M. I. K. Santala

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.

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.

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.

Fast particle generation and energy transport in laser-solid interactions

The generation of MeV electron and ion beams using lasers with intensities of up to 1020 W cm−2 is reported. Intense ion beams with high energies (up to 40 MeV and to 3×1012 protons >5 MeV) are observed. The properties of these particle beams were measured in considerable detail and the results are compared to current theoretical explanations for their generation.

The effect of high intensity laser propagation instabilities on channel formation in underdense plasmas

Experiments have been performed using high power laser pulses (up to 50 TW) focused into underdense helium plasmas (ne⩽5×1019 cm−3). Using shadowgraphy, it is observed that the laser pulse can produce irregular density channels, which exhibit features such as long wavelength hosing and “sausage-like” self-focusing instabilities. This phenomenon is a high intensity effect and the characteristic period of oscillation of these instabilities is typically found to correspond to the time required for ions to move radially out of the region of highest intensity.

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].

Diagnosis of peak laser intensity from high-energy ion measurements during intense laser interactions with underdense plasmas

Experiments were performed using high-power laser pulses (greater than 50 TW) focused into underdense helium, neon, or deuterium plasmas (ne [less-than-or-equal] 5 × 1019 cm−3). Ions having energies greater than 300 keV were measured to be produced primarily at 90° to the axis of laser propagation. Ion energies greater than 6 MeV were recorded from interactions with neon. Spatially resolved pinhole images of the ion emission were also obtained and were used to estimate the intensity of the focused radiation in the interaction region.

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.