Negative striations

Coulter,

Physica

J. R. M.

26

949-953

1960

NEGATIVE by J. Physics

Department,

STRIATIONS

R. M. COULTER

Queen’s University,

Belfast,

Northern

Ireland.

Synopsis Striations have

of high

been studied

techniques. cation

These

of positive

are not

usually

voltage,

rather

oscillations

velocity

in some

and

‘negative’ striations, observed

than

waves which at the

nega.tive

and positive

small

luminosity,

neon discharges

travelling

with rotating

seem to be associated occurs

mirror with

towards a frequency

near the head of the positive

anode.

It would

striations,

effect

appear

that

synchronization

the

anode,

and photomultiplier multipli-

column.

fluctuations between

They in tube

anode

spot

striations.

9 1. Introduction. When the positive column of a glow discharge contains striations moving towards the cathode 1)2) it is sometimes found that these are accompanied by striations of smaller luminosity and higher velocity travelling in the opposite direction. The latter, which have been termed ‘negative’ striations a) have speeds of the order of 1,000 mjsec and frequencies of the order of 1,000 set-1. The study of these negative waves is made difficult by virtue of their high velocity, and their nature and origin are still uncertain. Very few data relating to the properties of negative striations have been published, most being contained in the paper by Donahue and Dieke a). In an endeavour to obtain more information about this type of wavemotion some discharges in neon *) have now been examined in this laboratory with a photomultiplier, and by direct photography using a mirror rotating about an axis parallel to that of the column. Generally speaking, the results obtained with the mirror are easier to interpret than those obtained with the photomultiplier, but the latter gives a more accurate determination of striation velocity. We have observed negative striations only at the negative end of the column where an increase in striation frequency is found 4). They have not been detected at the anode end of the column or in the Faraday dark space. 0 2. Apfiaratus.

This

has

*) The British 0 xygtn Company’s helium as impurity, was used.

been

described

‘spectrally

--

in detail

elsewhere 5). The

pure’ neon, which contained

949 -

up to 0.1 percent

950

J. R. M. COULTER

discharge tube, 2.5 cm in diameter and 108 cm long, was mounted horizontally. The electrodes were solid zirconium cylinders, 4.5 mm in diameter and a few cm long. Both 50 cm. The striations

were moveable, allowing a maximum were photographed by light received

separation of by a camera

(f/l .9) after reflection from a flat rectangular stainless steel mirror (10 cm x x 5 cm), the axis of which was horizontal and parallel to the discharge tube. The speed of rotation of the mirror could be varied continuously in the range O-6,500 r.p.m. and a chosen speed maintained over long intervals of time. The tube, which was connected at the cathode end to a vacuum manifold, was baked at 450°C and the electrodes glowed inductively before admission of gas. The gas was kept clean by a liquid nitrogen trap directly connected to the tube, and no impurity could be detected in the visible spectrum during operation. 9 3. Results. (i) Velocity. Figure 1 is an enlarged reproduction of a typical rotating mirror photograph of the negative end of a column at 7 mm Hg pressure and 10 mA current. The length of the tube included in the photograph is 12 cm; the cathode was 3 cm further to the left and the anode 15.3 cm further to the right. The speed of rotation of the mirror was 12 r.p.s. Time increases vertically downwards and a horizontal line in the photograph gives the appearance of the negative end of the column at a particular instant. The frequency of the main positive striations (the bright streaks inclined down to the left) increases by a factor of 2 at a distance of 8 cm from the negative end of the column. Simultaneously, the speed of the waves becomes greater. These effects are described in some detail in another publication 5). Here we are more concerned with the appearance of the column at the point where an increase in striation frequency occurs. On examination of figure 1, it can be seen that streaks of lower intensity than the main striations are present, sloping slightly downwards to the right at the point where the striations divide. These negative streaks show that, in this region, striations of small luminosity are moving away from the Faraday dark space towards the anode. The streaks are inclined at a relatively small angle to the horizontal, which means that the disturbances are moving much more quickly than the positive striations, the respective velocities being about 1 170 and 120 mjsec. The magnitude of the velocity and the small distance over which striations can be observed, limit the accuracy with which the speed of the negative waves can be estimated. Their velocity is better determined by using a photomultiplier with output fed to a c.r.o.a)d). This method gave for the velocity of the negative striations in figure 1, 1260 m/set. The and negative striations. (ii) Interaction of positive positive striation labelled B in figure 1, proceeding towards the Faraday

NEGATIVE

STRIATIONS

951

dark space, meets the negative striation A near the point X and is considerably reduced in velocity. Photomultiplier records show that when a positive and negative striation meet, their motion often ceases for a short period, usually a few psec. Whatever the exact nature of the waves, it is highly probable that a plasma forms during this pause. From this plasma, B and A re-emerge at nearly the same instant, and are identified in figure 1 as B’ and A’ respectively *). B’ terminates in the intermittent spot C at the Faraday dark space boundary. A’ meets the next positive striation D, at Y, and disappears. D is slowed down by this interaction but is not

Fig.

1. Enlargement

of a time-resolved

photograph

in neon at 7 mm Hg pressure and 10 mA current. length

of tube included

of the negative

The tube voltage

is 12 cm and the time interval

end of a column was 340 volts.

from top to bottom

The

is

2.2 millisec.

stopped intil it meets another negative striation at 2. In the discharge described in figure 1, negative striations were not detected, either photographically or by the photomultiplier, to the anode side of the point Y. As striations remain for some time at places in the column where the two forms meet, e.g. X and 2, such meeting places are observed as maxima in the time integrated light, i.e. they look like a form of standing striae. The frequency of negative striations is the same as that of positive striations near the anode; since their velocity is greater by a factor of 10, their wavelength is longer than that of positive striations by the same

*) A’ cannot high contrast.

be seen in the reproduction because

some detail

has been sacrificed

in order to obtain

952

J. R. M. COULTER

factor. In figure 1, the wavelengths of positive and negative striations are about 7 cm and 70 cm respectively. Increase in the speed of rotation of the mirror, i.e. increase in the vertical time-scale of figure 1, did not yield further useful information about the nature of negative striations because of their feeble luminosity. It did, however, serve to check that there was no detectable periodic light fluctuation in either the Faraday dark space or the negative glow, a result which was confirmed by use of the photomultiplier. § 4. Discussion. For stable modes of oscillation, like that of figure 1, all positive striations meet and interact with negative striations at fixed points in the column, causing frequency multiplication and the formation of a kind of standing wave pattern in the negative end of the column 5). The frequency of negative striations must, therefore, be an integral multiple (from unity upwards) of the frequency of the positive striations, and it is probable that the two types are coupled in some way! As it is known 6) that positive moving striations may be generated by an oscillatory anode spot with a frequency of oscillation which is also an integral multiple of that of the striations, it at one time seemed possible 7) that synchronization of the oscillations in the positive column and this oscillation at the anode might be effected through the negative striations. However, since the negative waves do not apparently always travel to the anode (5 3), this mode of coupling appears to be ruled out in most instances. It would seem that it could only occur when the anode was so close to the Faraday dark space that it was inside that part of the column in which frequency multiplication occurs 4). On the whole, any feed-back required to synchronize anode spot oscillations and positive striations seems more likely to be provided by the small fluctuations in tube voltage which are commonly associated with these phenomena 8). These fluctuations are apparently generated in the discharge itself and conducted through the external circuit. Because of their high velocity (of order 105 cmjsec) it is unlikely that the negative waves are caused by the movement of low energy positive ions towards the anode in the reversed electric fields which have been reported in the column 9). They are more probably negative space-charges which move towards the anode in the normal mean electric field of a discharge 3). The magnitude of the velocity indicates that the space-charges could be bunches of electrons. The absence of negative striations in the anode end of the column might be accounted for by a debunching of the electrons due to loss of energy by inelastic collisions. The negative striations described here are not to be confused with the irregular disturbances of larger amplitude which are sometimes observed travelling in the same direction 4). The latter, which are clearly visible to

NEGATIVE

953

STRIATIONS

the eye, have speeds of the order of that of the positive

striations

but are

of lower frequency. In our experience they occur only when the distance between anode and cathode is not an integral number of halfwavelengths. When this condition is satisfied, no irregular fluctuations in the light intensity

are observed.

Acknowledgements. I would like to thank Professor K. G. Emeleus for his guidance and advice throughout the investigation. Acknowledgement is made to Mr. N. H. K. Armstrong for assistance with some of the experimental work. Received

15-7-60 REFERENCES

Pupp,

W., Physik.

1) 4

Druyvesteyn,

3)

Donahue,

2. 3tl (1935) 61, and an earlier series of pap-m.

M. J. and Penning, T. and Dieke,

F. M., Rev.

Mod. Phys.

I2 (1940)

G. H., Phys. Rev. 81 (1951) 248.

4)

Coulter,

J. R. M., Ph. D. thesis,

Belfast,

5)

Coul ter,

J. R. RI., J. Electronics

and Control

6)

Coulter,

J. R. M., c.a. Physica

24 (1958)

(1959). (1960),

in course of publication.

828.

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(New York;

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