Vacuum acceleration of electrons in a dynamic laser pulse

Abstract

A planar laser pulse propagating in vacuum can exhibit an extremely large ponderomotive force. This force, however, cannot impart net energy to an electron: As the pulse overtakes the electron, the initial impulse from its rising edge is completely undone by an equal and opposite impulse from its trailing edge. Here we show that planar-like "flying focus" pulses can break this symmetry, imparting relativistic energies to electrons. The intensity peak of a flying focus-a moving focal point resulting from a chirped laser pulse focused by a chromatic lens-can travel at any subluminal velocity, forwards or backwards. As a result, an electron can gain enough momentum in the rising edge of the intensity peak to outrun and avoid the trailing edge.Accelerating the intensity peak can further boost the momentum gain. Theory and simulations demonstrate that these dynamic intensity peaks can backwards accelerate electrons to the MeV energies required for radiation and electron diffraction probes of high energy density materials.Vacuum laser acceleration (VLA) exploits the large electromagnetic fields of highintensity laser pulses to accelerate electrons to relativistic energies over short distances [1][2][3][4][5][6][7][8][9][10][11][12].The field of an intense pulse can far surpass that in conventional radio-frequency (RF) or advanced plasma-based accelerators, and the underlying interaction-involving only an electron and the electromagnetic field-has an appealing simplicity. RF accelerators routinely improve beam quality and achieve unprecedented energies, but their low damage threshold constrains the maximum accelerating field. This necessitates physically and economically immense structures to accelerate electrons to the energies necessary for high energy density probes, radiation sources such as free electron lasers, or high-energy physics experiments [13][14][15][16]. Wakefield accelerators, on the other hand, employ plasma to sustain accelerating fields nearly 1000x that of RF accelerators [17][18][19][20][21][22][23]. The use of plasma, however, comes with its own set of challenges, such as tuning the laser or electron beam parameters to the plasma conditions, avoiding a myriad of instabilities, and creating long uniform plasma channels [24,25].

Nonlinear Thomson scattering with ponderomotive control

“…Matched NLTS represents the optimal case of NLTSPC. With a smaller ponderomotive velocity, the accelerated electron would outrun the intensity peak; with a larger, but still subluminal, ponderomotive velocity, the pulse would eventually overtake and outrun the accelerated electron, limiting the interaction length [42,45]. For large vector potentials (a 0 1), the optimal scalings (Table 1) result in a radiated power far greater and an emission cone far narrower than either drift-free or conventional NLTS (Figs.…”

“…By adjusting the chirp, the resulting intensity peak, and therefore the ponderomotive force, can travel at any velocity, either forward or backward with respect to the phase fronts, over distances much longer than a Rayleigh range [38,39]. Aside from extending the interaction length, a ponderomotive force that counter-propagates with respect to the phase fronts can accelerate an electron in NLTS [42] and provide unique insight into the corresponding quantum process, i.e., nonlinear Compton scattering [43].…”

“…The dashed lines indicate the "matched" and "drift-free" conditions of NLTSPC. Within the gray region, the ponderomotive force of the intensity peak accelerates the electron to a velocity greater than β I , and the electron outruns the intensity peak-a situation not considered here[42]. The insets depict the cycle-averaged electron trajectories (black lines) relative to the motion of the intensity peak (contours) for the drift-free and matched regimes.…”

Spatiotemporal control of laser intensity through cross-phase modulation

“…By exploiting space-time correlations in the amplitude or phase of a laser pulse, a number of recent techniques have created arbitrary-velocity intensity peaks that remain nearly propagation invariant over distances much longer than a Rayleigh range [1][2][3][4][5][6][7][8]. These features promise to revolutionize a wide range of laser-based applications, including high-power amplifiers [9], radiation sources [10][11][12], and compact accelerators [6,7,[13][14][15]. Nevertheless, each of the existing techniques requires an optical configuration that constrains properties of the intensity peak, such as its transverse or temporal profile, duration, or orbital angular momentum (OAM).…”

Optical Shock-Enhanced Self-Photon Acceleration

“…A photon accelerator based on SFF driven plasma could be optimized further to deliver higher frequencies, shorter pulses and better efficiency. By tailoring the initial axial density profile of the target gas and the spatiotemporal shaping together, a shock could be generated at higher densities, accelerating and then stabilizing over long-distances at lower densities [31,32], which could enable the generation of shorter extreme ultraviolet wavelengths, and possibly a table top-source of coherent soft x-rays. This report was prepared as an account of work sponsored by an agency of the U.S. Government.…”