Induction heating for desorption of surface contamination for high-repetition laser-driven carbon-ion acceleration

This study reports the first experimental demonstration of surface contamination cleaning from a high-repetition supply of thin-tape targets for laser-driven carbon-ion acceleration.The adsorption of contaminants containing protons,mainly water vapor and hydrocarbons,on the surface of materials exposed to low vacuum(>10^(−3)Pa)suppresses carbon-ion acceleration.The newly developed contamination cleaner heats a 5-μm-thick nickel tape to over 400℃in 100 ms by induction heating.In the future,this heating method could be scaled to laserdriven carbon-ion acceleration at rates beyond 10 Hz.The contaminant hydrogen is eliminated from the heated nickel surface,and a carbon source layer—derived from the contaminant carbon—is spontaneously formed by the catalytic effect of nickel.The species of ions accelerated from the nickel film heated to various temperatures have been observed experimentally.When the nickel film is heated beyond∼150℃,the proton signal considerably decreases,with a remarkable increase in the number and energy of carbon ions.The Langmuir adsorption model adequately explains the temperature dependence of desorption and re-adsorption of the adsorbed molecules on a heated target surface,and the temperature required for proton-free carbon-ion acceleration can be estimated.

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

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≥30TVm-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.