C.H. Lai

Photon acceleration from rest

In this paper we present a new mechanism for generating coherent radiation from a laser‐ionized gas‐filled capacitor array. The scheme directly converts a static electric field of wave number k0 into coherent radiation pulses. The physical mechanism is analogous to photon acceleration in plasma waves, and in this sense, photons are accelerated from zero frequency (zero energy) to a frequency ω2p/2k0c. The frequency (energy) of the photons is controlled by the gas pressure and/or capacitor spacing and has broad tunability over several orders of magnitude. We calculate the output frequency and power and compare theoretical predictions to 2D PIC simulations.

Tunable microwaves from Cerenkov wakes in magnetized plasma

Summary form only given, as follows. The Cerenkov wake excited by a particle beam or short laser pulse in a perpendicularly magnetized plasma is analyzed. The wake has both electrostatic and electromagnetic components and couples to a vacuum microwave at the plasma/vacuum boundary. The transmission coefficient depends on the boundary scale length and the ratio of /spl omega//sup c///spl omega//sub p/. The frequency of the forward going radiation is approximately /spl omega//sub p/, and its amplitude is /spl omega//sup c///spl omega//sub p/ times the amplitude of the wake excited in the plasma (for a sharp boundary). The possibility of tapering the magnetic field and density profile to optimize the output coupling is then discussed for a continuous boundary. We also use 1D PIC simulations to verify the scaling laws. Since plasma wakes as high as a few GeV/m are produced in current experiments, the potential for a high power (i.e., Gwatt) coherent microwave to THz radiation source exists.

A Cherenkov-emission microwave source

Summary form only given. In an unmagnetized plasma, there is no Cherenkov emission because the phase velocity /spl upsi//sub /spl phi// of light is greater than c. In a magnetized plasma, the situation is completely changed. There is a rich variety of plasma modes with phase velocities /spl upsi//sub /spl phi///spl les/c which can couple to a fast particle. In the magnetized plasma, a fast particle or particle beam excites a Cherenkov wake that has both electrostatic and electromagnetic components. Preliminary simulations indicate that at the vacuum/plasma boundary, the wake couples to a vacuum microwave with an amplitude equal to the electromagnetic component in the plasma. For a weakly magnetized plasma, the amplitude of the outcoupled radiation is approximately wc/w/sub p/ times the amplitude of the wake excited in the plasma by the beam, and the frequency is approximately w/sub p/. Since plasma wakes as high as 100 MeV/m are expected in near-term experiments, the potential for a high-power, coherent microwave to THz source exists. A brief overview of the scaling laws are presented, followed by 1-D and 2-D PIC simulations. Prospects for a tuneable microwave source experiment based on this mechanism at the UCLA plasma wakefield accelerator facility are discussed.

A DC to AC radiation converter based on plasma ionization

Summary form only given, as follows. We describe programs on a new class of high power radiation source. Rather than create electromagnetic fields, the present scheme frequency upshifts an existing static field (/spl omega/=0, k=k/sub 0/) by temporally varying the dielectric properties of a medium (i.e., ionizing a gas). An array of alternating capacitors is charged to a large voltage to produce a static electric field of the form E/spl sim/(E/sub 0/ sin k/sub 0/ Z) y. When the region between the capacitors is filled with a low density working gas and ionized by a short-pulse laser, a phased discharge current generates a radiation pulse following behind the ionizing laser. The frequency of the radiation generated scales as f=8 GHz (n/sub 0//10/sup 12/ cm/sup -3/) (1 cm/k/sub o/), where n/sub 0/ is the plasma density. The output frequency, pulse duration, bandwidth, arbitrary chirp, etc. can be controlled by varying the gas pressure and/or capacitor spacing. Output power is determined by the bias voltage, and high powers are in principal possible by using pulsed biases. At USC/UCLA proof-of-principle experiments are underway to test the predicted characteristics of this alternative source of high-power microwaves. Preliminary results are presented.

Photon acceleration from rest to the speed of light

A new mechanism for generating coherent radiation from a laser-ionized gas-filled capacitor array is presented. This scheme directly converts a static electric field of wave number k/sub 0/ into coherent radiation pulses. The physical mechanism is analogous to photon acceleration in plasma waves, and in this sense, photons are accelerated from zero frequency (zero energy) to a frequency /spl omega/p/sup 2//2k/sub 0/c. The photon frequency is tuned by gas pressure and/or capacitor spacing. In this paper we briefly describe the theory of this scheme and then present the preliminary experimental setup and results.

Simulations of the interaction between a light wave and an ionization front in a DC magnetic field

We use a 1-D particle-in-cell simulation code to investigate the interaction of a light wave with a step relativistic ionization front in the presence of an applied D.C. magnetic field either perpendicular or parallel to the incident wave. It is seen that four transmitted waves are generated in each case. The frequency upshifts and transmission coefficients of those transmitted waves are measured from the simulation and compared to theoretical predictions. Furthermore, the phenomenon of a density ripple associated with the free streaming mode in the perpendicular case is also observed in the simulation, and the modulation rates (?n/n0) are close to theoretical values.

Frequency upshifting by an ionization front in a magnetized plasma

Summary form only given, as follows. The interaction of a light wave with a relativistic ionization front in the presence of an applied DC magnetic field which is perpendicular or parallel to the incident wave has been investigated. In both cases, four transmitted modes are generated in the magnetized plasma by an incident linearly polarized wave. The frequency upshifts of the various modes have been calculated and compared to the unmagnetized case. The corresponding reflection and transmission coefficients have also been obtained. Finally, the density ripple associated with the free streaming mode in a magnetized plasma for the perpendicular case has been considered.