Streamer negative

Fig. 2.71. Negative streamer growth in a white naphthenic mineral oil (Marcol 70). Gap 1.27 cm voltage step 185 kV time in microseconds. (Reprinted from R. E. Tobeozon, The Liquid State and its Eiec-trical Properties, Plenum Press, New York, 1988.)...

A strong primary avalanche is able to amplify the external electric field and form a streamer. When the streamer chaimel cotmects the electrodes, the current may be significantly increased to form the spark. The avalanche-to-streamer transformation takes place when the internal field of an avalanche becomes comparable with the external one. If the gap is short, the transformation occurs only when the avalanche reaches the anode. Such a streamer that grows from anode to cathode and called the cathode-directed or positive streamer. If the gap and overvoltage are large, the avalanche-to-streamer transformation can take place far from the anode, and the anode-directed or negative streamer grows toward both electrodes. 

Figure 4-7. Illustration of the anode-directed (negative) streamer (a) propagation of the negative streamer (b) electric field near the streamer head.

Thus, the interaction of streamers in DBD can lead to the formation of an organized structure of microdischarges (similar to Coulomb crystals see Fridman Kennedy, 2004), which plays a significant (sometimes positive, sometimes negative) role in plasma-chemical applications. From this perspective it is important to analyze how highly organized the... 

Not only volumetric but also surface conductivity of a dielectric can promote DBD uniformity, if it is in an appropriate range. The memory effect can be suppressed by removing the negative charge spot formed by electrons of a streamer during the half-period of voltage... 

For a given liquid, positive streamers (originating from the anode) are always faster than the negative ... 

Most of these characteristics are also found with uniform fields, no matter what shape the voltage wave (Fig. 6 illustrates negative streamers). 

Electron scavengers, such as ethyl chloride and sulfur hexafluoride, considerably increase (by say one order-of-magnitude) the velocity of slow negative streamers (Fig. 7), no measurable effect being observed on the positive streamer velocities ... 

Low ionization potential compounds, for instance N, N -dime-thylaniline (DMA), does not change the negative streamer velocity, but speed up (by a factor 2 to 3) the positive streamers (Fig. 8) the velocity of positive streamer in mixtures of n-hexane and carbon tetrachloride vary in the way shown in Fig. 9. 

It was commonly accepted that only positive streamers were filamentary (with a high velocity), the negative ones being bush-like. In fact, the increase of negative streamer velocity by electron scavengers renders them more filamentary and they resemble more closely the positives... 

Devins et al., 1981) negative streamers can be very rapid (see Table 1). A study of the influence of the additives on velocity and shape of... 

Fig. 10. Current and light emission during streamer growth in cyclohexane. Gap 2 mm Point radius 3 pm. Point positive (at the left), VB = 21 kV. Point negative,...

Fig. 14. Influence of the pressure on the negative streamer growth in cyclohexane. Gap 1 mm Point radius 5 ym Voltage ...

It is possible, however, to get more useful information on streamer initiation the method consists of, under a rectangular voltage, measuring t, defined as the time at which a small optical perturbation (e.g., < 10 ym in diameter) is detectable by the Schlieren technique, or by light emission, or also by current measurements characteristic of the early streamer propagation. It was shown that when the rise time of the voltage was much lower than t, for several liquids, initiation times were shorter with the positive point than with the negative point, but from the V-t curves it was only deduced that one mechanism appeared to be rejected-cavitation, as will be discussed later - and it was not feasible... 

Streamers have recently been visualized with a positive point in cyclohexane (Denat et al., 1987) and in hexane with a negative point (MacCrath and Marsden, 1986). 

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