The experience of target fabrication with low-density and cluster heterogeneity is presented. Cluster plasma research is strongly dependent on target fabrication development and target structure characterization. Ten more target parameters should be measured for experiment interpreting in case of micro-heterogeneous plasma. Foam and foil targets, high-Z doped also, are produced and irradiated on the existing laser facilities. The density of 4.5 mg/cc cellulose triacetate in the form of regular three-dimensional polymer networks are achieved which is as low as plasma critical density for the third harmonic of iodine laser light. The possibilities of varying important target parameters, methods of their monitoring are discussed. Experiments with underdense foam targets with or without clusters irradiated on Prague Asterix Laser System (PALS) laser facility are analyzed preliminary for target optimization. Under-critical foams of varying structure (closed-cell foam or three-dimensional networks) and densities are reported for plasma experiments. Thermal and radiation transport in such targets are considered.

Experience of micro-heterogeneous target fabrication to study energy transport in plasma near critical density

The dynamics of Xe clusters with initial radius between 10 and 100 A irradiated by an IR subpicosecond laser pulse is investigated The evolution of the cluster is modeled with a relativistic time-dependent three-dimensional particle simulation model The focus of this investigation is to understand the energy absorption of clusters and how the absorbed energy is distributed among the various degrees of freedom The consequence of the initial cluster radius on the absorbed energy, average charge per atom, mean electron and ion energies, ionization, removal of electrons from the cluster, and cluster expansion was studied The absorbed energy per cluster scales as N5∕3, and the mean electron and ion energies scale as N1∕3 and N2∕3, respectively (N is the number of atoms per cluster) A significant fraction of the absorbed energy (∼90%) is converted into kinetic energy with comparable contribution to electrons and ions The energy balance suggests that smaller clusters are more efficient as radiators, while 

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Dynamics of a Xe cluster plasma produced by an intense ultrashort pulse KrF laser

Some preliminary views are presented to the topic “Fast High Density Plasma Blocks Driven by Picosecond Terawatt Lasers” of the UWS-International Workshop 1–4 December 2004 in Sydney, Australia, underlining the motivation to explain the difference between the relativistic and the subrelativistic effects of ps-laser pulse interaction with plasma at powers above TW. This refers to specifically selected experimental and theoretical presentations at the workshop containing results for explaining the differences but also the important applications for studies on the fast ignitor scheme for application on nuclear fusion energy. One of the aims with relativistic proton beams is to realize conditions of spark ignition, while the subrelativistic case implies the generation of fast plasma blocks eventually with the possibility to ignite a fusion flame in uncompressed solid DT fuel for a power station with high efficiency.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0263034605050627

Difference between relativistic petawatt-picosecond laser-plasma interaction and subrelativistic plasma-block generation

Harmonic emission from cluster nanoplasmas subject to short intense infrared laser pulses is studied. In a previous publication [M. Kundu et al., Phys. Rev. A 76, 033201 (2007)] we reported particle-in-cell simulation results showing resonant enhancements of low-order harmonics when the Mie plasma frequency of the ionizing and expanding cluster resonates with the respective harmonic frequency. Simultaneously we found that high-order harmonics were barely present in the spectrum, even at high intensities. The current paper is focused on the analytical modeling of the process. We show that dynamical stochasticity owing to nonlinear resonance inhibits the emission of high-order harmonics.

Harmonic emission from cluster nanoplasmas subject to intense short laser pulses

So far all analytic models of heating of clusters in the collisionless domain are based on linear or linearized concepts, like Mie resonance or other linear oscillations. They are characterized by constant eigenfrequencies lying well above the frequencies of most intense lasers. In this paper we show that nonlinear resonant excitation constitutes the basis of absorption in the collisionless domain, for individual electrons as well as for entire electron bunches. With increasing amplitude of the particles oscillating in their own space charge field their eigenperiod increases and at sufficient driver strength it equals and exceeds the constant laser period. At the crossing point, and essentially only there, resonant absorption takes place. Neither nonlinearities in themselves nor asymmetric excitation of the electron cloud of the cluster leads to irreversible energy gain.

Collisionless absorption in clusters out of linear resonance

Collective absorption of intense laser pulses by atomic clusters is studied by PIC simulations. The cluster is modeled in two-dimensional calculations as a cylindrical plasma column with a diameter of D = 6.4 nm and an initial electron density of ne0 = 1023 cm−3. The frequency and intensity dependence of absorption is discussed. It is found that nonresonant absorption by electron emission increases as a power law with the laser intensity. The absorbed energy per electron reaches a maximum of about Wmax = mωp2D2 (ωp: plasma frequency, m: electron mass) at the intensity where ionization saturates.

F. Greschik

Papers

Two-dimensional PIC simulation of atomic clusters in intense laser fields