[Utilizing the ultraintense JanUSP laser at LLNL]. 99-ERD-049 Final LDRD Report
Recent advances in laser and optical technologies have now enabled the current generation of high intensity, ultrashort-pulse lasers to achieve focal intensities of 10{sup 20}-10{sup 21} W/cm{sup 2} in pulse durations of 100-500fs. These ultraintense laser pulses are capable of producing highly relativistic plasma states with densities, temperatures, and pressures rivaling those found in the interiors of stars and nuclear weapons. Utilizing the ultraintense 100TW JanUSP laser at LLNL we have explored the possibility of ion shock heating small micron-sized plasmas to extremely high energy densities approaching 1GJ/g on timescales of a few hundred femtoseconds. The JanUSP laser delivers 10 Joules of energy in a 100fs pulse in a near diffraction-limited beam, producing intensities on target of up to 10{sup 21}W/cm{sup 2}. The electric field of the laser at this intensity ionizes and accelerates electrons to relativistic MeV energies. The sudden ejection of electrons from the focal region produces tremendous electrostatic forces which in turn accelerate heavier ions to MeV energies. The predicted ion flux of 1 MJ/cm{sup 2} is sufficient to achieve thermal equilibrium conditions at high temperature in solid density targets. Our initial experiments were carried out at the available laser contrast of 10{sup -7} (i.e. the contrast of the amplified spontaneous emission (ASE), and of the pre-pules produced in the regenerative amplifier). We used the nuclear photoactivation of Au-197 samples to measure the gamma production above 12MeV-corresponding to the threshold for the Au-197(y,n) reaction. Since the predominant mechanism for gamma production is through the bremsstrahlung emission of energetic electrons as they pass through the solid target we were able to infer a conversion yield of several percent of the incident laser energy into electrons with energies >12MeV. This result is consistent with the interaction of the main pulse with a large pre-formed plasma. The contrast of the laser was improved to the 10{sup -10} level by the insertion of two additional pockel cells to reduce the pre-pulse intensities, and by the implementation of a pulse clean up technique based on adding an additional pre-amplifier and saturable absorber which resulted in a reduction in the ASE level by a factor of approximately 1000. In FY00/01 we performed a series of experiments to investigate the mechanisms for ion generation and acceleration in thin foil targets irradiated at incident laser intensities above 10{sup 20} W/cm{sup 2}, and with the laser contrast at 10{sup -10}. Full details of this work can be found in the two accompanying papers: Energy spectrum and angular distribution of multi-MeV protons produced from ultraintense laser interactions, UCRL-JC-143112, P.K. Pate1 et al., and Enhancement of proton acceleration by hot electron re-circulation in thin foils irradiated by ultra-intense laser pulses, A.J. Mackinnon et al. UCRL-JC-145540. To obtain a more complete picture of the ion emission a range of detectors were developed and fielded including radiachromic films (measuring ion, electron, and x-ray dose), nuclear activation detectors (high energy protons), and single particle nuclear track detectors (protons and heavy ions). Significantly we found that a large fraction of the incident laser energy (greater than 1%) is coupled to highly energetic protons forming a well-collimated beam. The proton spectrum can be fit by an exponential distribution containing 10{sup 11} particles with a mean energy of 3 MeV and a high energy cutoff of 25 MeV. However, these particles appear to originate not from the interaction region at the front of the target but rather from a thin adsorption layer on the rear surface.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15003242
- Report Number(s):
- UCRL-ID-147510; TRN: US0402519
- Resource Relation:
- Other Information: PBD: 17 Apr 2002
- Country of Publication:
- United States
- Language:
- English
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