Optimization of the characteristics of a relativistic electron beam based on laser wake-field acceleration using a non-symmetric gas target profile

TL;DR

The paper addresses the challenge of producing compact, high-energy, high-charge electron beams with stable characteristics for biomedical and beam-direction applications. It introduces a novel non-symmetric nozzle to engineer a density downramp and a quasi-constant region in a nitrogen gas profile, promoting both downramp and ionization injection in laser wakefield acceleration, supported by 2D PIC simulations. Experimentally, it achieves a maximum electron energy of about $300\mathrm{MeV}$ and a charge-per-shot increase of at least an order of magnitude compared with previous symmetric-nozzle designs, while also showcasing a central high-dose region suitable for radiotherapy studies. The simulations corroborate the dual-injection mechanism and reproduce the energy scale, highlighting a tunable, high-dose electron source with potential for Betatron radiation and biomedical applications in compact laser-plasma facilities.

Abstract

We demonstrate a high-energy, high-charge, electron source produced by the irradiation of a novel gaseous target by an ultra-intense femtosecond laser pulse. By exploiting a nonsymmetrical nozzle, we increased the total charge of the electron beam by at least an order of magnitude with respect to our previous experiments using symmetrical nozzles. In addition, the maximum energy of the accelerated electrons was enhanced by a factor of two. The electrons are accelerated via the Laser Wake-Field Acceleration mechanism. Particle-in-cell simulations indicate that electrons are injected via the ionization and the downramp injection mechanisms. Our measurements indicate that the demonstrated electron source is a considerable candidate for high dose, Very High Energy Electrons applications, such as radiotherapy.

Figures (7)

• Figure 1: The experimental setup. The main beam of the Zeus laser focuses downstream of the NS nozzle exit (a) and interacts with the gas. The accelerated electrons travelling in the laser propagation direction enter the magnetic spectrometer (MS), and their resulting spectra are recorded by a CCD (b). The low-energy, secondary beam serves to probe the interaction via shadowgraphy (c).
• Figure 2: (a) A CAD section view of the NS nozzle with a graph at the top indicating the produced gas density profile (blue line), along the laser propagation direction. (b) Plane view of the particle density 300µm downstream the NS nozzle exit, obtained using a tomographic reconstruction method tomo_COUPERUSHIPP200453. (c) The density profile corresponding to the black dashed line of (b), namely the laser propagation axis at (Y=0mm).
• Figure 3: A series of typical electron spectra measurements for different backing pressures at the gas valve inlet, ranging from 25 to 55bar. Three representative shots per backing pressure are presented. The low energy limit of the spectrometer is 25MeV. The charge per pixel corresponds to the colour bar, and the angular divergences are displayed.
• Figure 4: (a) Total charge as a function of gas valve backing pressure. Measurements from 17-22 shots per backing pressure were averaged. (b) Energy spectrum of the highest energy shot at 45bar backing pressure.
• Figure 5: (a) The beam pointing angle spread of the electron source (blue marks) of several consecutive shots. Red mark indicates the expected zero point and (b) 2D colour map of the measured absolute dose on the RCF Film aggregated over 140 shots. A clear iso-dose distribution ranging from 1.5Gy the outer line to 2.5Gy the central iso-line localised on a region of 15 $\times$ 15mm is shown.