019 W cm−2 are also presented.

Production of radioactive nuclides by energetic protons generated from intense laser-plasma interactions

Nuclear activation has been observed in materials exposed to the ablated plasma generated from high intensity laser–solid interactions (at focused intensities up to 2×1019 W/cm2) and is produced by protons having energies up to 30 MeV. The energy spectrum of the protons is determined from these activation measurements and is found to be consistent with other ion diagnostics. The possible development of this technique for “table-top” production of radionuclides for medical applications is also discussed.

Energetic proton production from relativistic laser interaction with high density plasmas

Energetic protons up to 30 MeV have been measured from high intensity laser interactions (⩽5×1019 W/cm2) with solid density plasmas. Up to 1012 protons (> 2 MeV) were observed at the rear of thin aluminum foil targets and measurements of their angular deflection were made. Similar energies were measured from ions produced from the front of the foils. Nuclear activation and track detector measurements suggest that the protons measured at the rear originate from the front surface of the target and are bent by large magnetic fields that exist in the plasma interior, which are likely generated by a laser-produced beam of fast electrons.

High power laser production of short-lived isotopes for positron emission tomography

Positron emission tomography (PET) is a powerful diagnostic/imaging technique requiring the production of the short-lived positron emitting isotopes 11C, 13N, 15O and 18F by proton irradiation of natural/enriched targets using cyclotrons. The development of PET has been hampered due to the size and shielding requirements of nuclear installations. Recent results show that when an intense laser beam interacts with solid targets, megaelectronvolt (MeV) protons capable of producing PET isotopes are generated. This report describes how to generate intense PET sources of 11C and 18F using a petawatt laser beam. The work describing the laser production of 18F through a (p,n) 18O reaction, and the subsequent synthesis of 2-[18F] is reported for the first time. The potential for developing compact laser technology for this purpose is discussed.

Nuclear fusion: Fast heating scalable to laser fusion ignition

Rapid heating of a compressed fusion fuel by a short-duration laser pulse is a promising route to generating energy by nuclear fusion, and has been demonstrated on an experimental scale using a novel fast-ignitor geometry. Here we describe a refinement of this system in which a much more powerful, pulsed petawatt (1015 watts) laser creates a fast-heated core plasma that is scalable to full-scale ignition, significantly increasing the number of fusion events while still maintaining high heating efficiency at these substantially higher laser energies. Our findings bring us a step closer to realizing the production of relatively inexpensive, full-scale fast-ignition laser facilities.

Bright quasi-phase-matched soft-x-ray harmonic radiation from argon ions

Selective enhancement (>10(3)) of harmonics extending to the water window (approximately 4 nm) generated in an argon gas filled straight bore capillary waveguide is demonstrated. This enhancement is in good agreement with modeling which indicates that multimode quasi-phase-matching is achieved by rapid axial intensity modulations caused by beating between the fundamental and higher-order capillary modes. Substantial pulse energies (>10 nJ per pulse per harmonic order) at wavelengths beyond the carbon K edge (approximately 4.37 nm, approximately 284 eV) up to approximately 360 eV are observed from argon ions for the first time.

Measurements of ultrastrong magnetic fields during relativistic laser-plasma interactions

Measurements of magnetic fields generated during ultrahigh intensity (>1019 W cm−2), short pulse (0.7–1 ps) laser–solid target interaction experiments are reported. An innovative method is used and the results are compared with particle-in-cell simulations. It is shown that polarization measurements of the self-generated harmonics of the laser can provide a convenient method for diagnosing the magnetic field—and that the experimental measurements indicate the existence of peak fields greater than 340 MG and below 460 MG at such high intensities. In particular, the observation of the X-wave cutoffs and the observed induced ellipticity of the harmonics can provide a reliable method for measuring these fields. These observations are important for evaluating the use of intense lasers in various potential applications and perhaps for understanding the complex physics of exotic astrophysical objects such as neutron stars.

Ultrahigh-intensity laser-produced plasmas as a compact heavy ion injection source

The possibility of using high-intensity laser-produced plasmas as a source of energetic ions for heavy ion accelerators is addressed. Experiments have shown that neon ions greater than 6 MeV can be produced from gas jet plasmas, and well-collimated proton beams greater than 20 MeV have been produced from high intensity laser solid interactions. The proton beams from the back of thin targets appear to be more collimated and reproducible than are high-energy ions generated in the ablated plasma at the front of the target and may be more suitable for ion injection applications. Lead ions have been produced at energies up to 430 MeV.

Characterization of a gamma-ray source based on a laser-plasma accelerator with applications to radiography

The application of high intensity laser-produced gamma rays is discussed with regard to picosecond resolution deep-penetration radiography. The spectrum and angular distribution of these gamma rays is measured using an array of thermoluminescent detectors for both an underdense (gas) target and an overdense (solid) target. It is found that the use of an underdense target in a laser plasma accelerator configuration produces a much more intense and directional source. The peak dose is also increased significantly. Radiography is demonstrated in these experiments and the source size is also estimated.

Experimental studies of the advanced fast ignitor scheme

Guided compression offers an attractive route to explore some of the physics issues of hot electron heating and transport in the fast ignition route to inertial confinement fusion, whilst avoiding the difficulties associated with establishing the stability of the channel formation pulse. X-ray images are presented that show that the guided foil remains hydrodynamically stable during the acceleration phase, which is confirmed by two-dimensional simulations. An integrated conical compression/fast electron heating experiment is presented that confirms that this approach deserves detailed study.