RU2189679C2 - Process of pumping of active medium into continuous-wave gaseous-discharge laser with electrodes carrying dielectric coats - Google Patents

Abstract

FIELD: quantum electronics, development of continuous-wave gaseous- discharge lasers. SUBSTANCE: process can also find use in other fields where it is necessary to excite large volumes of spatially-homogeneous plasma, for instance, in plasmochemistry. Traditional process of pumping of active medium into continuous- wave gaseous-discharge laser with electrodes carrying dielectric coat includes excitation of glow discharge by harmonic electric field, simultaneous stabilization of glow discharge by pulse voltage of audio frequency. Pulse length does not exceed 10^-7 s. Amplitude U satisfies condition l≤2λ_i(e•U), where l is interelectrode space; e is electron charge; λ_i(e•U) - is length of free path of electron with energy e U relative to act of ionization. EFFECT: increased specific power injected into active medium of laser, raised power of coherent radiation picked off unit of volume of active medium.

Description

 The invention relates to the field of quantum electronics and can be used to create continuous discharge gas lasers. In addition, it can be applied in other areas where it is necessary to excite large volumes of spatially homogeneous plasma, for example, in plasma chemistry.

 Currently known methods for producing a spatially uniform large-volume plasma for pumping the active medium of gas-discharge lasers, or for plasma chemistry, are limited in terms of the specific power deposited in the discharge due to the formation of contracted structures as a result of the development of ionization instabilities [1].

 Closest to the proposed invention, selected by the inventors for the prototype, is a method of pumping the active medium of a gas-discharge laser with electrodes with dielectric coatings, including excitation of a glow discharge by a harmonic electric field [2].

 A plasma of such a discharge has a relatively high stability with respect to contraction due to the stabilizing role of dielectric coatings that perform the function of ballast resistances distributed in space.

 However, the presence of these coatings does not solve the problem of contraction in an adequate manner. The fact is that there are two types of ionization instabilities, the development of one of which (spatially homogeneous) is suppressed by dielectric coatings, and the other (spatially inhomogeneous) does not. The latter circumstance is explained by the fact that in the case of spatially inhomogeneous instabilities, the discharge current density at the ends of the plasma column remains unchanged and such instabilities cannot be stabilized by external (ballast) resistances. The presence of instabilities of this type significantly limits in the known method the possibility of increasing the level of specific power deposited in a spatially uniform large-volume plasma for pumping the active medium of cw gas-discharge lasers.

 Using the present invention, a technical result is achieved, consisting in increasing the specific power deposited in the active medium of the laser, and increasing the power of coherent radiation removed from a unit volume of the active medium.

The specified technical result is achieved by the fact that in the known method of pumping the active medium of a continuous gas-discharge laser with electrodes with dielectric coatings, which includes emitting a glow discharge by a harmonic electric field, the glow discharge is simultaneously stabilized by a pulse voltage of an audio frequency of a duration not exceeding 10 ^-7 s and amplitude U satisfying the condition l≤2λ _i (e • U), where:
l is the interelectrode distance,
e is the electron charge,
λ _i (e • U) is the mean free path of an electron with energy eU with respect to the ionization act.

, которая имеет место даже при относительно невысоких (порядка нескольких Вт/см³) величинах вкладываемой в разряд удельной мощности, падение напряжения на плазменном столбе тоже будет небольшим. Поэтому можно считать, что все напряжение U, подводимое к электродам во время действия импульса, практически полностью сосредоточится на катодном слое аномального тлеющего разряда, сформированного этим импульсом. В результате в данном слое образуется электронный пучок с энергией ≈ eU, который обеспечивает предварительную ионизацию лазерной смеси во всем разрядном промежутке и одновременно подавляет пространственно-неоднородные неустойчивости, поднимая тем самым пороговую величину вкладываемой в разряд удельной мощности, при которой еще не происходит контракция. При этом, очевидно, имеет место и повышение мощности когерентного излучения, снимаемого с единицы объема активной среды. Следует подчеркнуть, что импульсы напряжения осуществляют только предыонизацию и подавление пространственно-неоднородных неустойчивостей, накачка же активной среды происходит под действием гармонического электрического поля, подводимого к электродам одновременно с импульсами. Очевидно, что частота следования импульсов не должна быть меньше обратной величины характерного времени распада плазмы β_rn_e, где
β_r - коэффициент диссоциативной рекомбинации,
n_e - плотность электронов в плазме.The method is implemented as follows. Impulse voltage leads to the formation of an abnormal glow discharge during its operation. If the pulse duration is small (less or much less than 10 ^-7 s), the capacitance of the dielectric coatings, and hence the voltage drop across these coatings, will be relatively small. At the same time, due to the rather significant plasma conductivity

, which takes place even at relatively low (of the order of several W / cm ³ ) values of the specific power deposited in the discharge, the voltage drop across the plasma column will also be small. Therefore, we can assume that all the voltage U supplied to the electrodes during the action of the pulse will almost completely concentrate on the cathode layer of the anomalous glow discharge formed by this pulse. As a result, an electron beam with an energy of ≈ eU is formed in this layer, which provides preliminary ionization of the laser mixture in the entire discharge gap and simultaneously suppresses spatially inhomogeneous instabilities, thereby raising the threshold value of the specific power deposited into the discharge, at which contraction does not yet occur. In this case, obviously, there is also an increase in the power of coherent radiation removed from a unit volume of the active medium. It should be emphasized that voltage pulses carry out only preionization and suppression of spatially inhomogeneous instabilities, while the active medium is pumped by a harmonic electric field supplied to the electrodes simultaneously with the pulses. Obviously, the pulse repetition rate should not be less than the reciprocal of the characteristic plasma decay time β _r n _e , where
β _r is the coefficient of dissociative recombination,
n _e is the electron density in the plasma.

Usually β _r ~ 10 ^-6 ÷ 10 ^-7 cm ³ / s. Then, assuming for estimates n _e ~ 10 ¹¹ cm ^-3 , we find that the corresponding frequency belongs to the sound range and turns out to be of the order of 10-100 kHz.

Величину λ_i легко определить при заданном составе лазерной смеси, исходя из широко представленных в литературе значений сечений ионизации атомов и молекул электронным ударом:

где N_k и σ $\genfrac{}{}{0ex}{}{\text{(k)}}{\text{i}}$ - плотность и сечение ионизации k-ого компонента лазерной смеси. При этом σ $\genfrac{}{}{0ex}{}{\text{(k)}}{\text{i}}$ должны соответствовать энергии электрона, равной eU.The electron beam intensity decreases sharply (exponentially) with increasing distance from the electrode (more precisely, from the cathode layer). If the voltage pulses are alternating, then the electron beam is formed alternately from each of the two electrodes. Then the zone of the most effective action of this beam is determined by the condition

The value of λ _{i is} easy to determine for a given composition of the laser mixture, based on the values of the cross sections of ionization of atoms and molecules by electron impact, which are widely represented in the literature:

where N _k and σ $\genfrac{}{}{0ex}{}{\text{(k)}}{\text{i}}$ - density and ionization cross section of the kth component of the laser mixture. Moreover, σ $\genfrac{}{}{0ex}{}{\text{(k)}}{\text{i}}$ must correspond to the electron energy equal to eU.

 Using the proposed method allows to suppress contraction and to achieve the maximum possible values of specific powers deposited in the discharge and removed in the form of coherent radiation, the values of which are limited only by overheating of the laser mixture.

Sources of information
1. Lipatov N.I., Mineev A.P., Myshenkov V.I., Pashinin P.P., Prokhorov A.M. "Nonlinear structures in a gas discharge and excitation of the active medium in high-power gas-discharge molecular lasers." Proceedings of IOFAN, v. 17, p. 3-5, M., Science, 1989.

2. Yasui K., Kuzumoto M., Ogawa S., Tanaka M. and Yagi S. "Silent - Discharge Excited TEM ₀₀ 2.5kW CO ₂ Laser.IEEE Journal of Quantumn Electronics, vol. 25, 4, pp. 836- 839, 1989 - prototype.

Claims (1)

A method for pumping an active medium of a cw gas discharge laser with dielectric coatings, which includes emitting a glow discharge with a harmonic electric field, characterized in that the glow discharge is stabilized simultaneously by a pulse voltage with a pulse repetition rate β _r • n _e , where β _r is the dissociative recombination coefficient, n _e is the electron density in the plasma, and the amplitude U satisfying the condition l≤2λ _i (e • U), where l is the interelectrode distance, e is the electron charge, λ _i (e • U) is the mean free path of the electron throne with energy e • U in relation to the act of ionization.