Self-modulated laser wakefield accelerators as x-ray sources

Abstract

The development of a directional, small-divergence, and short-duration picosecond x-ray probe beam with an energy greater than 50 keV is desirable for high energy density science experiments. We therefore explore through particle-in-cell (PIC) computer simulations the possibility of using x-rays radiated by betatron-like motion of electrons from a self-modulated laser wakefield accelerator as a possible candidate to meet this need. Two OSIRIS 2D PIC simulations with mobile ions are presented, one with a normalized vector potential a 0 = 1.5 and the other with an a 0 = 3. We find that in both cases direct laser acceleration (DLA) is an important additional acceleration mechanism in addition to the longitudinal electric field of the plasma wave. Together these mechanisms produce electrons with a continuous energy spectrum with a maximum energy of 300 MeV for a 0 = 3 case and 180 MeV in the a 0 = 1.5 case. Forward-directed x-ray radiation with a photon energy up to 100 keV was calculated for the a 0 = 3 case and up to 12 keV for the a 0 = 1.5 case. The xray spectrum can be fitted with a sum of two synchrotron spectra with critical photon energy of 13 and 45 keV for the a 0 of 3 and critical photon energy of 0.3 and 1.4 keV for a 0 of 1.5 in the plane of polarization of the laser. The full width at half maximum divergence angle of the x-rays was 62 x 1.9 mrad for a 0 = 3 and 77 x 3.8 mrad for a 0 = 1.5.

Observation of Betatron X-Ray Radiation in a Self-Modulated Laser Wakefield Accelerator Driven with Picosecond Laser Pulses

“…µm, the back of the laser has expelled the electrons through relativistic self-focusing to form an ion channel. In this region, electrons are mainly accelerated by DLA [27], where, for our simulation parameters, DLA has been shown to be a physical effect and not a numerical artifact [29]. Figure 4B decaying spectrum extending out to 300 MeV is observed.…”

“…for a variety of conditions [27]. We illustrate the salient observations from one simulation that uses an a 0 = 3, τ =0.…”

Role of Direct Laser Acceleration of Electrons in a Laser Wakefield Accelerator with Ionization Injection

“…In this Letter, we show that in addition to wakefield acceleration, LWFAs can have all the conditions needed to transfer energy from the laser field to the electrons via a process known as direct laser acceleration (DLA) [4][5][6][7][8][9][10]. This process can be exploited to microbunch electron beams on attosecond timescales [11] or to enhance the x-ray radiation produced by betatron motion of electrons in LWFA [12,13].…”

“…The DLA process in LWFA can be optimized further [9,10] and must be considered in LWFAs that employ other injection schemes [13].…”

Betatron x-ray radiation from laser-plasma accelerators driven by femtosecond and picosecond laser systems

“…Electrons now can be trapped in several buckets and gain energy from both the longitudinal field of the plasma wave and also directly from the laser field in a process called direct laser acceleration (DLA) 17 if they overlap the drive laser. 18,19 Just like in the blowout regime, electrons trapped off axis in SMLWFA undergo betatron oscillations, 20 which in this case are reinforced by their overlap with the laser pulse. If the laser intensity is increased to above 10 20 W/cm 2 , the SMLWFA structure no longer exists, and electrons, accelerated by DLA alone, 21 also emit betatron x-rays.…”