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2 SIVD—a New Form of an SSVD

2 SIVD—a New Form of an SSVD

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30.2



Experimental Investigation



245



The studies into the discharge between plane electrodes have shown that, in the

presence of strong edge nonuniformity of the electric field in SSVD in CF4 and its

mixtures with C2H6, the discharge confines itself at the cathode edge in the region

of a maximum electric field enhancement. With an increase in the energy deposition

in SSVD plasma, spark channels begin to grow from the cathode edge bridging the

gap. Thus, in contrast to SSVD, in mixtures of SF6 with hydrocarbons, discharge in

CF4 does not extend over the entire gap, i.e., in this gas, the peculiarities typical of

SIVD do not manifest themselves despite a strong electronegativity of CF4.

A qualitatively similar situation is observed in developing SSVD in C3F8, mixtures

of C2HCl3 with C2H6 and mixtures of CCl4 with C2H6. In these gases, although an

initial breakdown also occurs at the edge of the discharge gap in the region of a

maximum electric field enhancement, SSVD further advances in the direction from

the cathode edge to the gap center wholly occupying the flat cathode surface similar

to the discharge in mixtures of SF6 with hydrocarbons. Hence, in the gases in

question, SIVD is also observed. However, SIVD stability in CCl4, C2HCl3 and in

their mixtures with C2H6 is appreciably lower than even in pure SF6 [5].

Conversely, the discharge stability in C3F8 is considerably higher than in SF6 and

mixtures of SF6 with hydrocarbons (at the same total pressures) (Fig. 30.1).

In Fig. 30.2, the voltage oscillograms of SIVD in C3F8 and mixtures of C2HCl3

with C2H6, respectively, are shown. It can be seen from the figures that restricting

SIVD sizes leads, as in SF6 and its mixtures with hydrocarbons, to increasing the

operating discharge voltage [4], with this phenomenon manifesting itself most

pronounced in C3F8. It seems plausible that this fact causes SIVD in C3F8 to exhibit

a higher stability than in other gases [5]. However, this question calls for further

examination.



Fig. 30.1 Current (I) and

voltage (U) oscillograms for

bounded SIVD in C3F8



246



30 SSVD in Strongly Electronegative Gases



Fig. 30.2 Current (I) and

voltage (U) oscillograms for

bounded SIVD in mixture of

C2HCl3 with C2H6



30.3



Conclusions



Parametric investigations of SIVD in C3F8, SF6 and mixtures of SF6 and C2HCl3

with hydrocarbons have revealed the distinguishing features as follows: (1) the

discharge with a uniform energy deposition is formed in a gap with a high edge

nonuniformity of the electric field; (2) increasing the energy putted into the

discharge leads to growing the volume it occupies; (3) on limitation on the transverse discharge sizes, the voltage oscillograms show a characteristic rise just after

the gap breakdown.

By these means, in the course of the studies performed, it was established that

SIVD is observed not only in SF6 and SF6-based mixtures, but in other strongly

electronegative gases: C3FS and mixtures of CCl4 and C2HCl3 with C2H6. Data on

the SIVD basic parameters in these gases were obtained. Further studies into the

mechanisms of development of this type of discharge, as well as seeking new gas

mixtures that allow SIVD to be obtained, are required.



References

1.

2.

3.

4.

5.



Y.D. Korolev, G.A. Mesyats, in Physics of Pulse Discharge in Gases (Moscow, 1991)

V.V. Apollonov et al., Proc. SPIE 3574, 374 (1998)

V.V. Apollonov et al., Quantum Electron. 25, 123 (1998)

V.V. Apollonov et al., Quantum Electron. 30, 207 (2000)

V.V. Apollonov et al., in Proceedings of the 10th Conference on Gas Discharge Physics,

Russia, 28, (2000)



Chapter 31



High-Energy Pulse and Pulse-Periodic

Nonchain HF/DF Lasers



Abstract In this chapter, the attempt has been made to gain an insight into the

physics of SIVD starting from the results of our investigations into nonchain HF

(DF) lasers performed in our laboratory and to analyze the potentialities for

increasing energy parameters of nonchain lasers.



31.1



Introduction



Nonchain HF(DF) chemical reaction-based lasers are the most suitable sources of

powerful coherent radiation in the 2.6 Ä 3.1 lm (HF laser) and 3.5 Ä 4.1 lm (DF

laser) spectral regions. Among different methods of initiating a nonchain reaction in

HF(DF) lasers, that of initiation by an SSVD seems to be one of the most attractive

[1]. The basic advantages of nonchain electric discharge HF(DF) lasers are high

radiation pulse power, possibility to operate at high pulse repetition frequencies,

simple design and convenience in use. However, for an appreciable length of time

(until 1996) these lasers were of limited usage because of their relatively low

maximum radiation energy (*10 J). Evidently, the problem of improving the

energy characteristics of similar lasers, as of most of other electric discharge lasers

operating at intermediate and high gas pressures, is readily connected to the challenge of performing SSVD itself. We have carried out special-purpose investigations of SSVD in the working mixtures of nonchain HF(DF) lasers aimed at

increasing their radiation energy, at least, to a level of several hundreds Joules

[2–12]. As a result of this study a number of special features of nonchain HF(DF)

lasers were found [3–12], which not only follow the traditional principles of

forming a volume discharge at intermediate and high gas pressures [13, 14], but are

largely contradictory to them. Specifically, ignition of SSVD without any preliminary ionization in SF6 and mixtures of SF6 with hydrocarbons (deuterocarbons)

was found to be possible [2, 3]. We called this form of discharge SIVD [5].

Realization of SIVD in large volumes allowed an increase in the radiation energy of

nonchain HF(DF) lasers of up to *400 J at electric efficiency of *4% [7–11].



© Springer International Publishing Switzerland 2016

V.V. Apollonov, High-Energy Molecular Lasers,

Springer Series in Optical Sciences 201, DOI 10.1007/978-3-319-33359-5_31



247



248



31



High-Energy Pulse and Pulse-Periodic Nonchain HF/DF Lasers



In this chapter, the attempt has been made to gain an insight into the physics of

SIVD starting from the results of our investigations into nonchain HF(DF) lasers

performed in our laboratory and to analyze the potentialities for increasing energy

parameters of nonchain lasers.



31.2



SIVD—a New Form of an SSVD



31.2.1 What Is an SIVD?

Following a common practice, now already classical, certain conditions should be

met for the realization of SIVD in dense gases, the most basic of which are as

follows: (1) primary electrons of the number densities of no less than 106–109 cm3

must be created within a gas volume through its preliminary ionization; (2) in the

special case that SSVD is employed for laser excitation, which imposes tight

constraints on uniformity of the active medium characteristics over the working

volume, the primary electron multiplication should occur in a uniform electric field,

which is usually provided by specially profiling the electrode surfaces. (Regardless

of lasers, SSVD can in principle be obtained in a highly nonuniform field, for

instance, in the rod–plane gap.) Clearly, the first condition cannot practically be met

in such strongly electronegative gas as SF6 [5, 7] because of great losses of primary

electrons, due to electron attachment, except for the special “photo triggered discharge” mode [15], which, however, is ineffective for energy input into molecular

gases owing to low over-voltages at the gap and, in the case of large discharge gaps

and active medium volumes (large-sized electrodes), is not realized at all. On

increase of the laser aperture and discharge volume, the problem also arises of to

how to meet the second condition, owing to both technical difficulties in the fabrication of large-sized intricate shape electrodes and in connection with rise of the

discharge circuit inductance caused by a useless growth in the transverse sizes of

the electrodes [3, 4].

Thus, it clearly follows from the simplified analysis presented here that the

possibilities for the creation of powerful electric discharge nonchain HF(DF) lasers

coming from the known physical principles of forming an SSVD in dense gases are

very limited. Moreover, when starting from these principles, all attempts to create

nonchain HF(DF) lasers with the radiation energy 1 kJ and above seem to hold no

promise. This evidence appeared to restrain the researcher’s efforts in the direction

of increasing energy parameters of nonchain HF(DF) lasers. Although there have

been numerous works concerned with nonchain HF(DF), their radiation energy

attained by 1996 was only slightly in excess of 10 J [16–18].

Incompleteness of the traditional notions of the physics of forming volume

discharges in dense gases came to be understood after we found the possibility of

obtaining an SSVD in SF6 and mixtures of SF6 with hydrocarbons (deuterocarbons)

in the systems of plane electrodes with high electric field enhancement at the edge



31.2



SIVD—a New Form of an SSVD



249



without any preionization in a gas [2, 3]. The sufficient condition for realization of

SSVD in this case was the presence of small-scale (*50 lm) roughness on the

cathode. We called this form of SSVD an SIVD [5]. SIVD is not dissimilar in

aspect to an ordinary SSVD with preionization. It comprises a set of diffuse

channels diverging in the direction of the anode and attached to bright cathode

spots. When overlapped, these diffuse channels show common diffuse glow [3, 5].

The SIVD current and voltage oscillograms are also typical of SSVD in electronegative gases [5].

It is worthy of note that in nonchain HF(DF) lasers with a rough cathode surface,

preionization does not influence only the discharge characteristics (i.e., glow

homogeneity, the presence of cathode spots, and the limiting energy deposited in

the discharge plasma), but also the output laser energy [3, 5].

It should also be mentioned that analysis of literature data is indicative of a

negligible role of preionization in forming SSVDs in nonchain lasers. For example,

in papers [16, 18], approximately the same output energies and efficiencies were

obtained. However in [16], where a set of metal rods connected to a common

busbar through a resistance (resistance uncoupling) served as the discharge gap

cathode, preionization was absent, whilst in [18] it was performed by a high-current

dielectric surface discharge whose spectrum displayed not only UV radiation, but

soft X-ray radiation as well.

By these means, we define SIVD as a form of SSVD obtained without

preionization in SF6 and SF6-based mixtures under SF6 pressures of 30–150 Torr

typical of HF(DF) lasers. Based on analysis of the experimental results, the following questions are considered below: (1) the key features of the development of

SIVD; (2) the physical mechanisms determining the possibility of the existence of

SIVD; (3) is SF6 a unique gas or SIVD can be observed in other gases?; (4) the

active medium characteristics and output characteristics of nonchain HF(DF) lasers

based on SIVD; and (5) the prospect for further increase of radiation energy of

nonchain lasers.

a. Dynamics of SIVD formation

The key features of SIVD can most clearly be inferred from its dynamics.

i. Experimental setup

The setup used for investigations of the dynamics of forming SIVD is represented schematically in Fig. 31.1a. An SIVD of *500 ns duration was initiated in

mixture SF6:C2H6 = 10:1 at the pressure of 33 Torr and interelectrode distance of

4 cm. As the electrodes there were a copper stripe of 0.5 mm thick and 16 cm long

(cathode) stood edgewise and a disk anode of diameter 6 cm rounded off along its

perimeter to a radius of 1 cm. The breakdown was force initiated at the gap edge by

a low-current spark restricted by resistance R = 900 Ohm. This spark could not in

principle provide a sufficient number of primary electrons in the gas volume;



250



31



High-Energy Pulse and Pulse-Periodic Nonchain HF/DF Lasers



Fig. 31.1 Scheme of the

experimental set-up for SIVD

dynamics investigation



however, it allowed the site of the primary gap breakdown to be spatially stabilized.

Luminosity of SIVD was recorded by a single frame camera with an exposure time

of 20 ns run with varying delay, T, relative to the instant of the gap breakdown.



31.2



SIVD—a New Form of an SSVD



251



The SIVD dynamics was also studied in the plane–plane gap geometry in

experiments with a sectioned cathode diagrammatically depicted in Fig. 31.1b. In

this case, the interelectrode distance, working medium pressure and setup electrical

scheme were the same as in the former experiment; however, as a cathode there was

a 0.2 mm flat disk rounded off along its perimeter to a radius of 1 cm. Isolated

conductors of 1 mm in diameter were inserted into holes 2 mm in diameter drilled

within the flat part of the cathode and spaced by a distance of *4 cm. The basic

cathode and these conductors were connected to a common bus. The current

through each the conductor was recorded by Rogowskii coils. One of the conductors (1) extended *1 mm above the cathode surface, which ensured a primary

gap breakdown just at this point whilst comparison of oscillograms of currents

through the initial (1) and control (2) conductors allowed the SIVD extension over

the gap to be followed.

Dynamics of a single diffuse channel were investigated with use of a setup

represented schematically in Fig. 31.1c. Diffuse channel was initiated by discharge

in the rod (cathode)–plane geometry in mixture SF6:C2H6 = 10:1 at pressures

p = 16.5 + 49.5 Torr and interelectrode distance d = 4 cm. The end of a 1.5 mm

diameter rod dressed with polyethylene insulation was used as a cathode and the

anode was a disk of diameter 10 cm. Limitation of the cathode surface ensured

development of no more than one cathode spot. The SIVD dynamics was followed

by a single frame camera as in the experiments performed by the scheme of

Fig. 31.1a. SIVD was also filmed with a video camera, which allowed the volume

occupied by the discharge to be exactly calculated as a function of the energy putted

into the plasma, the latter being calculated by the current and voltage oscillograms.

With the aim to increase a specific power input into the discharge plasma, SIVD in

the given gap geometry was bounded, in a number of experiments, by a glass tube

of diameter 6–8 mm (bounded SIVD [6, 11]) shown in Fig. 31.1c with dashed line.

This made possible to bring up specific energy depositions to *1 J/cm3.

ii. Experimental results

The frames of SIVD obtained by a single frame camera at different instants of

time relative to the instant of the gap breakdown are shown in Fig. 31.2. In

Fig. 31.3 the discharge voltage and current oscillograms corresponding to the

process described are shown. As is seen from Fig. 31.2, the gap is broken down at

the edge in the vicinity of an auxiliary electrode. At this instant in time, the SIVD

constitutes a single diffuse channel with an already developed cathode spot. Then,

near the first channel much less bright new channels appear, which temporally grow

in number while their brightness becomes gradually comparable to that of the first

channel, with the brightest channel being that located closer to the primary channel.

In time, all the channels become equally bright whilst the glow intensity of the first

channel noticeably decreases. On further development of the SIVD, increase in

glow intensity of diffuse channels at the gap edge removed from the region of the

first breakdown is observed. However, at T > 250 ns, the glow again becomes

homogeneous throughout the length of the cathode, and the glow within the region



252



31



High-Energy Pulse and Pulse-Periodic Nonchain HF/DF Lasers



Fig. 31.2 SIVD frames for different T-values



Fig. 31.3 Typical

oscillograms of voltage

(upper trace) and current

(bottom trace). Time scale

100 ns/div



of the first channel is recovered. Furthermore, discharge instability begins to

develop against the background of the total glow.

Thus, a very strange picture of the development of an SSVD is observed. Despite

a local gap breakdown and the drop of the voltage across the gap to its

quasi-stationary value (see Fig. 31.3) close to that of the static breakdown voltage

in SF6 [19], the initially formed channel is no more capable of passing all energy

restored in the capacitor through itself, as it takes place, for example, in air or

nitrogen [7] where a local gap breakdown, even in the form of a diffuse channel,

must be followed by discharge instability and, at a sufficiently high restored energy,

transition of the discharge from its diffuse to a constricted state. Instead, we see the

formation of new diffuse channels after appearance of the first one and extension of

an SIVD to the whole gap at the voltage close to the static breakdown value (see

Fig. 31.3); in so doing, judging from decrease of the brightness of the first channel

in time, the current through it not only rises, but, conversely, falls with appearing

new channels, i.e., an initially formed channel progressively quenches. ‘It should be

noted that the effect of current recovery, demonstrated by Fig. 31.2, which



31.2



SIVD—a New Form of an SSVD



253



Fig. 31.4 Oscillograms of

current through the initiating

(upper trace) and the control

(bottom trace) wires. The time

scale is 50 ns/div



manifests itself in equalizing the 'brightness’ of all channels after their initial drop is

observed only under sufficiently high deposited energies’. On decrease in energy or

growth of the cathode in size (accompanied, naturally, by increase the anode size),

this effect is absent. However, at high energies deposited in the discharge plasma

(150–200 J/1), typical of HF(DF) lasers, the SIVD extends to the whole gap so

quickly, that the single frame camera we used does not allow the process to be

resolved.

A similar picture of developing SIVD is observed in the discharge gaps with

plane–plane geometry too. In Fig. 31.4 are shown oscillograms of the currents

through initiating and control conductors (see Fig. 31.1b) obtained in the experiment with a sectioned cathode. It is seen from this figure that the current through a

control conductor begins to flow with a noticeable delay relative to the current

through the initiating conductor. Also seen is that the amplitude of the current

through the initiating conductor reduces, by the instant of appearance of the current

through a control conductor, by half, i.e., in the given experiment the effect of

quenching the first channel with appearing the next channels successively filling the

discharge gap is observed. The number of diffuse channels formed during the

discharge current pulse duration, as shown in [5, 7], is proportional to the specific

energy input into the discharge plasma. It is seen from the oscillograms in Fig. 31.4

that in the plane–plane discharge gap geometry, too, the effect of current recovery

in the first channel after its almost complete quenching takes place.

As in the experiments described above, increase of the volume occupied by

SIVD in the course of the energy input into plasma was observed in the rod–plane

geometry, i.e., the discharge volume immediately depended on the energy deposited

in this volume. In Fig. 31.5, the dependence of volume, V, occupied by an SIVD on

parameter W/p where W is the energy put into the discharge and p is the gas mixture



254



31



High-Energy Pulse and Pulse-Periodic Nonchain HF/DF Lasers



Fig. 31.5 Dependence of

volume, V, occupied by SIVD

on W/p, mixture SF6:

C2H6 = 10:1; p = 5 Torr (I);

p = 30 Torr (A): p = 45 Torr

(O)



pressure is given. It can be seen from Fig. 31.5 that the volume the discharge

occupies grows linearly with parameter W/p.

After the size of SIVD in the rod–plane geometry was restricted by a glass tube,

the discharge voltage and current oscillograms at energy depositions of up to

Win = 200 J/l showed no changes; however, at energy depositions in excess of

400 J/1, oscillograms of a bounded SIVD displayed appreciable characteristic

changes. Figure 31.6 depicts voltage and current oscillograms of bounded (Ub, h)

and unbounded (U, I) SIVDs in mixture SF6:C2H6 = 10:2 at pressure p = 33 Torr

and d = 4 cm. As may be inferred from this figure, on restriction of the SIVD

volume (equivalent to increasing a specific energy deposition), the voltage, just

after its initial drop under breakdown, rises simultaneously with the current even

during a certain period of time after the current passes its maximum. As a whole,



Fig. 31.6 Experimental

voltage and current

oscillograms for bounded (Ub,

Ib) and free (U, I) discharges,

time scale 50 ns/div



31.2



SIVD—a New Form of an SSVD



255



Fig. 31.7 Current (Ib) and voltage (Ub) oscillograms for a bounded SIVD; time scale 50 ns/div.

a −30 Torr C3F8; b mixture: 15 Torr C2HC13 + 3 Torr C2H6



Ub > U whilst Ib < I, i.e., an unusual situation is observed here—the gap conductivity decreases with increasing specific energy deposition. It should be highlighted that SF6 and SF6-based mixtures are not unique in this respect. Electric

discharges in other strongly electronegative gases and their mixtures exhibit similar

features. In Fig. 31.7a, b the current and voltage oscillograms of a bounded SIVD

in C3F8 and mixture C2HC13:C2H6, respectively, are shown. It can be seen from

Fig. 31.7a that oscillogram Ub shows in C3F8 a more pronounced bend than in SF6.

The above-listed features of the development of SIVD allow for the assumption

that mechanisms of current restriction in SF6 and SF6-based mixtures exist, which

make difficult the passage of all the stored energy through a single channel.

It seems that it is these mechanisms that, to a large extent, cause the existence of

such an unusual form of discharge as SIVD, as well as the possibility of its generation in the gaps with a high edge nonuniformity of the electric field.



31.2.2 The Mechanisms of Restriction of a Current Density

in Diffuse Channels of SIVD in SF6 and Mixtures

of SF6 with Hydrocarbons and Deuterocarbons

The results presented here are indicative of the presence of certain restriction

mechanisms of the current density in diffuse channels in SF6 and SF6-based mixtures. (The same effect can be found in some other strongly electronegative gases;

however, data for most of them on the rate of coefficients necessary for interpreting

these effects even at a qualitative level are absent.) It is natural to link the presence

of these mechanisms to such a distinguishing feature as SF6 owing to its high

electronegativity.



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