Enhanced ion acceleration from transparency-driven foils demonstrated at two ultraintense laser facilities

Abstract

Laser-driven ion sources are a rapidly developing technology producing high energy, high peak current beams. Their suitability for applications, such as compact medical accelerators, motivates development of robust acceleration schemes using widely available repetitive ultraintense femtosecond lasers. These applications not only require high beam energy, but also place demanding requirements on the source stability and controllability. This can be seriously affected by the laser temporal contrast, precluding the replication of ion acceleration performance on independent laser systems with otherwise similar parameters. Here, we present the experimental generation of >60 MeV protons and >30 MeV u−1 carbon ions from sub-micrometre thickness Formvar foils irradiated with laser intensities >1021 Wcm2. Ions are accelerated by an extreme localised space charge field ≳30 TVm−1, over a million times higher than used in conventional accelerators. The field is formed by a rapid expulsion of electrons from the target bulk due to relativistically induced transparency, in which relativistic corrections to the refractive index enables laser transmission through normally opaque plasma. We replicate the mechanism on two different laser facilities and show that the optimum target thickness decreases with improved laser contrast due to reduced pre-expansion. Our demonstration that energetic ions can be accelerated by this mechanism at different contrast levels relaxes laser requirements and indicates interaction parameters for realising application-specific beam delivery.

Laser-driven high-energy proton beams from cascaded acceleration regimes

“…A dedicated pre-study was conducted, where the target thickness was varied over a wide range and the acceleration performance was compared to numerical simulations. An optimal target thickness between 200 − 300 nm was determined, where electron expulsion from the target bulk due to RIT led to extremely localised space charge fields [25]. These results served as the basis for the present study.…”

“…These advanced concepts enabled compact highintensity lasers with ultrashort pulses to achieve comparable performance levels with significantly reduced laser energy (few J) and repetition rates relevant for practical applications [18][19][20]. Recent experimental and theoretical results revealed that multiple acceleration mechanisms coexist during the laser-plasma interaction [21][22][23][24][25] and the current energy record for laser-driven proton acceleration near-100 MeV was achieved by a hybrid combination of processes [23], albeit still using a high-energy laser.…”

“…Previous research has shown that plasmaacceleration can be enhanced when the laser main pulse arrival coincides with the onset of target transparency [23,25,[29][30][31][32][33][34], making this a promising way to improve particle beam parameters such as energy and directionality. The moment where the initially opaque target becomes transparent to the laser is termed the onset of relativistically induced transparency (RIT) [35] and occurs when the plasma frequency drops below the laser frequency due to the relativistic mass increase of the electrons.…”

“…Considering the extremely short effective acceleration duration provided by the ultrashort laser pulses, we anticipate a combination of different mechanisms [21,22,25].…”

“…The influence of the preceding laser light on a 270 nm thick plastic foil and the resulting expansion was simulated by a 2D hydrodynamic code. The simulation started 67 ps before the arrival of the laser main pulse, when laser induced breakdown is known to occur for the target and temporal laser contrast conditions of this experiment [25,37]. The results of this first simulation stage, ending 1 ps prior to the laser main pulse, were used as input for a subsequent 3D PIC simulation, which studied the high-intensity interaction (details in Methods).…”