Yen-Yu Chang

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.

Single-shot visualization of evolving, light-speed structures by multiobject-plane phase-contrast imaging

We demonstrate a single-shot method of visualizing the evolution of light-speed, laser-generated structures as they propagate over hundreds of Rayleigh lengths (typically ≥10 cm) through a tenuous medium. An ultrashort probe pulse crosses the objects path at a small angle (θ<5°) and a specific time delay. Copies of the phase-modulated probe are then relay-imaged to separate detectors from selected object planes along the propagation path. A phase-contrast technique based on Kerr effect and nonlinear absorption converts phase to intensity modulation, improving sensitivity in tenuous media. A continuous record of the probe phase modulation along the propagation path is reconstructed.

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.

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.

Single-shot, ultrafast diagnostics of light-speed plasma structures and accelerating GeV electrons

We have experimentally demonstrated ultrafast diagnostics to visualize the laser wakefield acceleration process in a single-shot mode. We measured the Faraday rotation of a probe pulse due to the magnetic field induced by GeV electrons in low-density plasmas. In addition, we improved the temporal resolution of Frequency Domain Streak Camera (FDSC) to ∼10 fs by broadening the bandwidth of the probe beam, enabling visualization of the bubble dynamics. A prototype experiment using the broad bandwidth FDSC was performed.

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.

Single-Shot Visualization Of Evolving Laser- Or Beam-Driven Plasma Wakefield Accelerators

We introduce Frequency-Domain Tomography (FDT) for visualizing sub-ps evolution of light-speed refractive index structures in a single shot. As a prototype demonstration, we produce single-shot tomographic movies of self-focusing, filamenting laser pulses propagating in a transparent Kerr medium. We then discuss how to adapt FDT to visualize evolving laser- or beam-driven plasma wakefields of current interest to the advanced accelerator community. For short (L ∼ 1 cm), dense (ne ∼ 1019 cm−3) plasmas, the key challenge is broadening probe bandwidth sufficiently to resolve plasma-wavelength-size structures. For long (L ∼ 10 to 100 cm), tenuous (ne ∼ 1017 cm−3) plasmas, probe diffraction from the evolving wake becomes the key challenge. We propose and analyze solutions to these challenges.