Self-generated surface magnetic fields inhibit laser-driven sheath acceleration of high-energy protons

Abstract

High-intensity lasers interacting with solid foils produce copious numbers of relativistic electrons, which in turn create strong sheath electric fields around the target. The proton beams accelerated in such fields have remarkable properties, enabling ultrafast radiography of plasma phenomena or isochoric heating of dense materials. In view of longer-term multidisciplinary purposes (e.g., spallation neutron sources or cancer therapy), the current challenge is to achieve proton energies well in excess of 100 MeV, which is commonly thought to be possible by raising the on-target laser intensity. Here we present experimental and numerical results demonstrating that magnetostatic fields self-generated on the target surface may pose a fundamental limit to sheath-driven ion acceleration for high enough laser intensities. Those fields can be strong enough (~10 5 T at laser intensities ~10 21 W cm –2 ) to magnetize the sheath electrons and deflect protons off the accelerating region, hence degrading the maximum energy the latter can acquire.

Identification of Coupling Mechanisms between Ultraintense Laser Light and Dense Plasmas

“…Such surface quasi-static fields have already been reported in multiple studies of the interaction of intense lasers with dense plasmas (see e.g. [59][60][61][62]), and can be induced by a variety of physical processes (see e.g. [63][64][65][66]).…”

Proton beam emittance growth in multipicosecond laser-solid interactions

“…In addition, the presence of a low density preformed plasma on the rear surface of the target in advance of the peak of the laser pulse has been shown to enhance Weibel-like instability formation that can lead to strong modulations in the accelerated proton beam [45,46]. Even without filamentation, recent work by Nakatsutsumi et al using higher intensities and thinner targets demonstrated that strong azimuthal magnetic fields on the rear surface can inhibit proton acceleration and impact the beam quality [47]. With these simulations, we investigate the influence of pulse duration on emittance growth mechanisms.…”

“…However if l s ?λ D , the field is reduced to = E k T el B e s [34]. Alternatively, strong magnetic fields can inhibit sheath formation by confining or scattering hot electrons away from the central axis [47]. In figure 8, the maximum proton energies observed in the experiment are compared with predictions based on the analytical model described by Schreiber et al [50].…”

P3: An installation for high-energy density plasma physics and ultra-high intensity laser–matter interaction at ELI-Beamlines

“…The parameter space of focused intensity and pulse duration for the experiments using EPMs and tightly focused OAP (only for I > 10 21 W=cm 2 ). Squares represent EPM experiments that have been done [126,127], and circles represent the parameters of tightly focused OAP. The value for the Texas PW stems from an approximate estimate based on typical laser parameters in conjunction with a f/#1 OAP.…”