Abstract
Photoionization by a femtosecond, terawatt laser pulse generates a plasma column in a neutral ambient gas. Velocity of electrons, pushed by the laser ponderomotive force along the column surface, couples to the the radial density gradient at the column boundary, generating an azimuthally polarized THz rotational current (RC). The same mechanism produces the low-frequency RC in a leaky plasma channel. Applying external voltage to the channel induces a radially non-uniform electron flow (direct current) and a constant, azimuthally polarized magnetic field. Coupling them to the electron density perturbations adds two more terms to the RC. The surface RC in the plasma column supports a broadband, evanescent THz signal accompanying the wake. A few millimeters away from the column, rapid evanescence of high-frequency components turns this THz signal into a radially polarized, single-cycle pulse.
Highlights
Multi-dimensional electron plasma oscillations [1] had been on the radar of advanced accelerator community for decades
A particular interest was expressed in electron plasma waves (EPWs), dubbed “laser wakes”, driven by the ponderomotive force of a terawatt (TW), femtosecond laser pulse [2]–[7]
B is of the order of a few percent, meaning that the direct current (DC)-induced magnetic field is of the same order of magnitude as the electric field in the linear wake
Summary
Multi-dimensional electron plasma oscillations [1] had been on the radar of advanced accelerator community for decades. The process may go in reverse: The EPW excited in the stratified plasmas may be converted into EM waves, generating at every point of the density down-ramp an EM wave with a frequency equal to the local Langmuir frequency [16, 17] For this kind of wave transformation to occur, electron velocity in the EPW must have a component orthogonal to the density gradient [9]. At every point of the density side-ramp, this current converts the EPW into a radially polarized EM mode with a frequency equal to the local Langmuir frequency Continuum of these modes forms a broadband THz EMP accompanying the wake [18].
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