Joey's Future Description

Laser-plasma accelerators (LPAs) use intense, ultrashort TW (PW) pulses to drive high-amplitude charge-density waves in a plasma, which facilitate the capture and acceleration of electrons to MeV (GeV) energies [1] within just a few mm (cm). Such compact acceleration is possible because laser-driven plasma waves can sustain electric gradients on the order of GV/cm, compared to the ~1 MV/cm breakdown fields that limit the metallic cavities of RF-accelerators. LPAs therefore have the potential to shrink the size and cost of certain applications whose requirements are only currently met by large accelerator facilities.

Professor Mike Downer leads ongoing investigations to understand and utilize laser wakefield acceleration (LWFA) of electrons in his 30 TW laser laboratory (UT3) and the Texas PetaWatt (TPW) Laser facility. Although the TW- and PW-driven LPAs rely on nearly the same theoretical mechanisms, the two accelerators are experimentally quite different. These two experiments therefore present uniquely different challenges and opportunities, often requiring a unified experimental, computational and theoretical effort. For this reason, the LPA in UT3 functions both as an independent experimental setup as well as a testbed for experiments in the TPW facility. The research conducted by the Downer LWFA group are motivated by the following goals: push the frontier of demonstrated energies from LWFA; enhance the quality, quantity and control of accelerated electrons; explore novel solutions in LWFA metrology; and characterize LPA-generated secondary sources (i.e. x-/γ-rays, positrons, and neutrons) while examining their utility for applications.

Snapshot of electron density showing electrons trapped in a plasma bubble during LWFA, from a 3D envelope model VORPAL particle-in-cell simulation [2,3]. The wakefield structure propagates from left to right behind the laser pulse (not shown). Electrons inside the bubble are accelerated by the bubble's electric field and attain GeV energy in cm-scale distances for TPW parameters.

An active area of research in the Downer group has been the development of a technique for generating high energy, collimated, femtosecond x-ray sources by Compton backscattering intense laser light from LPA electrons. The technique we’ve implemented uses only a single intense pulse to first drive a LPA, then form a plasma mirror (PM) on a thin film at the LPA exit, and finally retro-reflect from the PM to stimulate x-ray emission from the electrons it just accelerated. We applied this technique on the 1mm-long LPA in UT3 to demonstrate an energy-tunable x-ray source (75-200 keV) by tuning the electron energy from 50-100 MeV [4]. The high energies and ultrashort duration of this source may be advantageous for applications such as: low-dose medical/industrial imaging of extended objects; targeted radiotherapy; and time-resolved radiography. Once demonstrated in UT3, we applied this PM technique to the TPW-driven 7cm-long LPA, which was the first of its kind to accelerate electrons to 2 GeV [5], producing a brilliant 5-85 MeV γ-ray source [6]. Notably, this energy range encompasses the entire Giant Dipole Resonance for medium to heavy nuclei. Potential applications include medical radioisotope production, sterilization of food and medical equipment, interrogation of potentially threatening cargo, and studies in exotic nuclear phenomena like photo-fission.

[1] E. Esarey et al. Physics of laser-driven plasma-based electron accelerators. Reviews of Modern Physics. 2009;81(3):1229-1285.

[2] C. Nieter and J. R. Cary, J. Comput. Phys. 196, 448 (2004).

[3] B. M. Cowan, et al., J. Comput. Phys. 230, 61 (2011).

[4] H.-E. Tsai et al. Compact tunable Compton x-ray source from laser-plasma accelerator and plasma mirror. Physics of Plasmas. 2015;22(2):023106.

[5] X. Wang et al. Quasi-monoenergetic laser-plasma acceleration of electrons to 2 GeV. Nature Communications. 2013;4.

[6] J.M. Shaw et al. Bright 5-85 MeV Compton γ-ray pulses from GeV laser-plasma electron accelerator and plasma mirror. (manuscript in preparation)