Self-initiated volume discharge pumps chemical lasers

The nonchain HF(DF) laser is a safe and conveniently operated chemical laser; it can be pumped by a discharge that forms and spreads automatically to fill the discharge volume.

Th 139771

The nonchain HF(DF) laser is a safe and conveniently operated chemical laser; it can be pumped by a discharge that forms and spreads automatically to fill the discharge volume.

Hydrogen fluoride (HF)/deuterium fluoride (DF) chemical lasers—so-called HF(DF) lasers—are compact and efficient sources of infrared radiation, with emission wavelength ranges of 2.6 to 3.4 µm for HF and 3.5 to 4.4 µm for DF. Some HF(DF) lasers operate via an electron-beam-induced chain chemical reaction; while these lasers are very efficient, the radiative activity of the high-energy electron beam results in safety issues. Another type of HF(DF) laser—the nonchain electric-discharge-controlled laser—is safer and more convenient to operate.

We at the Prokhorov General Institute of Russian Academy of Sciences have investigated a method of pumping nonchain HF(DF) lasers that is a type of self-controlled volume discharge without preionization. Called a self-initiated volume discharge, the pumping method is intended for nonchain HF(DF) lasers that use sulfur hexafluoride-ethane (SF6-C2H6) and other SF6-based mixtures. We established that, under proper conditions, a discharge forms, then spreads spontaneously to fill the volume, with the volume of the self-initiated volume discharge increasing in proportion to the energy deposited in the plasma. When the discharge volume is confined by a dielectric surface, the discharge voltage increases simultaneously with the current. The hypothesized current-limitation mechanism—the dissociation and electron attachment of SF6 molecules—is an effect of great importance to the future development of this type of laser.

Rough cathode

In our study of a nonchain HF(DF) laser with preionization by soft x-rays, it was observed that the self-sustained volume-discharge characteristics and the laser output characteristics can be made independent of the presence of preionization if the cathode surface is treated by depositing small-scale inhomogeneities on it. In other words, if the cathode has a rough surface, preionization is not essential to obtain a self-sustained volume-discharge in the mixtures of an HF(DF) laser.

The feasibility of achieving a self-sustained volume discharge without preionization and with an energy input that is homogeneous throughout the volume under conditions of a high-edge inhomogeneity of the electric field in the discharge gap has been demonstrated. These results make it possible to increase the aperture of a nonchain HF(DF) laser based on SF6-hydrocarbon (deuterocarbon) mixtures up to 30 cm and to obtain a radiation energy of about 500 J at an electric efficiency of approximately 4%.1, 2

A self-sustained volume discharge without preionization—that is, a self-initiated volume discharge—in SF6 and in mixtures that contain it has several unusual properties for volume discharges. It opens unique opportunities for the development of simple, ecologically safe, and effective nonchain HF(DF) lasers that can be operated in both pulsed and repetitively pulsed regimes.

The dynamics of self-initiated volume discharge were studied in discharge gaps with different geometries. The self-initiated volume discharge consists of a set of overlapping diffuse channels linked to bright cathode spots that expand toward the anode and generate an overall diffuse glow. Although the electric field strengthens appreciably at the edge of the discharge gap, the region is scarcely noticeable, and the cathode spots cover the entire surface of the cathode.

Voltage and current oscillograms of self-initiated volume discharge are similar to those for self-sustained volume discharge in electronegative gases. Energy is deposited in the self-initiated volume-discharge plasma at a quasi-steady-state voltage. In mixtures of SF6 and hydrocarbons in which the proportion of the latter is not greater than 17%, the voltage depends only slightly on their partial pressure.

The differences between self-initiated volume discharge and self-sustained volume discharge are manifest in the dynamics of their development over time. In contrast to self-sustained volume discharge, a self-initiated volume discharge is struck initially in the zone of maximum amplification of the electric field on the gap edge, taking the form of one or several diffuse channels linked to the cathode spots. The radiation from the discharge in the remainder of the gap is not initially detected. The channels formed first initiate the appearance of subsequent channels; the self-initiated volume discharge then spreads across the gap perpendicular to the direction of the electric field at a constant voltage, gradually filling the entire gap. The total number of spots on the cathode increases from the instant of the electrical breakdown of the gap virtually in proportion to time—that is, in proportion to the energy injected into the discharge.

Dissociation and electron attachment

Characteristics of a self-initiated volume discharge include consecutive formation of diffuse channels (with a simultaneous decrease in the current through the channels that had arisen earlier) during the development of the self-initiated volume discharge in a gap with planar geometry, the expansion of the diffuse-glow zone in the rod-plane gap, and an increase in the discharge voltage as specific energy is increased. As already mentioned, these processes are apparently determined to a large extent by the mechanisms of the confinement of the current in the conducting channel that depends on the specific energy input. We consider two possible mechanisms that lead to this state of affairs, namely, the dissociation of SF6 by electron impact and the attachment of electrons to vibrationally excited SF6 molecules.

Th 139771
FIGURE 1. A streak camera reveals the shape of an electric discharge in a system of plane electrodes linked by a glass plate in air (top) and in a 10:1 SF6:C2H6mixture (bottom).
Click here to enlarge image

Discharges in air and in an SF6:C2H6 = 10:1 mixture at a pressure of 65 Torr can be obtained in a system of plane electrodes linked by a dielectric plate (see Fig. 1). As expected, the discharge in air develops in the form of an electric-spark breakdown of the gap along the surface of the dielectric plate, a volume discharge being entirely absent. In an SF6:C2H6 mixture, the discharge is of the volume type, its external appearance and oscillograms being completely indistinguishable from a self-initiated volume discharge obtained in the same system in the absence of a plate (with the exception that streak-camera images show a black band at the site where the plate is located).

For a density-normalized electric-field strength E/N (where E is the field strength and N is the number of neutral molecules per unit of volume) close to the critical value, virtually all the energy deposited in SF6 is consumed in dissociation, with most dissociated SF6 becoming SF4 + 2F. Because the ionization potential of F atoms (17.42 eV) exceeds that for SF6 (15.7 eV), F atoms contribute very little to ionization. With regard to the attachment of electrons, the formation of F ions by any of the possible mechanisms is known to be incapable of competing, as shown by estimates with the attachment of electrons to SF6 molecules. The F excitation threshold is 12.7 eV. The component of the main doublet with a threshold energy of approximately 0.05 eV can be disregarded, because it is in a region of intense excitation of the SF6 terms by electron impact. Furthermore, inelastic processes involving SF4 molecules likely do not in any significant way influence the energy spectrum of the electrons in the discharge.

The influence of the dissociation of SF6 might affect only the decrease in E/N with an increase in N as the specific energy input increases. It is possible to show that allowance for additional factors will significantly complicate the description without fundamentally altering the result: the dissociation of SF6 by electron impact and the attachment of electrons to vibrationally excited SF6 molecules might indeed serve as the mechanisms of the limitation of the current in the conducting channel in the active media of a HF laser. At the same time it is important to note that resorting to the mechanisms of current limitation in the conducting channel is not by itself sufficient for the complete understanding of the processes observed in the experiment; in particular, the propagation of the discharge into the interior of the gap in the direction perpendicular to the applied field.

Current limiting

Th 139773
Click here to enlarge image

FIGURE 2. Streak-camera images show the progression of a self-initiated volume discharge over time (T). A single diffusion channel appears first; new channels then appear near the first channel at lower brightness. The new channels multiply and disperse from the middle to the periphery of the gap, gradually achieving the brightness of the original channel. Next, the brightness of the first channel decreases as the brightness of the channels at the gap periphery increases; then, the glow in the first channel is restored—an effect termed a current return. Finally, a discharge instability develops.

Streak-camera images at different instants of time of a self-initiated volume discharge in a gap with a knife-edge cathode show spreading of current away from the initial discharge channel (see Fig. 2). As new channels appear, the current flowing through the earlier-formed channels diminishes. The volume occupied by the self-initiated volume discharge increases almost linearly with respect to the energy deposited in the plasma; when the discharge volume is confined by a dielectric surface, the discharge voltage increases simultaneously with the current.

Such findings confirm assumptions that, in the conducting channel of a discharge in SF6 and SF6-based mixtures (and also in several other strongly electronegative gases), current-limiting mechanisms exist that prevent the transfer of all the deposited energy through a single channel. The hypothesized mechanisms for this allow a volume discharge to form spontaneously. These results show great promise for scaling self-initiated volume-discharge–based HF(DF) lasers to kilojoule-level energy outputs.

REFERENCES

  1. V. V. Apollonov et al., Elektron. (Moscow) 24, 213 (1997) [Quantum Electron. 27, 207 (1997)].
  2. V. V. Apollonov et al., Elektron. (Moscow) 25, 123 (1998) [Quantum Electron. 28, 116 (1998)].

VICTOR V. APOLLONOV is a professor at the Prokhorov General Physics Institute of the Russian Academy of Sciences (RAS), Vavilov St., Moscow, 119991, Russia; e-mail: vapollo@kapella.gpi.ru.

More in Detectors & Imaging