Linear energy transfer of proton clusters.

In conventional particle accelerators, protons are produced in long pulses, in which the average inter-proton distance is in the order of tens of centimeters or more. Therefore, the radiobiology of conventionally accelerated protons is primarily governed by the interaction of a single proton with the cell. In a laser-plasma interaction scheme, the accelerated protons come as a single bunch of particles (less than 1 ps in duration) with inter particle distances that are many orders of magnitude shorter than those in conventional particle accelerators. As laser-accelerated protons traverse the medium, they not only interact with each other, but also with the host medium. It is shown that when the average distance between protons in a cluster is less than or equal to their velocity divided by the characteristic frequency of the collective excitations supported by the medium, the cluster's linear stopping power increases and can reach several times that of sparsely distributed protons. As a result, the elevated radio biological effectiveness of the proton cluster may take place and conditions for its experimental observation are presented.

Energy optimization procedure for treatment planning with laser-accelerated protons.

A simple analytical model is found that predicts the exact proton spectrum needed to obtain a spread-out-Bragg peak (SOBP) distribution for laser-accelerated proton beams. The theory is based on the solution to the Boltzmann kinetic equation for the proton distribution function. The resulting analytical expression allows one to calculate the SOBP proton energy spectra for the different beamlet sizes and modulation depths that can be readily implemented in the calculation of energy and intensity modulated proton dose distributions. Since the practical implementation of energy modulation for proton beams is realized through the discrete superposition of individual Bragg peaks, it is shown that there exists an optimal relationship between the energy sampling size and the width of the initial proton energy distribution.

Coulomb explosion effect and the maximum energy of protons accelerated by high-power lasers.

The acceleration of light ions (protons) through the interaction of a high-power laser pulse with a double-layer target is theoretically studied by means of two-dimensional particle-in-cell simulations and a one-dimensional analytical model. It is shown that the maximum energy acquired by the accelerated light ions (protons) depends on the physical characteristics of a heavy-ion layer (electron-ion mass ratio and effective charge state of the ions). In our theoretical model, the hydrodynamic equations for both electron and heavy-ion species are solved and the test-particle approximation for the light ions (protons) is applied. The heavy-ion motion is found to modify the longitudinal electric field distribution, thus changing the acceleration conditions for the protons.

Two-beam modulation instability in noninstantaneous nonlinear media.

The development of modulation instability for two beams copropagating in a medium with noninstantaneous nonlinear response is investigated. The frequency dependence of the experimentally observed spectral broadening reveals the presence of two-beam coupling, resulting from the time-delayed Raman part of the third-order nonlinear susceptibility. We present an analysis of the experimental data, which yields a value of tau=27(1) fs for the lifetime of the optical phonons in silica fibers. To our knowledge, this is the first experimentally determined value for this important material parameter.

Higher-order stimulated Brillouin scattering with nondiffracting beams.

We report on an experimental investigation of stimulated Brillouin scattering pumped with a Bessel beam. Owing to the extended interaction length along the diffraction-free propagation, higher-order Stokes components are generated in a bulk Brillouin-active medium with odd and even orders propagating in opposite directions. The spatial, spectral, and temporal properties of the interacting waves are discussed.

A Dense Grid of Reference Iodine Lines for Optical Frequency Calibration in the Range 571-596 nm

A high precision dense grid of reference lines in the hyperfine structure of the B-X system of molecular iodine (127I2) is presented. A simple parametrization has been derived, predicting the hyperfine line positions of the "t" components for vibrational bands (13-1) up to (18-1) and rotational quantum numbers J = (9-140). The analysis in this paper is based on Doppler-free saturation spectroscopy spectra used for calibration of more than 100 new components in the hyperfine structure of 127I2. The data presented contains a prediction (with 2 MHz accuracy) for the positions of 1584 "t" components in the rotational structure of iodine, covering the wavelength interval 571-596 nm. Copyright 1998 Academic Press. Copyright 1998Academic Press