Particle Acceleration Mechanism Of Laser Driven Self-formed Plasma Channel

With the development of laser technology in decades, ultra power (TW) and ultra short pulse (fs) laser has already been realized. Since then, accelerating charged particles by laser has been more and more interested. One of the most remarkable points is plasma wake-field acceleration by laser-plasma interaction. Relevant theoretical and experimental works have got huge progress. The electron beam with quasi-energy about GeV and low emittance has been produced. It is considered that using plasma as'compact'high field accelerators to replace conventional accelerators is a probable regime at present.When the ultra-intense and ultra-short laser pulse interacts with plasma, the ponderomotive force produced by laser will push the electrons in plasma away from their equilibrium positions and produce a charge-separation column field including wake field and sheath field. If the charged particles can get rid of their equilibrium positions and be trapped in the charge separated field, they can accelerated into high energy by the large gradient of the electrical field.Generally speaking, relatively low density plasma, for example n0～10-2-10-3nc is used to study the laser-electron acceleration, here, n0 and nc is the plasma initial and critical densities, respectively. At the same time, the over-dense plasma with n0～101nc is used to investigate the ion acceleration by laser-illuminated on a foil target. Between these two parameter areas, it is believed that the plasma with relative high density n0～10-1 nc, which is classified as a near-critical density plasma is inappropriate both to electrons acceleration because the increasing of the plasma density will shorten the acceleration length by the risk of dephasing and protons acceleration since the produced charge-separation coulomb field is not strong enough. However, nowadays laser system can produce ultra-short pulses with intensities above I=1021W/cm2, with such a condition, many non-linear effects may appear and the phenomena may much differ from the previous. Based on 2.5 dimensional (2D3V) PIC(ParticIe-In-Cell) simulations which take relativistic effect into consideration, we investigate the physical mechanism of laser interacting with near-critical density plasma and analyze the difference between low density and near-critical density plasma. Some new physical phenomena have been found including laser self-confinement, self-formed plasma channel under ultra short pulse laser and a continuous coulomb explosion in plasma channel. We found, under the condition of near-critical density plasma, a self-confinement of laser pulse can reduce the diffraction effect of laser and elongates the maintenance length. It can also excite a strong charge-separation coulomb field about hundreds GV/cm by the ponderomotive force. The reflux electrons are bended to longitude rapidly before they oscillate to balance the charge, thus the unbalanced positive ions will undergo a column explosion process which can accelerate ions to about 6 MeV in the backwards, while a self-formed plasma channel is produced which do benefit the acceleration of the electron beam. The mechanism of near-critical density plasma acceleration is a development and supplement of laser-plasma wake field acceleration.