proton and ion laser plasma acceleration 179
shown that the required laser intensities are on the order of
10 21 W/cm 2 .
Various ways to improve TNSA have been considered,
starting from the brute force method to increase the laser intensity, to more subtle methods involving the enhancement of
the laser energy transfer to electrons and increase of the electron density. Reduction of foil thickness and reduced mass
targets, enhanced coupling by a conically shaped target and
use of nano-particle structured targets, among others, have
all been used with some degree of success.
Despite the mentioned improvements, the TNSA mechanism still has a major disadvantage due to the shape of the
spectrum of accelerated particles — the number of particles
at the high end of the spectrum remains very low.
9.5.2 Hole-boring radiation pressure acceleration regime
The radiation pressure acceleration mechanism is based on
the effects equivalent to radiation pressure, which light exhibits on a mirror when it reflects from its surface, as illustrated in Fig. 9.13. The radiation pressure for perfect reflection is proportional to the laser intensity I L as
2 I L
P L =
(9.6)
c
Similar pressure can be applied to a thin foil, upon whose
surface plasma quickly forms. The radiation pressure effect FIGURE 9.13
is transmitted into the plasma by electrons via the pondero- Radiation pressure acceleramotive force. Displaced electrons produce space charge that tion concept.
creates a steady pressure, which in its turn transfers the effect
to the ions.
Two versions of radiation pressure mechanisms have received distinct names — the hole-boring and the light-sail
mechanism.
In the case of hole-boring, the space charge due to electrons acts on ions that are pushed into the overdense plasma,
initially compressing the foil and then pushing a region of
the foil forward, as illustrated conceptually in Fig. 9.14.
The derivation of the approximate scaling rules for the
FIGURE 9.14
Hole-boring radiation pressure laser acceleration of protons.
shown that the required laser intensities are on the order of
10 21 W/cm 2 .
Various ways to improve TNSA have been considered,
starting from the brute force method to increase the laser intensity, to more subtle methods involving the enhancement of
the laser energy transfer to electrons and increase of the electron density. Reduction of foil thickness and reduced mass
targets, enhanced coupling by a conically shaped target and
use of nano-particle structured targets, among others, have
all been used with some degree of success.
Despite the mentioned improvements, the TNSA mechanism still has a major disadvantage due to the shape of the
spectrum of accelerated particles — the number of particles
at the high end of the spectrum remains very low.
9.5.2 Hole-boring radiation pressure acceleration regime
The radiation pressure acceleration mechanism is based on
the effects equivalent to radiation pressure, which light exhibits on a mirror when it reflects from its surface, as illustrated in Fig. 9.13. The radiation pressure for perfect reflection is proportional to the laser intensity I L as
2 I L
P L =
(9.6)
c
Similar pressure can be applied to a thin foil, upon whose
surface plasma quickly forms. The radiation pressure effect FIGURE 9.13
is transmitted into the plasma by electrons via the pondero- Radiation pressure acceleramotive force. Displaced electrons produce space charge that tion concept.
creates a steady pressure, which in its turn transfers the effect
to the ions.
Two versions of radiation pressure mechanisms have received distinct names — the hole-boring and the light-sail
mechanism.
In the case of hole-boring, the space charge due to electrons acts on ions that are pushed into the overdense plasma,
initially compressing the foil and then pushing a region of
the foil forward, as illustrated conceptually in Fig. 9.14.
The derivation of the approximate scaling rules for the
FIGURE 9.14
Hole-boring radiation pressure laser acceleration of protons.