High-Density Dynamics of Laser Wakefield Acceleration from Gas Plasmas to Nanotubes

Abstract

The electron dynamics of laser wakefield acceleration (LWFA) is examined in the high-density regime using particle-in-cell simulations. These simulations model the electron source as a target of carbon nanotubes. Carbon nanotubes readily allow access to near-critical densities and may have other advantageous properties for potential medical applications of electron acceleration. In the near-critical density regime, electrons are accelerated by the ponderomotive force followed by the electron sheath formation, resulting in a flow of bulk electrons. This behavior represents a qualitatively distinct regime from that of low-density LWFA. A quantitative entropy index for differentiating these regimes is proposed. The dependence of accelerated electron energy on laser amplitude is also examined. For the majority of this study, the laser propagates along the axis of the target of carbon nanotubes in a 1D geometry. After the fundamental high-density physics is established, an alternative, 2D scheme of laser acceleration of electrons using carbon nanotubes is considered.

Laser Beat-Wave Acceleration near Critical Density

“…However, when 𝑛𝑛 𝑒𝑒 is approaches the critical density, additional physics must come into play. Nicks et al showed electron energy scaling with respect to laser intensity (1 > 𝑎𝑎 0 ≥ 0.1) for 0.1𝑛𝑛 𝑐𝑐𝑐𝑐𝑖𝑖𝑐𝑐 and 0.3𝑛𝑛 𝑐𝑐𝑐𝑐𝑖𝑖𝑐𝑐 in their Figure 5a,b [7]. Nicks et al also showed that a somewhat discontinuous maximum energy gain of electrons as a function of 𝑎𝑎 0 (around 1) is observed.…”

“…As mentioned above, it is conventional wisdom that the maximum electron acceleration energy achieved using laser wakefield schemes is proportional to �𝜔𝜔 0 𝜔𝜔 𝑝𝑝 ⁄ � 2 (due to the relativistic dynamics in underdense plasma). For this reason, we sought to see the limitations of the BWA with respect to density, similar to what was done by Nicks et al [7]. We ran eight BWA simulations like those shown in Figure 2, with a pump and seed intensity given by 𝑎𝑎 0 , 𝑎𝑎 1 = 0.1 and Gaussian pulse width of 100 fs, just below the nonrelativistic regime.…”

“…Valenta et al determined that electron densities of roughly 0.1n c were necessary for high repetition rate, low energy, and short pulse lasers [6]. More recently, Nicks et al further explored how one can achieve bulk acceleration of electrons by exploring the maximum energy achieved for different near-critical densities, laser intensities, and laser pulse widths [4,7]. We note that in the nearcritical densities the gas plasma is replaced by other materials such as nanomaterials [8].…”

Long-term future particle accelerators

“…For more details see Ref. [68]. The current state-of-the-art of the RF techniques is limited to gradients on the order of 100 MV/m.…”

“…For example, recent theoretical and numerical studies have shown the feasibility of obtaining tens of TV/m fields in nanomaterials by excitation of non-linear plasmonic modes driven by a high density sub-micron particle bunch [62,63]. To obtain multi-TV/m fields other techniques rely on the ablation of solids [67] or the induced electron field emission from nanomaterials [68].…”