Hai-En Tsai

Quasi-monoenergetic laser-plasma acceleration of electrons to 2 GeV

Laser-plasma accelerators of only a centimetre’s length have produced nearly monoenergetic electron bunches with energy as high as 1 GeV. Scaling these compact accelerators to multi-gigaelectronvolt energy would open the prospect of building X-ray free-electron lasers and linear colliders hundreds of times smaller than conventional facilities, but the 1 GeV barrier has so far proven insurmountable. Here, by applying new petawatt laser technology, we produce electron bunches with a spectrum prominently peaked at 2 GeV with only a few per cent energy spread and unprecedented sub-milliradian divergence. Petawatt pulses inject ambient plasma electrons into the laser-driven accelerator at much lower density than was previously possible, thereby overcoming the principal physical barriers to multi-gigaelectronvolt acceleration: dephasing between laser-driven wake and accelerating electrons and laser pulse erosion. Simulations indicate that with improvements in the laser-pulse focus quality, acceleration to nearly 10 GeV should be possible with the available pulse energy.

Self-injected petawatt laser-driven plasma electron acceleration in 10 17 cm −3 plasma

We report observation of electron self-injection and acceleration in a plasma accelerator driven by the Texas petawatt laser at 10¹⁷ cm⁻³ plasma density, an order of magnitude lower density than previous self-injected laser-plasma accelerators.

Self-aligning concave relativistic plasma mirror with adjustable focus

We report an experimental-computational study of the optical properties of plasma mirrors (PMs) at the incident laser frequency when irradiated directly at relativistic intensity ( 10 18 < I 0 < 10 19 W / cm 2 ) by near-normally incident ( 4 ° ), high-contrast, 30 fs, 800 nm laser pulses. We find that such relativistic PMs are highly reflective ( 0.6 – 0.8 ) and focus a significant fraction of reflected light to intensity as large as ∼ 10 I 0 at distance f as small as ∼ 25 μ m from the PM, provided that pre-pulses do not exceed 1014 W/cm2 prior to ∼ 20 ps before arrival of the main pulse peak. Particle-in-cell simulations show that focusing results from denting of the reflecting surface by light pressure combined with relativistic transparency and that reflectivity and f can be adjusted by controlling pre-plasma length L over the range 0.5 ≲ L ≲ 3 μ m. Pump-probe reflectivity measurements show that the PMs focusing properties evolve on a ps time scale.

Generation of dark-current-free quasi-monoenergetic 1.25 GeV electrons by laser wakefield acceleration

We report electron acceleration to 1.25 GeV by petawatt-laser-driven wakefield acceleration at plasma density 5×1017 cm3. Electron beams are dark-current-free, quasi-monoenergetic, highly collimated (<;1mrad divergence), contain tens of pC and have excellent pointing stability.

Single-shot visualization of evolving plasma wakefields

We measure the evolution history of terawatt-laser driven plasma wakefields in the nonlinear bubble regime using an all-optical frequency-domain streak camera (FDSC) technique. The longitudinal profiles of the plasma “bubble” at different propagation distances within a 3 mm long ionized helium gas target are imaged in single shots, illustrating formation, propagation and coalescence of the bubble. 3D particle-in-cell (PIC) simulations validate the observed density-dependent bubble evolution, and correlate it with generation of a quasi-monoenergetic ∼ 100 MeV electron beam. To visualize petawatt-laser- or e-beam driven plasma wakefields, FDSC is extended to multi-object-plane imaging (MOPI) to measure evolution of wakefield transverse profiles over acceleration distance up to ∼ 1 m in a single shot.

GeV Electrons and High brightness Betatron X-rays from Petawatt-Laser-Driven Plasma Accelerators

We identify three regimes of correlated GeV-electron/keV-betatron-X-ray generation by a laser-plasma accelerator driven by the Texas Petawatt laser, and relate them to variations in strength of blowout, injection geometry and beam loading.