An intense focused laser pulse propagating in underdense plasma generates a plasma electron density wave behind it. The resulting plasma wave that is called laser wakefield supports high electric fields, and electrons can be accelerated to high energies in these fields within a very short length. The advantage of a laser wakefield accelerator is that plasmas support much higher electric fields compared to conventional accelerators.
When an ultra short relativistic intense focused laser pulse drives a plasma wave, the ponderomotive force of the laser expels all of the electrons transversely, and forms a region completely devoid of electrons behind the driving laser pulse as shown in the movie below. Plasma electrons can then be injected into this plasma “bubble”, and accelerated to multi-GeV.
A brand new and mysterious gate in the laser-plasma acceleration has been opened recently…
Figure 1. Laser Wakefield and Direct Acceleration [3]. (a) Electron spatial distribution color-coded by electrons’ relativistic factor. DLA electrons with large oscillation have higher energy. (b) The plasma bubble structure and the injected DLA and non-DLA electrons. (c) Two representative electrons, DLA electron (blue) and non-DLA electron (red). DLA electron gains some energy from laser field (dashed line) and more energy from the wakefield (solid line). (d) Electron final energy spectrum has two peaks, DLA peak and non-DLA peak. Laser Wakefield and Direct Acceleration almost double the electrons’ energy.
We proposed the merging concept of the laser wakefield acceleration and the direct laser acceleration, so called Laser Wakefield and Direct Acceleration see Figure 1 and Figure 2. Direct laser acceleration accelerates electrons directly by laser electromagnetic wave in the plasma structures through the betatron resonance. When the plasma structure is a plasma bubble, laser wakefield acceleration and direct laser acceleration work synergistically, that is, electrons gain some energy from the laser field and more energy from the wakefield. Because of the high energy and large oscillation of the electrons, laser wakefield and direct acceleration is promising not only for the production of high energy electron beam but also for the generation of copious radiations.
Figure 2. Electron self-injection [9]. From (a) to (b), the plasma bubble expands with the diffraction of the laser pulse and the electrons are self-injected. From (b) to (c), the plasma bubble contracts with the focusing of the laser pulse and the self-injected electrons are still in the plasma bubble and accelerated.
In conclusion, our research focuses on the computational and theoretical studies of laser-plasma accelerations, including the laser wakefield acceleration, the direct laser acceleration, the dynamics of electron injection into the plasma bubble see Figure 2 and Figure 3.
Figure 3. Ionization Injection Dynamics [2]. Five ionization produced electron trajectories are plotted on top of the wake potential. Blue: on axis ionization injected electron with small oscillation. Red: off axis ionization injected electron with large oscillation. Green: Ricochet electron with large initial transverse momentum after exiting laser pulse. Brown: on axis deep trapping electron. Purple: ionization produced run away electron. Red electron and green Ricochet electron are typical laser wakefield and direct acceleration electrons.
Selected publications:
[1] V. N. Khudik, A. Arefiev, X. Zhang and G. Shvets, Universal scalings for laser acceleration of electrons in ion channels, Phys. Plasmas 23, 103108 (2016)
[2] X. Zhang, V. N. Khudik, A. Pukhov and G. Shvets, Laser wakefield and direct acceleration with ionization injection, Plasma Phys. Control. Fusion 58, 034011 (2016)
[3] X. Zhang, V. N. Khudik, and G. Shvets, Synergistic Laser-Wakefield and Direct-Laser Acceleration in the Plasma-Bubble Regime, Phys. Rev. Lett. 114, 184801 (2015)
[4] Z. Li, H.-E. Tsai, X. Zhang, C.-H. Pai, Y.-Y Chang, R. Zgadzaj, X. Wang, V. Khudik, G. Shvets and M. C. Downer, Single-Shot Visualization of Evolving Laser Wakefields Using an All-Optical Streak Camera, Phys. Rev. Lett. 113, 085001 (2014)
[5] Xiaoming Wang, Rafal Zgadzaj, Neil Fazel, Zhengyan Li, S. A. Yi, Xi Zhang, Watson Henderson, Y.-Y. Chang, R. Korzekwa, H.-E. Tsai, C.-H. Pai, H. Quevedo, G. Dyer, E. Gaul, M. Martinez, A. C. Bernstein, T. Borger, M. Spinks, M. Donovan, V. Khudik, G. Shvets, T. Ditmire & M. C. Downer, “Quasi-monoenergetic laser-plasma acceleration of electrons to 2 GeV,” Nature Commun. 4:1988, doi: 10.1038/ncomms2988 (2013)
[6] S. A. Yi, V. Khudik, C. Siemon, and G. Shvets , “Analytic model of electromagnetic fields around a plasma bubble in the blow-out regime,” Phys. Plasma. 20, 013108 (2013)
[7] S. A. Yi, V. Khudik, S. Kalmykov and G. Shvets, “Hamiltonian analysis of electron self-injection and acceleration into an evolving plasma bubble,” Plasma Phys. Contr. Fus. 53, 014012 (2011).
[8] P. Dong, S. A. Reed, S. A. Yi, S. Kalmykov, G. Shvets, M. C. Downer, N. H. Matlis, W. P. Leemans, C. McGuffey, S. S. Bulanov, V. Chvykov, G. Kalintchenko, K. Krushelnick, A. Maksimchuk, T. Matsuoka, A. G. R. Thomas, and V. Yanovsky, “Formation of Optical Bullets in Laser-Driven Plasma Bubble Accelerators,” Phys. Rev. Lett. 104, 134801 (2010).
[9] S. Kalmykov, S. A. Yi, V. Khudik and G. Shvets, “Electron Self-Injection and Trapping into an Expanding Plasma Bubble,” Phys. Rev. Lett. 103, 135004 (2009).
[10] N. H. Matlis, S. Reed, S. S. Bulanov, V. Chvykov, G. Kalinitchenko, T. Matsuoka, P. Rousseau, V. Yanovsky, A. Maksimchuk, S. Kalmykov, G. Shvets, and M.C. Downer, “Snapshots of laser wakefields,” Nature Phys. 2, 749 (2006).