Virtual Laser Plasma Laboratory Research group of Prof. Dr. Alexander Pukhov

Bubble regime of electron acceleration

The concept of laser-plasma electron acceleration has the decisive advantage over conventional accelerators:plasma supports electric fields orders of magnitude higher than the breakdown-limited field in radio-frequency cavities of conventional linacs. It is expected that the relativistic laser-plasma will finally lead to a compact high energy accelerator

The electrons can be accelerated either directly by the laser pulse or in the laser wake field plasma wave.  Laser Wake Field Acceleration (LWFA) works for laser pulses shorter than the plasma wavelength. When driven into the highly non-linear wave breaking regime, we could show recently 
 [A. Pukhov and J. Meyer-ter-Vehn, Laser wake field acceleration: The highly non-linear broken-wave regime, Appl. Phys. B 74, 355 (2002)] that ultra-short bunches of electrons with superior properties 
are produced. Large amounts of electrons are self-trapped and accelerated to relativistic energies (gamma-factors of 100 - 1000) with high efficiency.Very dense, low-emittance pulses are created. The particular breakthrough is that quasi-monoenergetic electron beams have been predicted.

Just two years later, in 2004, three experimental groups have independently observed monoenergetic beams of electron from short pulse interactions with underdense gas jets.

Our group has simulated the experiment done at LOA, Palaiseau (France):The experiment demonstrated the generation of high quality electron beams from ultraintense laser-plasma acceleration. Extremely collimated beams with 10 mrad divergence and 0.5 nC (+/- 0.2 nC) of charge at 170 MeV +/- 20 MeV have been observed. Contrary to all previous results obtained from laser-plasma accelerators, the electron energy distribution was  quasi-monoenergetic. The number of high energy electrons (170 MeV) is increased by at least three orders of magnitude with respect to previous works. This high performance was obtained in the bubble acceleration regime: a single laser beam was used to generate a plasma bubble, able to trap and accelerate plasma electrons to high energy.

The formation of a plasma bubble is optimized when the plasma is fully resonant with the laser pulse, i.e., when the transverse and longitudinal dimensions of the laser pulse are about the plasma wavelength. In this case, the ponderomotive force of the laser expels violently all electrons, creating a solitary cavity (or plasma bubble) around the laser pulse. This cavity carries very high electric fields, on the order of the wavebreaking fieldE₀=2pm_e/el_p, where m_e and e are respectively the electron mass and charge (E₀=100 GV/m for a n_e=10¹⁸ cm^-3 electron plasma density). In spite of these extremely high fields, the cavity maintains its structural integrity and propagates as long as the laser intensity stays high. In addition, the cavity is able to trap electrons: as the laser propagates, it fills up with electrons which accumulate in a given region of phase space, causing the energy distribution of electrons to become quasi-monoenergetic .

For more information, please. look at:

Faure J, Glinec Y, Pukhov A, Kiselev S, Gordienko S, Lefebvre E, Rousseau JP, Burgy F,Malka V A laser-plasma accelerator producing monoenergetic electron beams, NATURE 431, 541 (2004)