Modeling of laser wakefield acceleration atCO2laser wavelengths

Abstract

The upgraded Accelerator Test Facility (ATF) CO 2 laser located at Brookhaven National Laboratory offers a unique opportunity to investigate laser wakefield acceleration (LWFA) with a 10:6-m laser, a wavelength where little experimental work exists. While long laser wavelengths have certain advantages over short wavelengths, our modeling analysis has uncovered another important effect. The upgraded ATF CO 2 laser will have a pulse length as short as 2 ps. At a nominal plasma density of 10 16 cm ÿ3 , this pulse length would normally be considered too long for resonant LWFA, but too short for self-modulated LWFA. However, our model simulations indicate that a well-formed wakefield is nevertheless generated with electric field gradients of E z * 2 GV=m assuming 2.5 TW laser peak power. The model indicates pulse steepening is occurring due to various nonlinear effects. It is possible that this intermediate laser pulse length mode of operation may permit the creation of well-formed, regular-shaped wakefields, which would be needed for staging the LWFA process. Discussed in this paper are the model, its predictions for an LWFA experiment at the ATF, and the pulse steepening effect.

Quasistatic capillary discharge plasma model

“…This model supports the staged electron laser acceleration -laser wakefield (STELLA-LW) experiment being performed at the Brookhaven National Laboratory Accelerator Test Facility (BNL-ATF) [13]. This experiment will be testing the pseudoresonant LWFA [9] and seeded SM-LWFA [10] schemes. For the conditions at the ATF, these methods require relatively low plasma densities ( 10 16 -10 17 cm ÿ3 ).…”

“…There is another common operating regime called selfmodulated LWFA (SM-LWFA) [6 -8], which is a more complex process relying on nonlinear effects occurring between the laser pulse and the plasma. There are also hybrid approaches, such as pseudoresonant LWFA [9], where t L is longer than dictated by Eq. (1), but laser pulse steepening effects occurring within the plasma effectively create Fourier components of a shorter laser pulse so that Eq.…”

Seeded self-modulated laser wakefield acceleration

“…(4)-(7) from [13] except for the addition of a new term j b in Eq. (3) that represents the current density of the seed e-beam bunch.…”

“…A model to simulate LWFA generation using a CO 2 laser pulse was developed earlier in conjunction with analysis of another scheme called pseudoresonant LWFA [13]. This model is well suited for also simulating seeded SM-LWFA; however, it required two modifications.…”

“…As done in [13], we make a quasistatic approximation [14] by defining dimensionless coordinates moving with the laser pulse variables k p0 z ÿ ct; k p0 z; k p0 r ? ; (6) where k p0 !…”