The ion acceleration driven by a laser pulse at intensity I= 10(20)-10(22) W/cm(2) x (microm/lambda)(2) from a double layer target is investigated with multiparametric particle-in-cell simulations. For targets with a wide range of thickness l and density n(e), at a given intensity, the highest ion energy gain occurs at certain electron areal density of the target sigma = n(e)l, which is proportional to the square root of intensity. In the case of thin targets and optimal laser pulse duration, the ion maximum energy scales as the square root of the laser pulse power. When the radiation pressure of the laser field becomes dominant, the ion maximum energy becomes proportional to the laser pulse energy.
We present a first-principles calculation for an optical dielectric breakdown in a diamond, which is induced by an intense laser field. We employ the time-dependent density-functional theory by solving the timedependent Kohn-Sham equation in real time and real space. For low intensities, the ionization agrees well with the Keldysh formula. The calculation shows a qualitative change of electron dynamics as the laser intensity increases, from dielectric screening at low intensities to optical breakdown at and above 7 ϫ 10 14 W / cm 2. Following the pulse, the electrons excited into the conduction band exhibit a coherent plasma oscillation that persists for tens of femtoseconds.
The electron, positron, and photon acceleration in the first cycle of a laser-driven wakefield is investigated. Separatrices between different types of the particle motion (trapped, reflected by the wakefield and ponderomotive potential, and transient) are demonstrated. The ponderomotive acceleration of electrons can be largely compensated by the wakefield action, in contrast to positrons and positively charged mesons. The electron bunch energy spectrum is analyzed. The maximum upshift of an electromagnetic wave frequency during reflection from the wakefield is obtained.
Ion tail formation by neutral beam (D°) injection and second harmonic ion (D + ) cyclotron heating in a 50:50 D-T plasma is investigated on the basis of a local Fokker-Planck calculation. The deformation of the deuteron velocity distribution function is examined analytically and numerically. The effectiveness of the tail formation is estimated from the enhancement of the D-T fusion reactivity, (ov) = / dv f (v) G(v) (f is the deuteron distribution and G is the 'ov-function', averaged over the isotropic triton distribution). The profile of the integrand typically exhibits two humps in velocity space. This results in a large reactivity enhancement for high energy beam injection. For a radiofrequency (RF) induced tail, a 'wing' or smeared tail rather than a hump is formed. ICRF waves couple well with the beam induced tail ions and enhance the reactivity, especially when the beam is injected perpendicularly to the magnetic field. The efficient use of the power supplied by beam and RF waves to enhance the reactivity is also discussed.
Laser acceleration promises innovation in particle beam therapy of cancer where an ultra-compact accelerator system for cancer beam therapy can become affordable to a broad range of patients. This is not feasible without the introduction of a technology that is radically different from the conventional accelerator-based approach. The laser acceleration method provides many enhanced capabilities for the radiation oncologist. It reduces the overall system size and weight by more than one order of magnitude. The characteristics of the particle beams (protons) make them suitable for a class of therapy that might not be possible with the conventional accelerator, such as the ease for changing pulse intensity, the focus spread, the pinpointedness, and the dose delivery in general. A compact, uncluttered system allows a PET device to be located in the vicinity of the patient in concert with the compact gantry. The radiation oncologist may be able to irradiate a localized tumor by scanning with a pencil-like particle beam while ascertaining the actual dosage in the patient with an improved in-beam PET verification of auto-radioactivation induced by the beam therapy. This should yield an unprecedented flexibility in the feedback radiotherapy by the radiation oncologist. Laser accelerated radiotherapy has a unique niche in a current world of high energy accelerator using synchrotron or cyclotron.Comment: 26 pages, 8 figures, 2 tables, 69 references. International Symposium on Laser-Driven Relativistic Plasmas Applied for Science, Industry and Medicine, Kyoto, Japan, 17-20 September (2007
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