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 ≳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.

New algorithm using L1 regularization for measuring electron energy spectra.

Retrieving the spectrum of physical radiation from experimental measurements typically involves using a mathematical algorithm to deconvolve the instrument response function from the measured signal. However, in the field of signal processing known as "Source Separation" (SS), which refers to the process of computationally retrieving the separate source components that generate an overlapping signal on the detector, the deconvolution process can become an ill-posed problem and crosstalk complicates the separation of the individual sources. To overcome this problem, we have designed a magnetic spectrometer for inline electron energy spectrum diagnosis and developed an analysis algorithm using techniques applicable to the problem of SS. An unknown polychromatic electron spectrum is calculated by sparse coding using a Gaussian basis function and an L1 regularization algorithm with a sparsity constraint. This technique is verified by using a specially designed magnetic field electron spectrometer. We use Monte Carlo simulations of the detector response to Maxwellian input energy distributions with electron temperatures of 5.0 MeV, 10.0 MeV, and 15.0 MeV to show that the calculated sparse spectrum can reproduce the input spectrum with an optimum energy bin width automatically selected by the L1 regularization. The spectra are reproduced with a high accuracy of less than 4.0% error, without an initial value. The technique is then applied to experimental measurements of intense laser accelerated electron beams from solid targets. Our analysis concept of spectral retrieval and automatic optimization of energy bin width by sparse coding could form the basis of a novel diagnostic method for spectroscopy.

Compact Thomson parabola spectrometer with variability of energy range and measurability of angular distribution for low-energy laser-driven accelerated ions.

This article reports the development of a compact Thomson parabola spectrometer for laser-accelerated ions that can measure angular distribution with a high energy resolution and has a variable measurable energy range. The angular-resolved energy spectra for different ion species can be measured in a single shot, and the sampling angle can be selected from outside the vacuum region. The electric and magnetic fields are applied to the ion dispersion by using a permanent magnetic circuit and annulus sector-shaped electrodes with a wedge configuration. The compact magnetic circuit consists of permanent magnets, fixed yokes, and movable yokes. The magnetic flux is intentionally leaked to the movable yokes, allowing the magnetic field to be adjusted from 53 mT to 259 mT. The annulus sector-shaped electrodes with a wedge configuration provide better trace separation for high-energy ions, retain the lower-energy part of the ion signal, and subject ions passing through all pinholes to an equivalent Lorentz force. The magnetic and electric fields are designed for measuring protons and carbon ions with an energy range of 0.1-5 MeV. The spectrometer allows for the adjustment of the observable energy range afterward according to the parameters of the accelerated ion.