Relativistic Ionization Front Research Articles

Ultra-short laser-produced optical microcavities and ionization fronts

A class of nonlinear optical effects related to fast field ionization in an interference pattern is investigated by numerical simulations. Interference between counter-propagating ultra-short pulses slightly below the ionization threshold produces a layered distribution of free-electron density. In a dense dielectric target, this effect allows us to trap light between plasma layers creating a sort of optical microcavity. Other peculiar features include frequency upshift, pulse lengthening and self-generated relativistic ionization fronts.

Frequency upshifting of electromagnetic radiation via oblique incidence on an ionization front

Frequency upshifting of electromagnetic radiation impinging on a relativistically moving ionization front is theoretically investigated. Unlike previous works in this field treating the case of normal incidence and qualitatively similar case of oblique incidence of a transverse electric polarized wave, oblique incidence of a transverse magnetic polarized wave on the front is considered. The peculiarities of the case under consideration are connected with the generation of Langmuir waves behind the front and Brewster's phenomenon. We present a complete analysis of the incident wave transformation including analysis of the frequencies and amplitudes of the waves excited ahead of and behind the front. Special emphasis is made on energy transformation in the case when a wave packet is incident on the front. In particular, we show that even for negligible angles of incidence, energy losses via transformation into Langmuir waves may be very high (up to /spl sim/60%). In general, generation of Langmuir waves may play a significant role in the plasma-based radiation sources with relativistic ionization fronts.

Generation of microwave pulses from the static electric field of a capacitor array by an underdense, relativistic ionization front

The dc to ac radiation converter is a new device in which a relativistic ionization front directly converts the static electric field of an array of alternatively biased capacitors into a pulse of tunable radiation. In a proof-of-principle experiment frequencies between 6 and 21 GHz were generated with plasma densities in the 1012 cm−3 range and a capacitor period 2d=9.4 cm. In the present experiment, short pulses with frequencies between 39 and 84 GHz are generated in a structure with 2d=2 cm. The frequency spectra of these pulses are measured using a diffraction grating. The spectra are discrete, and their center frequency varies linearly with the gas pressure prior to ionization (or plasma density), as expected from theory. Their relative spectral width is around 18%, consistent with the expected number of cycles (six) contained in the pulses. An upper limit of 750 psec (bandwidth detection limited) is placed on the pulses length. The emitted frequency increases from 53 to 93 GHz when the capacitors are connected by pair to obtain a effective array period of 4 cm.

Experimental Observation of Short Microwave Generation via Relativistic Ionization Front Produced by CO2 Laser

A short microwave pulse is generated by the interaction between the relativistic ionization front produced by CO2 laser and a static, zero-frequency electric field. The zero-frequency electric field is produced by an alternately biased capacitor array. Laser light propagation between electrodes produces the ionization front propagating with the velocity equal to the laser group velocity. The maximum frequency of the generated microwave is determined to be in the range of 8.2 to 17.6 GHz by several waveguides with specific cut-off frequency and horn antenna combination.

Generation of ultrashort, discrete spectrum microwave pulses using the dc to ac radiation converter

The output radiation of a dc to ac radiation converter is characterized. A relativistic ionization front passing through a capacitor array of period d=1 cm produces short pulses of tunable radiation between 39 and 84 GHz with a gas pressure between 0 and 30 mT. The frequency spectra of the produced pulses are discrete and exhibit full widths at half maximum between 12% and 28%, consistent with the expected width for six cycles’ pulses. An upper bound of 750 ps (detection bandwidth limited) is placed on the pulse widths. These are the shortest pulses produced by a source of coherent radiation in this frequency range.

Experimental Evidence of Photon Acceleration of Ultrashort Laser Pulses in Relativistic Ionization Fronts

The frequency up-shifts of ultrashort laser pulses (65 fs) propagating in opposite directions (20\ifmmode^\circ\else\textdegree\fi{} and 160\ifmmode^\circ\else\textdegree\fi{}) with respect to a relativistic ionization front (interface gas plasma) are measured for the first time. Up-shifts of the order of 25 nm are observed. A very good agreement is found with a two-dimensional ray-tracing theory.

Demonstration of Microwave Generation from a Static Field by a Relativistic Ionization Front in a Capacitor Array.

We present the results of a proof-of-principle experiment to demonstrate the generation of tunable radiation from a laser-ionize gas-filled capacitor array. This scheme directly converts a static electric field of wave number {ital k}{sub 0} into coherent radiation pulses of frequency {omega}{sup 2}{sub {ital p}}/2{ital k}{sub 0}{ital c}, where {omega}{sub {ital p}} is the plasma frequency. The radiation frequency can be tuned by varying gas pressure and/or capacitor spacing. In this experiment, well-polarized, short (less than 5 ns) microwave pulses have been generated over a frequency range of 6 to 21 GHz. The frequency of the detected signal, as measured with cut-off waveguides, scales linearly with the plasma density, and the relative power of the signal scales quadratically with the dc bias voltage in agreement with the theory. {copyright} {ital 1996 The American Physical Society.}

Conversion of dc Fields in a Capacitor Array to Radiation by a Relativistic Ionization Front.

A mechanism for generating coherent radiation of high power, variable duration, and broad tunability over several orders of magnitude from a laser-ionized gas-filled capacitor array is described. The scheme directly converts a dc electric field "wave" into a coherent electromagnetic wave train when a relativistic ionization front passes between the plates. The frequency and duration of the radiation is controlled by the gas pressure and/or capacitor spacing. Output frequency and power are calculated and compared to 2D particle-in-cell simulations.

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 (Δn/n0) are close to theoretical values.

Frequency upshifting by an ionization front in a magnetized plasma

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 is considered. 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 are calculated and compared to the unmagnetized case. The corresponding reflection and transmission coefficients are also obtained. Finally, the density ripple associated with the free streaming mode in a magnetized plasma for the perpendicular case is discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Generation of tunable radiation using an underdense ionization front.

It is shown that when a light pulse is incident upon an underdense relativistic ionization front, the light pulse can be simultaneously upshifted in frequency and compressed in duration. The reflection and transmission coefficients are examined. Examples for both overdense and underdense fronts are given to show that a relativistic ionization front can generate tunable radiation from millimeter to ultraviolet wavelengths.

Frequency up-shifting of laser pulses by copropagating ionization fronts.

A method for up-shifting the frequency of a subpicosecond laser pulse that utilizes the interaction of the pulse with a co-propagating relativistic ionization front is examined. The induced frequency shift is found to initially scale linearly with the propagation time \ensuremath{\tau}. Asymptotically, the frequency scales as ${\mathrm{\ensuremath{\tau}}}^{1/2}$. Phase-slippage limitations may be overcome by appropriately increasing the plasma density as a function of \ensuremath{\tau}, thus allowing for substantially higher-frequency shifts.