Monoenergetic ion beam from an exploding foil
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Monoenergetic Ion Beam from an Exploding Foil V. Yu. Bychenkova and V. F. Kovalevb a Lebedev
Physical Institute, Russian Academy of Sciences, Leninskiœ pr. 53, Moscow, 119991 Russia b Institute for Mathematical Modeling, Russian Academy of Sciences, Miusskaya pl. 4a, Moscow, 125047 Russia Received March 13, 2006; in final form, April 3, 2006
Abstract—An analytic model is proposed that describes the generation of a monoenergetic beam of light ions during the Coulomb explosion of a stratified target irradiated by an ultrashort high-power laser pulse. The spatiotemporal and energy parameters of the ions of a thin coating on a heavy foil are obtained. PACS numbers: 52.27.Ny, 52.30.-q, 52.38.Kd DOI: 10.1134/S1063780X06110134
It is known that a Coulomb explosion of a microplasma produced by a short intense laser pulse provides conditions for efficient ion acceleration to high energies [1–3]. The energy spectrum of the accelerated ions is typically broad and has a cutoff at a certain energy. However, for some practical applications, such as hadron therapy [4] and fast ignition of targets in confinement fusion research [5], it is necessary that not only the ions be accelerated to high energies (>100 MeV) but also that the accelerated ion beams be monoenergetic. The latter can be achieved by coating the back surface of a thin foil of high-Z elements with a light material. In this case, the electrons escaping from the foil under the action of laser radiation create a strong charge-separation electric field that decreases insignificantly across the coating and accelerates the ions of the coating to essentially the same energy [6, 7]. The ion acceleration dynamics and the shape of the ion energy spectra in the interaction of intense electromagnetic pulses with multicomponent clusters were investigated numerically in [8, 9] and experimentally in [10]. When the laser intensity is so high that all the target electrons escape into a vacuum, the ions are accelerated due to a Coulomb explosion. For a foil made of a highZ material, the time scale on which the ions of the light coating are accelerated is too short for the foil ions to expand appreciably. In the present report, we show that the problem of a Coulomb explosion of a stratified target, with no mixing of different ion species, is one that can be solved analytically (like the problems in [11, 12]), without recourse to the given field approximation [7]. We present the relevant analytic solution that describes the formation of an ion beam accelerated up to ultrarelativistic energies and having a small energy spread. Assuming that the ions of a plane-stratified plasma target are cold, we use a collisionless hydrodynamic
model that is based on the equations ∂t n ∂t u
(α)
(α)
+ ∂xn
+u
(α)
∂ x E = 4π
(α) (α)
∂xu
u
(α)
Z eE -, = --------------(α) 3 M γ
(α)
∑Z
= 0,
(α)
en
(α)
(1)
,
α
n
(α) t=0
(α)
= n 0 ( x ),
u
(α) t=0
= U
(α)
( x ),
x
E
t=0
∫ ∑Z
= 4πe d x 0
(α)
(α)
n0 ( x )
α
and in which the expansion dynamics of a charge
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