A scheme for electron self-injection in the laser wakefield acceleration is proposed. In this scheme, the transverse wave breaking of the wakefield and the tightly focused geometry of the laser beam play important roles. A large number of the background electrons are self-injected into the acceleration phase of the wakefield during the defocusing of the tightly focused laser beam as it propagates through an underdense plasma. Particle-in-cell simulations performed using a 2D3V code have shown generation of a collimated electron bunch with a total number of 1.4×109 and energies up to 8MeV.
Ion acceleration by ultrashort circularly polarized laser pulse in a solid-density target is investigated using two-dimensional particle-in-cell simulation. The ions are accelerated and compressed by the continuously extending space-charge field created by the evacuation and compression of the target electrons by the laser light pressure. For a sufficiently thin target, the accelerated and compressed ions can reach and exit from the rear surface as a high-density high-energy ion bunch. The peak ion energy depends on the target thickness and reaches maximum when the compressed ion layer can just reach the rear target surface. The compressed ion layer exhibits lateral striation which can be suppressed by using a sharp-rising laser pulse.
The study of temperature relaxation and ion acoustic wave excitation in localized regions of intense laser field ͑hot spots͒ has been carried out for the conditions of inhibited electron thermal transport. We consider the fast energy deposition in a single cylindrical hot spot and describe the following temperature and density relaxation using a perturbation approach and the nonlocal electron transport equations. We find a significant increase in the temperature relaxation time for hot spots with sizes comparable to the electron mean free path. Thermal decay of the hot spot together with the pondermotive effect of the laser field on ions could also be a source of large density fluctuations in currently exploited laser-produced plasmas. ͓S1063-651X͑97͒06312-5͔PACS number͑s͒: 52.40. Nk, 52.35.Mw, 52.35.Nx
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