Modified Thomson spectrometer design for high energy, multi-species ion sources

Abstract

A modification to the standard Thomson parabola spectrometer is discussed, which is designed to measure high energy (tens of MeV/nucleon), broad bandwidth spectra of multi-species ions accelerated by intense laser plasma interactions. It is proposed to implement a pair of extended, trapezoidal shaped electric plates, which will not only resolve ion traces at high energies, but will also retain the lower energy part of the spectrum. While a longer (along the axis of the undeflected ion beam direction) electric plate design provides effective charge state separation at the high energy end of the spectrum, the proposed new trapezoidal shape will enable the low energy ions to reach the detector, which would have been clipped or blocked by simply extending the rectangular plates to enhance the electrostatic deflection.

Characterisation of deuterium spectra from laser driven multi-species sources by employing differentially filtered image plate detectors in Thomson spectrometers

“…a) Electronic mail: s.kar@qub.ac.uk Laser accelerated ion beams from solid targets are typically multi-species, either due to the chemical composition of the target material or due to the layer of contamination over the target (typically composed of water vapour and hydrocarbons) 12,13 . One of the diagnostics commonly used to characterize these beams is the Thomson Parabola Spectrometer (TPS) 14,15 , which has the unique ability to energy resolve the ion spectra while discriminating ions with different charge-to-mass ratio.…”

Recent developments in the Thomson Parabola Spectrometer diagnostic for laser-driven multi-species ion sources

“…However, for the case of high-energy ion sources, the different species can also overlap at high ion energies, even when they possess different Z/A, due to the small deflection of the energetic particles by the fields. The traces for two given species will merge together when the width of the traces, defined by the size of the pinhole, is larger than the separation between them [21,44]. Analytically, this separation can be expressed as…”

“…However, an increase in separation between the plates in this configuration reduces the field strength commensurately, compromising the net separation between neighbouring species at high ion energies. An alternative method is using trapezoidally shaped electric plates, as suggested by Gwynne et al [21]. With such electrodes, the high-energy ions will effectively experience a longer electric field region, while the slower ions will be able to reach the detector by passing over the wedged side of the electrode.…”

“…For non-relativistic speeds (v z c), the general expression can be approximated by performing a first-order Taylor expansion around γ = 1. Introducing the kinetic energy (E = γ − 1 m 0 c 2 ), one can obtain the formulae matching with the non-relativistic equations commonly used for this diagnostic [21,22].…”

“…In light of Eq. 4.1, there are three ways to increase the separation between the two traces: (1) Increasing the electric field, (2) enlarging the electrode length (L E ) [21,22], or (3) placing the detector farther away (D E ) [22]. However, the increase in D E in most cases is limited by the size of the vacuum chambers, where the diagnostic has to be placed , whereas the maximum electric field attainable is limited by the vacuum conditions inside the chamber (∼ 20 kV/cm is typically attainable in a laser-plasma interaction chamber at ∼ 10 −4 mbar).…”