Thomas Sokollik

Demonstration of self-truncated ionization injection for GeV electron beams.

Ionization-induced injection mechanism was introduced in 2010 to reduce the laser intensity threshold for controllable electron trapping in laser wakefield accelerators (LWFA). However, usually it generates electron beams with continuous energy spectra. Subsequently, a dual-stage target separating the injection and acceleration processes was regarded as essential to achieve narrow energy-spread electron beams by ionization injection. Recently, we numerically proposed a self-truncation scenario of the ionization injection process based upon overshooting of the laser-focusing in plasma which can reduce the electron injection length down to a few hundred micrometers, leading to accelerated beams with extremely low energy-spread in a single-stage. Here, using 100 TW-class laser pulses we report experimental observations of this injection scenario in centimeter-long plasma leading to the generation of narrow energy-spread GeV electron beams, demonstrating its robustness and scalability. Compared with the self-injection and dual-stage schemes, the self-truncated ionization injection generates higher-quality electron beams at lower intensities and densities, and is therefore promising for practical applications.

Quasimonoenergetic collimated electron beams from a laser wakefield acceleration in low density pure nitrogen

A laser wakefield acceleration (LWFA) experiment is performed using 30 TW, 30 fs, and 800 nm laser pulses, focused onto pure nitrogen plasma having relatively low densities in the range of 0.8×1018 cm−3 to 2.7×1018 cm−3. Electron beams having a low divergence of ∼ 3  mrad (full-width at half-maximum) and quasi-monoenergetic peak energies of ∼ 105  MeV are achieved over 4-mm interaction length. The total electron beam charge reached to 2 nC, however, only 1%–2% of this (tens of pC) had energies >35 MeV. We tried different conditions to optimize the electron beam acceleration; our experiment verifies that lower nitrogen plasma densities are generating electron beams with high quality in terms of divergence, charge, pointing stability, and maximum energy. In addition, if LWFA is to be widely used as a basis for compact particle accelerators in the future, therefore, from the economic and safety points of view we propose the use of nitrogen gas rather than helium or hydrogen.

Stable laser–plasma accelerators at low densities

We report stable laser wakefield acceleration using 17–50 TW laser pulses interacting with 4 mm-long helium gas jet. The initial laser spot size was relatively large (28 μm) and the plasma densities were 0.48–2.0 × 1019 cm−3. High-quality 100–MeV electron beams were generated at the plasma density of 7.5 × 1018 cm−3, at which the beam parameters (pointing angle, energy spectrum, charge, and divergence angle) were measured and stabilized. At higher densities, filamentation instability of the laser-plasma interaction was observed and it has led to multiple wakefield accelerated electron beams. The experimental results are supported by 2D particle-in-cell simulations. The achievement presented here is an important step toward the use of laser-driven accelerators in real applications.

Resonantly enhanced betatron hard x-rays from ionization injected electrons in a laser plasma accelerator

Ultrafast betatron x-ray emission from electron oscillations in laser wakefield acceleration (LWFA) has been widely investigated as a promising source. Betatron x-rays are usually produced via self-injected electron beams, which are not controllable and are not optimized for x-ray yields. Here, we present a new method for bright hard x-ray emission via ionization injection from the K-shell electrons of nitrogen into the accelerating bucket. A total photon yield of 8 × 108/shot and 108 photons with energy greater than 110 keV is obtained. The yield is 10 times higher than that achieved with self-injection mode in helium under similar laser parameters. The simulation suggests that ionization-injected electrons are quickly accelerated to the driving laser region and are subsequently driven into betatron resonance. The present scheme enables the single-stage betatron radiation from LWFA to be extended to bright γ-ray radiation, which is beyond the capability of 3rd generation synchrotrons.

Generation of electron beams from a laser wakefield acceleration in pure neon gas

We report on the generation of quasimonoenergetic electron beams by the laser wakefield acceleration of 17–50 TW, 30 fs laser pulses in pure neon gas jet. The generated beams have energies in the range 40–120 MeV and up to ∼430 pC of charge. At a relatively high density, we observed multiple electron beamlets which has been interpreted by simulations to be the result of breakup of the laser pulse into multiple filaments in the plasma. Each filament drives its own wakefield and generates its own electron beamlet.

Periodic spectral modulations of low-energy, low-charge-state carbon ions accelerated in an intense laser–solid interaction

We report experimental observation of periodic modulations in the energy distribution of C1+ ions dominantly accelerated in the interaction of a 15 μm thick tape target with intense laser pulses of intensities ∼1018 W/cm2 in a defocused configuration. Moreover, the influence of laser intensity on the acceleration of low- and high-charge-state species of carbon ions is observed. Two-dimensional (2D) particle-in-cell simulations elucidate the dynamics of ionization-dependent acceleration of different species in different laser focusing conditions. By comparison, 1D simulations suggest that the modulations of C1+ ions are due to the longitudinal recirculation dynamics of hot electrons in the target, which modulates the sheath field for acceleration of C1+ ions.We report experimental observation of periodic modulations in the energy distribution of C1+ ions dominantly accelerated in the interaction of a 15 μm thick tape target with intense laser pulses of intensities ∼1018 W/cm2 in a defocused configuration. Moreover, the influence of laser intensity on the acceleration of low- and high-charge-state species of carbon ions is observed. Two-dimensional (2D) particle-in-cell simulations elucidate the dynamics of ionization-dependent acceleration of different species in different laser focusing conditions. By comparison, 1D simulations suggest that the modulations of C1+ ions are due to the longitudinal recirculation dynamics of hot electrons in the target, which modulates the sheath field for acceleration of C1+ ions.

High-Quality Laser-Driven Electron Beams by Ionization Injection in Low-Density Nitrogen Gas Jet

We report a laser wakefield acceleration of electron beams up to 130 MeV from laser-driven 4 mm long nitrogen gas jet. Using moderate laser intensity (~ 35× 10¹⁸ W·cm^-2) and relatively low plasma densities (0.8×10¹⁸ cm^-3 -2.7×10¹⁸ cm^-3), we have achieved stable laser propagation and consequently stable acceleration of electron beams. We experimentally studied the dependence of the electron beam parameters on the laser beam energy. Evidence of the ionization-induced electron injection has been recognized from the characteristic long-tail beam energy spectrum, however, the majority of the electrons were contained in quasi-monoenergtic peaks.

Generation of high-quality electron beams from a laser-based advanced accelerator*

At Shanghai Jiao Tong University (SJTU) we have established a research laboratory for advanced acceleration research based on high-power lasers and plasma technologies. In a primary experiment based on the laser wakefield acceleration (LWFA) scheme, multi-hundred MeV electron beams of reasonable quality are generated using 20–40 TW, 30 femtosecond laser pulses interacting independently with helium, neon, nitrogen and argon gas jet targets. The laser-plasma interaction conditions are optimized for stabilizing the electron beam generation from each type of gas. The electron beam pointing angle stability and divergence angle as well as the energy spectra from each gas jet are measured and compared.