A larger spectral region the phase rotation can be

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Unformatted text preview: entially in test beam lines operating at 1 Hz repetition (Nishiuchi et al., 2010a) with a view to future biomedical applications (see Sec. V.D). Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 773 IV. OTHER ACCELERATION MECHANISMS A. Radiation pressure acceleration EM waves carry momentum, which may be delivered to a nontransparent (either absorbing or reflecting) medium. This is the origin of radiation pressure16 whose expression for a plane, monochromatic EM wave of intensity I and frequency ! normally incident on the plane surface of a medium at rest, is given by I I (27) Prad ¼ ð1 þ R À T Þ ¼ ð2R þ AÞ ; c c where R, T , and A are the reflection, transmission, and absorption coefficients (with R þ T þ A ¼ 1) defined as a function of the refractive index and thus of the wave frequency, e.g., as done in the derivation of Fresnel formulas (Jackson, 1998). Radiation pressure is related to the total steady PF on the medium (see Secs. II.A and II.B.1). Being proportional to the inverse of the particle mass, the PF effectively acts on the electrons. At the surface of an overdense plasma the electrons are pushed inward by the PF, leaving a charge separation layer and creating an electrostatic, backholding field that in turn acts on the ions and leads to their acceleration. In the case of normal incidence of a plane wave on a flat surface the PF density is the cycle-averaged value of the J Â B force. In the following discussion of RPA we refer to such a case unless otherwise stated and consider only the steady action of radiation pressure. As discussed in Sec. II.B.1, the oscillating component of the J Â B force drives a sweeping oscillation at 2! of the density profile and causes strong absorption and hot electron generation, except in the case of circular polarization for which the oscillating component vanishes. In the latter case, on the time scale of ion motion it may be assumed that the electrons are mostly in a mechanical equilibrium so that the PF and electrostatic force locally balance each other. 1. Thick targets: Hole boring regime The intense radiation pressure of the laser pulse pushes the surface of an overdense plasma inward, steepening the density profile. For a realistic laser beam of finite width, the radiation pressure action drives a parabolic deformation of the plasma surface allowing the laser pulse to penetrate deeply into the target; this process is commonly named hole boring, even when referring to a planar geometry, and it is associated with ion acceleration at the front side of the target. Note that in the literature different definitions, such as ‘‘sweeping acceleration’’ (Sentoku et al., 2003) or ‘‘laser piston’’ (Schlegel et al., 2009), are also used to refer to essentially the same process. The recession velocity of the plasma surface, also named the HB velocity vHB , may be simply estimated by balancing the EM and mass momentum flows in a planar geometry (Denavit, 1992; Wilks et...
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