I. Watts

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.

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.

Feasibility study of high harmonic generation from short wavelength lasers interacting with solid targets

Abstract The generation of the third and fourth harmonics from the interaction of a 1 ps, ultraviolet (UV), krypton fluoride (KrF) laser with a solid surface is investigated. The conversion efficiency is seen to increase linearly with Iλ2, with a transition from specular harmonic emission to emission into 2π steradians occurring between 1015 and 1016 W cm−2 μm2. The diffuse emission is strongly dependent on the incidence angle of the laser, with the peak in emission at around 30° being consistent with measurements for resonance absorption. Finally, the conversion efficiencies are found to be in agreement with particle-in-cell (PIC) simulations including appropriate density scalelengths.

Using self-generated harmonics as a diagnostic of high intensity laser-produced plasmas

The interaction of high intensity laser pulses (up to I~1020 W cm−2) with plasmas can generate very high order harmonics of the laser frequency (up to the 75th order have been observed). Measurements of the properties of these harmonics can provide important insights into the plasma conditions which exist during such interactions. For example, observations of the spectrum of the harmonic emission can provide information of the dynamics of the critical surface as well as information on relativistic non-linear optical effects in the plasma. However, most importantly, observations of the polarization properties of the harmonics can provide a method to measure the ultra-strong magnetic fields (greater than 350 MG) which can be generated during these interactions. It is likely that such techniques can be scaled to provide a significant amount of information from experiments at even higher intensities.

Laser induced nuclear reactions

In the last decade the intensities of light fields which can be produced in a laser focus increased by four orders of magnitude from 1016 to 1020 W/cm2. Intensities exceeding 1018 W/cm2 allow for the production of relativistic laser plasmas, that is the quiver energy of plasma electrons reaches the electron rest mass. These plasmas are sources of a whole spectrum of energetic particles, such as highly relativistic electrons, hard bremsstrahlung [13], protons with energies up to a few hundred MeV [7,14], neutrons [4,11,13] and deuterons [4, 17]. These particles can be used to induce nuclear reactions like photo-fission (γ,f) [3,6,8,12], neutron generation by (γ,n)- (p,n)- or (d,n)-reactions, neutron capture or fusion [4, 17].

Nuclear diagnostics of high intensity laser plasma interactions

Nuclear activation has been observed in materials exposed to energetic protons and heavy ions generated from high intensity laser-solid interactions (at focused intensities up to 5×1019 W/cm2). The energy spectrum of the protons is determined through the use of these nuclear activation techniques and is found to be consistent with other ion diagnostics. Heavy ion fusion reactions and large neutron fluxes from the (p, n) reactions were also observed. The reduction of proton emission and increase in heavy ion energy using heated targets was also observed.