B. Ramakrishna

The TARANIS laser: A multi-Terawatt system for laser-plasma investigations

The multi-Terawatt laser system, terawatt apparatus for relativistic and nonlinear interdisciplinary science, has been recently installed in the Centre for Plasma Physics at the Queen’s University of Belfast. The system will support a wide ranging science program, which will include laser-driven particle acceleration, X-ray lasers, and high energy density physics experiments. Here we present an overview of the laser system as well as the results of preliminary investigations on ion acceleration and X-ray lasers, mainly carried out as performance tests for the new apparatus. We also discuss some possible experiments that exploit the flexibility of the system in delivering pump-probe capability.

MeV negative ion generation from ultra-intense laser interaction with a water spray

MeV negative oxygen ions are obtained from a water spray target irradiated by high intensity (5 × 1019 W/cm2) and ultrashort (50 fs) laser pulses. Generation of negative ions is ascribed to electron-capture processes that the laser-accelerated high-energy positive ion experiences when it interacts with atoms in the spray. This mechanism implies the existence of a large number of MeV neutral oxygen atoms, which is consistent with indirect experimental evidence.

Laser-driven quasimonoenergetic proton burst from water spray target

A narrow band proton bursts at energies of 1.6±0.08 MeV were observed when a water spray consisting of ∅(150 nm)-diameter droplets was irradiated by an ultrashort laser pulse of about 45 fs duration and at an intensity of 5×1019 W/cm2. The results are explained by a Coulomb explosion of sub-laser-wavelength droplets composed of two ion species. The laser prepulse plays an important role. By pre-evaporation of the droplets, its diameter is reduced so that the main pulse can interact with a smaller droplet, and this remaining bulk can be ionized to high states. In the case of water, the mixture of quite differently charged ions establishes an “iso-Coulomb-potential” during the droplet explosion such that protons are accelerated to a peak energy with a narrow energy spread. The model explains this crucial point, which differs critically from usual Coulomb explosion or ion sheath acceleration mechanisms.

Complementary ion and extreme ultra-violet spectrometer for laser-plasma diagnosis

Simultaneous detection of extreme ultra-violet (XUV) and ion emission along the same line of sight provides comprehensive insight into the evolution of plasmas. This type of combined spectroscopy is applied to diagnose laser interaction with a spray target. The use of a micro-channel-plate detector assures reliable detection of both XUV and ion signals in a single laser shot. The qualitative analysis of the ion emission and XUV spectra allows to gain detailed information about the plasma conditions, and a correlation between the energetic proton emission and the XUV plasma emission can be suggested. The measured XUV emission spectrum from water spray shows efficient deceleration of laser accelerated electrons with energies up to keV in the initially cold background plasma and the collisional heating of the plasma.

Experimental investigation of hole boring and light sail regimes of RPA by varying laser and target parameters

Temporal evolution of plasma jets from micrometre-scale thick foils following the interaction of intense (3???1020?W?cm?2) laser pulses is studied systematically by time resolved optical interferometry. The fluid velocity in the plasma jets is determined by comparing the data with 2D hydrodynamic simulation, which agrees with the expected hole-boring (HB) velocity due to the laser radiation pressure. The homogeneity of the plasma density across the jets has been found to be improved substantially when irradiating the laser at circular polarization compared to linear polarization. While overdense plasma jets were formed efficiently for micrometre thick targets, decreasing the target areal density and/or increasing the irradiance on the target have provided indication of transition from the ?HB? to the ?light sail (LS)? regime of RPA, characterized by the appearance of narrow-band spectral features at several MeV/nucleon in proton and carbon spectra.

New developments in energy transfer and transport studies in relativistic laser?plasma interactions

Two critical issues related to the success of fast ignition inertial fusion have been vigorously investigated in a co-ordinated campaign in the European Union and the United States. These are the divergence of the fast electron beam generated in intense, PW laser–plasma interactions and the fast electron energy transport with the use of high intensity contrast ratio laser pulses. Proof is presented that resistivity gradient-induced magnetic fields can guide fast electrons over significant distances in (initially) cold metallic targets. Comparison of experiments undertaken in both France and the United States suggests that an important factor in obtaining efficient coupling into dense plasma is the irradiation with high intensity contrast ratio laser pulses, rather than the colour of the laser pulse itself.

Ion source development and radiobiology applications within the LIBRA project

In view of their properties, laser-driven ion beams have the potential to be employed in innovative applications in the scientific, technological and medical areas. Among these, a particularly high-profile application is particle therapy for cancer treatment, which however requires significant improvements from current performances of laser-driven accelerators. The focus of current research in this field is on developing suitable strategies enabling laser-accelerated ions to match these requirements, while exploiting some of the unique features of a laser-driven process. LIBRA is a UK-wide consortium, aiming to address these issues, and develop laser-driven ion sources suitable for applicative purposes, with a particular focus on biomedical applications. We will report on the activities of the consortium aimed to optimizing the properties of the beams, by developing and employing advanced targetry and by exploring novel acceleration regimes enabling production of beams with reduced energy spread. Employing the TARANIS Terawatt laser at Queens University, we have initiated a campaign investigating the effects of proton irradiation of biological samples at extreme dose rates (> 109 Gy/s).

Observation of Quasi Mono-Energetic Protons in Laser Spray-Target Interaction

Laser driven ion acceleration arises from charge separation effects caused by an ultrahigh intensity laser pulse. Limited mass targets confine the accelerated electrons within the target size and prevent the large area spreading seen in extended foil targets. Furthermore, if the target size is smaller than the laser wavelength and focal spot diameter, homogeneous heating of the target is ensured. Observation of quasi‐monoenergetic protons in the interaction of a high intensity high contrast laser pulse at 5×1019 W/cm2 with 150 nm—diameter water droplets is investigated. An ensemble of such objects is formed in a spray. Quasi mono energetic proton bursts of energy E∼1.6 MeV are observed and are associated with a specific ionization and explosion dynamics of the spheres.

Magnetic collimation of petawatt driven fast electron beam for prospective fast ignition studies

Collimated transport of fast electron beam through solid density matter is one of the key issues behind the success of the fast ignition scheme by means of which the required amount of ignition energy can be delivered to the hot spot region of the compressed fuel. Here we report on a hot electron beam collimation scheme in solids by tactfully using the strong magnetic fields generated by an electrical resistivity gradient according to Faradays law. This was accomplished by appropriately fabricating the targets in such a way that the electron beam is directed to flow in a metal which is embedded in a much lower resistivity and atomic number metal. Experimental results showed guided transport of hot electron beam over hundreds of microns length inside solid density plasma, which were obtained from two experiments examining the scheme for petawatt laser driven hot electron beam while employing various target configurations.

Field dynamics and filament growth following high-intensity laser-solid interactions

The electromagnetic fields generated following the interaction of a 3 × 1019 W cm−2 infrared pulse with a gold wire have been investigated using the well-documented proton radiography diagnostic1. The pulse is observed to drive a transient current along the length of the wire, the spread velocity of which is confidently measured via a novel experimental arrangement to be ∼c. Within a temporal window of 20 ps, the current rises to its peak magnitude ∼104 A before decaying to below measurable levels. Both PIC simulation results and simple theoretical reasoning are used to demonstrate that the ultrafast target charging and transient current observed are a direct product of the permanent escape of a small fraction of the laser-accelerated hot electron population to vacuum.