Section 2

PLASMA EDGE EXPERIMENTS - ACTIVE PROBES

Journal of Nuclear Materials 111 & 112 (1982) 11-22 North-Holland Publishing Company

11

HIGH SPEED CINE FILM STUDIES OF PLASMA BEHAVIOUR AND PLASMA SURFACE INTERACTIONS IN TOKAMAKS

D.H.J. GOODALL

Culham Luboratoty, Abingdon, Oxon OX14 3DB, UK (Euratom / VKA EA Fusion Association)

High speed tine photography is a useful diagnostic aid for studying plasma behaviour and plasma surface interactions. Several workers have filmed discharges in tokamaks including ASDEX, DITE, DIVA, ISX, JFTZ, TFR and PLT. These films are discussed and examples given of the observed phenomena which include plasma limiter interactions, diverted discharges, disruptions, magnetic islands and moving glowing objects often known as UFOs. Examples of plasma structures in ASDEX

and DITE not previously published are also given. The paper also reports experiments in DITE to determine the origin of UFOs.

1. Introduction

High speed tine photography of tokamak discharges is a simple and effective way of obtaining information on plasma behaviour and plasma surface interactions. The use of time markers and event markers on the film enables visual events to be correlated with other di- agnostic data e.g. plasma movement, density, loop volt- age and MHD activity. Such filmed discharges can therefore yield much more information than is possible with still photography, especially when studying disrup- tive behaviour where gross changes are observed in times less than one millisecond.

In practice a disadvantage of tine film is the delay between exposing and viewing the film. Video recording offers the possibility of immediate playback but at present it is not possible to make high speed colour recordings. Some progress has been made with high speed monochrome recording and full frame pictures of 192 by 240 pixels (picture elements) at 2000 frames per second have been reported [ 1,2]. Even at this definition the data transfer is demanding and requires a 34 track tape recorder with the tape running at 240 inches per second. Such a system might well be valuable for routine instant replay of plasma discharges, using colour tine film with its higher definition, sensitivity and framing rate, when interesting phenomena are observed. Only observations using tine film will however be reported here.

Discharges have been filmed in a number of toka- maks including PLT, DIVA, JFT2, ISX, TFR, ASDEX and DITE but there has been little published discussion

of the observations. This paper will concentrate mainly on ASDEX and DITE discharges which represent poloidal and bundle divertors and toroidal and poloidal limiters.

2. Filming requirements

In normal tine cameras, the exposure takes place while the film is stationary, limiting film speeds to 500 pictures per second, as at higher speeds the discontinu- ous motion makes too much demand on the mechanical properties of the film. Films of plasma discharges re- quire film speeds exceeding 1000 frames per second and preferably in the range 3000-5000 fps with even higher speeds for filming fast events such as disruptions. Several commercial cameras are available which are suitable for such speeds, using rotating prisms which sweep the picture in the direction in which the film is travelling, thus producing a stationary image on the continuously moving film. By starting the camera motors 0.5 to 1.0 s before the discharge the film has time to accelerate to the required speed at the start of the discharge. The film capacity of the camera is important at high speeds and long discharge times if the whole discharge is to be filmed. To film the DITE discharge (0.2 s) at 3000 pictures per second requires a capacity of 100 feet while ASDEX (3 s) and JET (10 s) will require 300 and 800 feet of film respectively.

Apart from the length of film, the limit to the film speed will be determined by the light available from the plasma and the f number of the camera optics. Film

0022-3 115/82/0000-0000/$02.75 0 1982 North-Holland

12 D. H.J. Good&l / High speed tine film studies

speeds of 8000 pictures per second at f/2 were found to be possible with DITE discharges. It was however found to be necessary to use a reversal film with a speed of 400 ASA in order to obtain a suitable exposure at these speeds. Some regions of the torus will, however, emit light at high intensity e.g. limiter surfaces, gas puffing positions etc. and film speed will not therefore be a problem when filming at these locations.

For correlations between filmed events and other diagnostics it is essential to provide time markers on the edge of the film. A I kHz oscillator driving neon lamps or light emitting diodes, providing markers every mil- lisecond, is very suitable for this purpose and interpola- tion between markers gives the required accuracy of timing. Single events can be marked on the film where a signal from another diagnostic is available which may be synchronised with a visual event.

When the plasma is viewed through a tangential window, the cross section of the plasma can be observed and it is particularly interesting if the region near limiters can be filmed. It is essential to locate the camera in a region of low magnetic field if damage to the camera and erratic running is to be avoided. This is best achieved by viewing the plasma through a long large bore tangen- tial tube with a window outside the toroidal field coils. Where this is impossible and some separation is neces- sary between the camera and the window, the field of view may be restricted by either the window aperture or the collimating effect of a tube joining the window to the torus. This can be avoided by using an optical relay system which should be carefully designed to avoid vignetting and reducing the effective aperture of the camera.

3. Observations

3.1. Limiter discharges

Limiter discharges have been filmed in several tokamaks with various types of limiters including poloidal, toroidal. rail and mushroom limiters.

Discharges with large poloidal limiters in ASDEX and DITE are very similar in appearance with a bright region near the limiter surface facing the plasma. This region of intense visible radiation is probably due in part to the excitation of recycled plasma at the limiter

Fig. 1. Discharges with large poloidal limiters. (a) DITE (I, = 100 kA, max%,=2.2X109 mm3); (b) ASDEX (I,=240 kA. max ?ie =3.1 X lOI me3).

D. H. J. Goodall / High speed tine film studies 13

but is dominated by the excitation of sputtered, evaporated or thermally desorbed atoms. The colour of the glow is a blue white, unlike the red colour of the H, radiation from the outer region of the plasma. The glowing region has a width of 1.5 to 2 cm which does not change significantly during the discharge except for a broadening during disruptions or at the end of the current pulse. Discharges in DITE show a correlation between TiI and TiII radiation from the plasma and the intensity of the glow around the titanium fixed limiters. Similarly Fe and 0 radiation has been observed from the glow around a stainless steel toroidal limiter in ASDEX [3] but in neither case was a full spectral analysis made. Fig. 1 shows two large poloidal limiter discharges for (a) DITE (inner limiter) [4] (b) ASDEX (inner limiter) [5]. In DITE limiter discharges a flutter- ing in the outer region of the plasma is frequently observed. With the exception of ASDEX where some fluttering was observed near the outer limiter in di- verted discharges and possibly in ISX near limiters, this flutter has not been observed in other tokamaks but this may be due to the available films lacking the necessary definition or viewpoint to show this phenomenon.

In addition to the large fixed poloidal limiters, DITE has a pair of movable graphite limiters with each limiter subtending an angle of 80 compared with 144 for each individual fixed limiter. Fig. 2 shows a single gra- phite limiter in DITE when pushed in beyond the fixed limiter to a minor radius of 20 cm. The glow around the fixed limiter has disappeared and the scrape-off layer of the movable limiter can be seen. When projected the film of the discharge from which fig. 2 was taken [4] shows a distinct dark region or void near the inner wall and at a position representing a rotation from 60-90 from the geometric centre of the limiter. This angle is reasonably well correlated with the rotation expected from the safety factor of the discharge at the limiter, qa, which suggests that the void is the shadow of the limiter having made one rotation around the major cir- cumference of the torus. Increasing the minor radius of the carbon limiter moves the void towards the torus wall as expected. Fig. 3 shows the calculated q values from the plasma current compared with the q value given by the void position.

There are many examples of films of strong plasma limiter interactions. One example is shown in fig. 4 for the ASDEX outer limiter, where numerous large glow- ing objects from the limiter enter the plasma causing cooling and a high impurity content and finally disrup- tions which prematurely end the discharge [S]. Brights- pots were also observed on this limiter throughout this particular discharge. The origin of these spots is uncer-

Fig. 2. A discharge in DITE at t = 12 ms with an adjustable graphite limiter at 20 cm minor radius (I, =80 kA. max A, = 2.7X lOI mP3).

/ /

/

+I-

-& / ,4

+ Y t++

++

/ /

2 3 L 5 6

q (plasma current)

Fig. 3. The q of the discharge calculated from the limiter shadow position.

14 D. H. J. Goodall / High speed cme film studies

Fig. 4. Plasma surface interactions at the outer ASDEX poloidal limiter (I,=225 kA, maxii,=1.9X109 mw3).

tain but since at the end of the discharge they disappear rapidly without going through a glowing red stage their behaviour is inconsistent with incandescent hot spots. Arcing, unless due to multiple arcing, is also unlikely because the stationary spots persist for most of the discharge, unlike arcs which are of short duration and move in magnetic fields [8]. The position above the median plane makes bombardment by runaway electron beams unlikely since runaway damage is expected to occur on this plane. The stainless steel limiter used in this discharge has a segmented active face with individ- ual elements 4 cm long and the bright spots appear to correspond to individual segments. The restricted heat flow between segments could result in some elements having a high temperature producing an enhanced ther- mal desorption and a localised glow near the limiter surface.

Strong plasma limiter interactions have also been filmed for mushroom limiters in ISX [6] and JFT2 [7]. Both ISX and JFT2 teams report arcing on the limiter and glowing particles originating from the limiter are also seen. Arcing in DITE has been shown to occur mainly at the start of the discharge and to have a duration - 200 ps (8.91. Arcs would therefore appear on single frames of a tine film exposed at 3000 fps and can therefore only be detected by a careful frame-by-frame study. Since arcs occur predominantly on the limiter in DITE [lo]. they are difficult to see against the limiter glow. Some examples have however been seen early in the discharge when light levels are low and the arcs are the brightest objects present. Fig. 5 shows an example of such arcs.

Fig, 5. Arcs near the fixed limiter in DITE, (a) f =O. (b) I =0.3 ms. (c) t =0.6 ms, (d) I ~0.9 ms.

D. H. J. Goodall / High speed tine film studies 15

3.2. Diverted discharges

Two poloidal divertor experiments, ASDEX and DIVA, have been filmed, fig. 6 shows the cross section of both machines. In both cases the visible scrape-off layer near the separatrix can be clearly seen close to the inner wall in the case of ASDEX and the outer wall for DIVA. In ASDEX very little visible radiation is seen from the plasma during a diverted discharge in constrast to the limiter discharges previously described. The il- lumination seen in films of ASDEX diverted discharges originates mainly from the gas puffing position, where the excitation of the gas entering the torus acts as a light source, illuminating the torus walls. This is evident from the shadows cast on the torus surface and direct ob- servation of the gas puffing position which appears on film as a very bright source, fig. 7 [I 11. During an ASDEX diverted discharge the light from the scrape-off layer due to recycling gas and impurities becoming ionised in a region a few cm wide, slowly becomes brighter as the discharge continues. This visible scrape- off layer is usually very stable and discharges lasting 3 s

2.0 c

1.6;

5

i 1.2 :

E _

g 0.6

: 1

ct.41

t O0-

\Getter Panel

Dwertor _ Coils -<

1 -L- , __I 1 0.4 0.6

Radius (Metres)

-

Fig. 6. The cross-section of DIVA (top figure) and ASDEX

Fig. 7. Illumination from the gas puffing position in ASDEX showing filaments.

have been filmed which show no visible change apart from the beginning and end of the pulse. For stable discharges such as this, the visible radiation is dominated

Fig. 8. The ASDEX divertor scrape-off layer 2 s after the start of the discharge (I, =240 kA. max A, =2.7X lOI mM3).

16 D.H.J. GoodaN / High speed cm film studies

by recycled hydrogen and has the characteristic red colour of H, radiation. When the discharge is less stable and plasma surface interactions occur, this colour changes to a blue white, as impurities from the wall become excited near the separatrix. Fig. 8 shows an example of an ASDEX diverted discharge at an elapsed time of 2 s.

Fig. 9 is from a film of a diverted discharge in DIVA [ 121 which shows a similar visible scrape-off layer which can be seen entering the divertor chamber and terminat- ing at the target plate.

Diverted discharges in DITE using the MKI bundle divertor are also very different from limiter discharges in appearance but unlike ASDEX and DIVA there is still visible radiation from the outer region of the plasma. This is consistent with measurements of density which show only a slow decrease in density with radius [ 13,141. The wide scrape-off layer is considered to be due to the long connection length to the divertor target which is characteristic of bundle divertors. Fig. 10 shows the cross section near the fixed limiters for a diverted discharge. Unlike the limiter discharge of fig. l(a) the outer region of the plasma consists of slow moving, overlapping, crescent shaped structures of varying in- tensity with some very bright narrow regions [4]. Fig. 11 shows the computed cross sections of magnetic flux bundles at the same toroidal position as fig. 10 where the value of N indicates the number of rotations of the bundles around the major circumference of DITE be- fore entering the divertor chamber. The shape of these flux bundles and their evolution with time during the

Fig. 9. The divertor scrape-off layer in DIVA.

Fig. 11. Computed flux bundles for the MkI bundle divertor for q = 3.1 ( r = 18.5 cm). The numbers indicate the number of complete revolutions around the major circumference of the

Fig. 10. A diverted discharge in DITE (I, = 82 kA. max ii, = 1.8X lOI mP7, Ed = 1.5 T).

\ / / \ 1 :f / d 2 ,/ , . _ P _. // . I._ _// __-t- .___ -- torus before entering the divertor. The solid and dotted lines differentiate between a clockwise and an anticlockwise rotation respectively.

D. H.J. Goodall / High speed tine film studies 17

discharge are very like the observed structures in films of diverted discharges. Flux bundles making a small number of revolutions around the torus before entering the divertor would produce a region of low density and temperature which in the film are seen as a dark cres- cent shape. The computations also show that certain regions for a wide range of q values, undergo many revolutions before entering the divertor and can be considered to be undiverted. Such regions will have a higher density and temperature compared with the sur- rounding diverted plasma. There will therefore be a higher rate of ionisation and excitation so that these areas will show up as bright areas in the film. Thin, bright, almost stationary shapes, are in fact observed in the films at the position predicted by the computations. The existence of slow moving regions of differing den- sity can explain the discontinuities in the ion flux in diverted discharges in DITE observed by Erents using a thermal desorption probe [ 131.

4. Plasma behaviour

4. I. Initial stages of rhe discharge

10 15 20 q

Fig. 13. The number of cells m as a function of q.

by as much as 15 cm are observed producing a localised increase in the limiter glow where they make contact with the limiter. As the plasma current increases these structures disappear and reform several times, decreas- ing in number as the current increases (fig. 12(a)-12(g)). The number of cells is correlated with the value of the safety factor q at the limiter, as shown in fig. 13. This shows that the effective mode number m changes in a regular way as q falls. The values for m are in the range 0.9- 1.3 q, where q = (a/R)( B,/B,). During the plasma current plateau the cells are normally not visible, al- though for one abnormal, low current, resistive dis- charge such phenomena persisted for most of the dis- charge. These cells are seen at the start of most DITE discharges including discharges where the adjustable limiters have been used to produce a discharge of small minor radius. They have not however been observed in diverted discharges but may well be hidden by the complex structures seen in diverted plasmas.

During the initial phase of the DITE discharge, radial cell like phenomena which penetrate the plasma

4.2. Disruprions

a a

Fig. 12. Transient phenomena during the initial stage of a DITE discharge; (a) I =2.2 ms, (b) 3.2 ms. (c) 3.6 ms, Cd) 4.0 ms. (e) 4.2 ms, (f) 4.6 ms, (g) 5.6 ms.

When the plasma disrupts and expands towards the wall an increased plasma surface interaction is observed with a glow appearing around components behind the limiter and near the wall similar to the limiter glow during a normal discharge. This increased interaction is well correlated with fluctuations in the loop voltage and plasma position even for minor disruptions as shown in fig. 14 for DITE.

The plasma behaviour during soft disruptions with its characteristic fast expansion towards the wall and the slow contraction afterwards has been modelled by Lisitano [15] and Turner and Wesson [ 161. The sudden

18 D. H.J. Goodall / High speed tine film studies

outwards r

Horizontal Plasma Positlon

2.0cm n

150

kA)

100

,

50

Neutral Injectton Period

Plasma Current

1 , I I I + + t + T1rrt 1 0 40 80 120 Ii+ 200 t (ms)

Fig. 14. The correlation between visible disruptions in DITE (vertical arrows). and loop voltage signals. plasma position and Ti II radiation. The occurrence of individual UFOs is shown by the horizontal lines.

influx of impurities after the plasma contact with the wall causes cooling by an increase in the plasma radi- ation which in turn causes a reduction in the plasma minor radius. Precursors to disruptions are seen in DITE as an increase in MHD activity from coil sensors with an accompanying break up in the glow around the limiter in filmed discharges.

4.3. Magnetic islands

Fig. 15 shows examples of phenomena in the outer regions of the plasma which have been interpreted as magnetic islands. The islands have an elongated shape with a dark centre. Although superficially resembling the limiter shadows previously discussed, they tend to be more elongated, with an appreciable variation in poloidal position in times less than 0.3 ms. In contrast the poloidal position of the limiter shadow is very stable and changes slowly with plasma current and the re- sultant change in q. The islands are most frequently observed during disruptions when multiple structures are often seen in the length of the visible part of the minor circumference.

Fig. 15. Magnetic islands in a DITE discharge, 173 ms after the start (I, = 190 kA, max A, =6.8X lOI mm3).

4.4. Filaments

The bright filaments shown in fig. 7 in ASDEX at the gas puffing position throughout the discharge are

D. H. J. Goodall / High speed tine film studies 19

very similar to filaments which appear when a pulse of methane is injected into the plasma in the field of view of the camera [5]. This suggests that the filaments are always present but are normally invisible and are il- luminated by the presence of atoms or ions emitting visible radiation. The filaments fluctuate in position independently of the filming speed, suggesting a wide frequency spectrum [3]. Similar fluctuations at the edge of the plasma have been observed with Langmuir probe measurements [14] and in films as a flutter near the limiter. The marked similarlity between the ASDEX filament fluctuations and the DITE flutter suggests that they are the same phenomenon. The difference in ap- pearance may be due to observing the filaments in cross section in DITE and toroidally in the ASDEX films of the gas puffing position. Toroidal filaments are, how- ever, sometimes seen in films of DITE discharges, usu- ally during unstable conditions. Plasma current fila- ments induced by electron thermal instabilities in tokamak like plasmas have been proposed [ 17,181 which appear to be consistent with this phenomena.

5. UFOs

Bright moving macroscopic particles (UFOs) have been observed in many tokamaks, often originating from limiters, particularly during abnormal discharges. In DITE. however, UFOs in the cameras field of view do not originate from the visible limiter and occur mainly in the lower half of the torus. Fig. 16 shows an example of UFOs in an unstable discharge. The UFOs in DITE have a velocity of a few metres per second and

Fig. 16. UFOs in an unstable DITE discharge, 309 ms after the start when I, =35 kA (I, = 150 kA).

appear to travel predominantly around the torus in the direction of the plasma current. Examples of UFOs travelling in the opposite direction are, however, often observed in the same discharge. Similarly when UFOs are observed falling from the top of the torus, examples of UFOs rising can also be seen. For some DITE discharges with neutral injection, UFOs originating in the injector beam line during the injection period have been observed. In one discharge a beam of UFOs from the bottom of the beam line travelled in straight lines in the direction of the beam with a lifetime T- 20 ms. After the neutral injector period the normal UFO mo- tion around the torus was observed. This indicates a momentum transfer from the neutral beam to the UFOs.

Results from DITE in table 1 show the average num- ber of UFOs per discharge and their mean lifetime for the 10% of the torus volume filmed. Unlike most of the other tokamaks where UFOs have been observed, the vacuum vessel of DITE had been gettered with titanium during the period of observation. It is well known that as the layer of titanium builds up on the walls of a titanium gettered vacuum vessel, titanium flakes detach from the surface, particularly if the walls are con- taminated. Such flakes have been observed to move in plasmas and fig. 17 shows a still photograph taken with a long exposure in the CLEO tokamak [ 191.

To demonstrate that flakes from the walls in DITE can produce UFOs similar to those observed during normal discharges in DITE, discharges were filmed dur- ing which the torus received a mechanical shock. The shock was produced by a 1.5 kg weight falling 60 cm which could be triggered to make the impact at any desired time during the discharge. Impacts after the start of the discharge produced showers of UFOs ap- pearing at the time expected for flakes falling freely under gravity to reach the discharge (fig. 18). Impacts before the discharge sometimes produced disruptions at a time when freely falling flakes would have reached a minor radius of 10 cm. Showers timed to be at the

Table 1

Average UFO lifetime

Average number of UFOs per discharge

Discharges with neutral injection

Discharges without neutral injection

3.2 ms 2.9

2.3 ms 3.3

20 D.H.J. Goodall / High speed tine film studies

Fig. 17. Moving titanium flakes in the CLEO stellerator oper- ating in the tokamak mode.

plasma centre at the start of the discharge were ejected

from the plasma in less than 5 ms.

In addition to the mechanical shock experiments

Fig. 18. UFOs 3 ms after the start of the discharge. produced by a mechanical shock to the wall in DITE.

Ti Pellets 3mm long

150 - and diameter as shown

0.5.1.0.2.0.3.0mm

0 40 60 120 160 (1,) Time (ms)

Fig. 19. Premature termination of DITE discharges caused by dropping titanium pellets into the plasma.

films were taken in DITE of titanium pellets dropping into the plasma. The cylindrical samples which ranged from 0.25 mm to 3 mm diameter and 2 to 3 mm in length, were released from the fixed limiter position at 26 cm at various times during the discharge. Samples timed to be at the torus centre at the start of the discharges caused an immediate extinction of the plasma and samples released at 80 ms before the start of the discharges produced disruptions as shown in fig. 19. Falling freely under gravity the samples would have reached a minor radius in the range lo-13 cm at the time of disruption. For a given sample size and release time, the disruption time was determined by the plasma density and the higher the density the later the time of disruption. The experiments with mechanical shock and pellet dropping in DITE show that large showers and pellets can cause disruptions and pellets in particular produce violent disruptions which extinguish the plasma. Titanium flakes falling from the divertor chamber in PDX have also terminated discharges [20]. The UFOs from the shock and pellet experiments, however, appear to be substantially dominated by gravitational forces unlike the UFOs occuring spontaneously in normal discharges.

6. Discussion

The observations reported show a wide range of phenomena which may have important implications in understanding plasma behaviour. The observed limifer shadows could for example yield information on per- pendicular and parallel diffusion since visible dif-

D. H.J. GoodaN / High speed tine film studies 21

ferences in the plasma are evident after at least two rotations around the major circumference of DITE. The bundle divertor also shows visible plasma features per- sisting after several rotations. Discrete limiter and diver- tor shadows will of course have important effects on surface physics probes in the outer regions of the plasma and should be taken into account when short poloidal or mushroom limiters are used. The fluctuating fila- ments observed in DITE and ASDEX also merit further study as they may well be a universal characteristic of tokamak discharges. They are easily seen in ASDEX near the gas puffing position or after impurity injection. Although both the gas puffing and impurity injection positions are on the outer wall of ASDEX, the DITE fluctuations are seen at the inner wall. This indicates that the filaments, which are present for most dis- charges, occur over the entire plasma surface. The spa- tial fluctuations of the filaments suggest a possible contribution to cross field diffusion. If they do make a significant contribution, the fact that the filaments are more prominent in some discharges than others in DITE and their intensity changes during the discharge, offers the possibility of some measure of control when their behaviour is understood.

The phenomena observed in the plasma i.e. the ini- tial plasma cells and magnetic islands are also signifi- cant and will be analysed in more detail in another paper [22]. The cells in particular may help to explain why plasma skin currents do not persist as predicted by present theory [23] which would predict skin currents in JET lasting several seconds.

The limiter glow is another striking feature of the discharge filmed in many tokamaks. It is interesting to compare the observed width of the glow (- 2 cm) in DITE with an estimated plasma penetration depth of neutrals from the limiter. This depth will be determined by the depletion of neutrals as ionisation occurs, and the ions drift along field lines away from the immediate vicinity of the limiter. Taking the observed values of T, and n, at the limiter in DITE [14] of 10 eV and 10 m 3, the penetration depth for H. 0 and Ti atoms can be estimated. It is assumed that scattered H atoms have energies in the range 50-100 eV, sputtered, evaporated or thermally desorped atoms have energies of 2 eV and 0.05 eV respectively. For an e-folding length h for n, in DITE of 1 cm, and taking the reaction rates from Lotz [21 J, it can be shown that sputtered Ti has a penetration depth - 2cm and desorbed 0- 3.5 cm. Sputtered 0 and scattered H have much longer length far exceeding the observed width of the glowing region. This estimate and the spectroscopic results in section 3.1 are con- sistent with the production of a limiter glow by excited

sputtered Ti neutral atoms with a contribution from

thermally desorbed oxygen. UFOs are an obvious source of plasma impurities

and their contribution to the impurity concentration must be assessed. The following discussion is a summary of a more detailed account to be published separately

v41. The size of a UFO can be estimated from its ob-

served lifetime which in DITE has a maximum value - 20 ms. For a titanium spherical UFO of radius r, the lifetime will be determined by its thermal capacity and the incident energy. It can be shown that for an ion density of lOI me3 and T, 100 eV, a 20 ms lifetime gives a value for r of 30 pm.

Estimates of the size of a UFO which can follow magnetic field lines can also be made. Particles travel- ling under the influence of magnetic forces require an appropriate Q/m ratio for the maximum gyroradius in DITE which is -0.2 m. For a titanium UFO with the maximum possible charge consistent with its tensile strength, the maximum value of the UFO radius = 0.25 pm. This value of r is inconsistent with the value calculated from the observed lifetime of UFOs and even allowing for the uncertainty in the value of T, used in this calculation, it seems unlikely that UFOs are mag- netically confined. Observations of UFO paths show no evidence for helical motion.

Electrostatic forces in the plasma permit UFOs of the required size but it is not possible to explain the direction of motion from the direction of the electric field in the plasma. It is therefore likely that the UFO driving force is non uniform ablation, momentum trans- fer or both.

For discharges without neutral injection, mass mo- tion of the plasma around the major circumference of a tokamak is thought to be related to the motion of MHD waves. Measurements in tokamaks have shown a char- acteristic frequency - 10 kHz for n = 1, m=2 waves [25]. For a wavelength X = 2nR a velocity in DITE (R = 1.17 m) of 6 X lo4 ms- is obtained. Taking this velocity as the ion velocity, the size of a UFO can be estimated assuming its motion is determined only by momentum transfer. For the same ion density and UFO velocity previously taken, a UFO radius of 60 pm is obtained which is consistent with the value estimated from its lifetime. Similar calculations for the UFOs which originated in the neutral injection beam line estimate the UFO radius of a UFO driven by momen- tum transfer from the beam to be r - 20 pm.

There is some direct evidence for the size of UFOs in DITE from debris removed from the torus. Many exam- ples have been found of titanium spheres of 40 pm

22 D. H.J. Goodall / High speed tine film studies

Fig. 20. Titanium spheres found in DITE.

diameter. A few molybdenum spheres presumably from discharges when molybdenum limiters were used have also been observed. Examples of the spheres are shown in fig. 20 and they may well represent UFOs which have been ejected from the plasma.

A spherical UFO of 20-30 pm radius if completely evaporated will produce 2 to 7 X 1Ol5 atoms. The typi- cal level of 5 UFOs per discharge in DITE and the upper limit for UFO size. will produce a total impurity per discharge of lOI to 4 X lOI atoms. This is ap- proximately equal to the typical measured titanium concentrations in DITE [26]. UFOs of the estimated size would therefore make a significant contribution to the impurity content if completely ablated.

7. Conclusions

High speed tine photography has been shown to record interesting and sometimes previously unknown plasma phenomena and plasma surface interactions. It is particularly important to use tine in conjunction with other diagnostics if the maximum value is to be ex- tracted from the films. Similarly study of the films by a wide range of specialists will undoubtedly broaden the understanding of the recorded phenomena. The use of a tine camera as a routine diagnostic should prove to be a valuable and inexpensive aid in plasma experiments.

Acknowledgements

The author would like to thank J.W.M. Paul and the DITE team, M. Keilhacker and the ASDEX team for

providing facilities to film discharges and the support of G.M. McCracken in undertaking this work. Many thanks are due to R.E. Clausing (ISX), H. Otsuka (DIVA, JFT2), H. Niedermeyer (ASDEX) and P.C. Johnson (CLEO) for information and permission to use films and photographs taken by them. Discussions with J. Wesson on the observed plasma phenomena have also been very useful. Particular thanks are due to J. Dowl- ing (Harwell) for invaluable assistance in filming and editing material, G.E. Austin for the construction and installation of apparatus and last but not least T. Bend- ing who spent many hours in the dark analysing film and counting UFOs.

References

[l] W.E. Hyzer, Industrial Research and Development Feb. (1981) p. 181.

[2] H. Lida, Opt. Eng. 20 (1981) 688. [3] H. Niedermeyer (ASDEX) private communication (1982). [4] Culham Laboratory Film Tokamak Discharges in DITE.

produced by D.H.J. Goodall (1982). [5] Culham Laboratory Film Plasma Discharges in the

ASDEX Tokamak produced by D.H.J. Goodall (198 1). [6] R.E. Clausing et al., J. Nucl. Mater. 103 & 104 (1981) 309. [7] H. Ohtsuka, Private communication (1982). [8] D.H.J. Goodall and G.M. McCracken, Nucl. Fusion, 19

(1979) 1396. [9] D.H.J. Goodall et al., J. Nucl. Mater. 93 (1980) 154.

[IO] D.H.J. Goodall, J. Nucl. Mater. 76 (1978) 492. [I l] Frame from tine film supplied by H. Niedermeyer

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