P/3 UK

Review of Controlled Thermonuclear Research at A.E.I.Research Laboratory

By T. E. Allibone,* D. R. Chick,* G. P. Thomsonf and A. A. Ware*

At temperatures of several hundred million degrees,in deuterium, the energy generated by nuclear re-actions will exceed the radiation loss. This factsuggested to one of us (G.P.T.), in 1946, that usefulpower might be generated from such reactions, pro-vided that the loss of energy to the walls could besufficiently reduced by magnetic fields. Experimentswere begun at Imperial College, London, in February1947, to produce very large currents in a low-pressuregas discharge. The containment of the gas was to beachieved by the pinch effect and a toroidal dischargetube was used to eliminate end losses. Currents in therange 1 x 104-2 x 104 amp were induced in the gas bydischarging a condenser of maximum energy 400joules through a primary winding coupled with thegas.1 These experiments were the first to demonstratea marked pinch effect in a gas discharge, althoughinertial effects caused the contracting discharge tooverswing and expand again, followed by furthercontractions and expansions.2

In order to achieve temperatures of several milliondegrees and demonstrate a thermonuclear reaction itwas clear from the Bennett3 relation, 2NkT =72,that currents of the order 105 amp were required.(N is the total number of particles per unit length ofthe discharge, k Boltzmann's constant, T the tempera-ture and/ the current.) When, however, the condenserenergy was increased in 1950 to 18,000 joules, seriouscrazing and evaporation of the small Pyrex glass dis-charge tubes occurred and it became necessary todevelop superior materials to contain the largecurrents.

At this juncture thermonuclear research was madesecret by the Government and, because of the un-desirabüity of conducting secret research at auniversity, the staff and apparatus were moved to theAssociated Electrical Industries Research Laboratory,Aldermaston, in 1951. In the development of suitabledischarge tubes, quartz and porcelain were tried, butit soon became apparent that only metals could with-stand the heat pulse involved. With metal tubes, anew problem was encountered; arc breakdown

* Associated Electrical Industries Limited, ResearchLaboratory, Aldermaston, Berkshire.

t Corpus Christi College, University of Cambridge, Cam-bridge.

occurred across the gap which must be left in thetube to avoid a short-circuit. This arc breakdownhas been the main problem in the development ofmetal discharge tubes. In addition, as the researchwas extended to currents of longer duration the funda-mental problem of the discharge instability wasencountered. Most of the research at A.E.I, has beendirected towards solving these two main problems,namely the arc breakdown and the dischargeinstability.

DEVELOPMENT OF METAL DISCHARGE TUBESThe quartz and porcelain discharge tubes used had

tube diameters 3-5 cm and torus diameters 20-30 cm.With pulsed oscillatory currents of amplitude about3 x 104 amp and frequency 20 kc/s, the quartz tubewas found to vaporise appreciably after 5 microsecondsand the porcelain after about 20 microseconds. Asimple calculation showed that these times were con-sistent with most of the heat generated in the gasbeing passed rapidly to the tube walls and the wallsvaporising when their surfaces approached boilingpoint. The time for which a given surface can with-stand a heat flux of q calories cm"2 sec"1 is given by:

t = X/f (1)where Л = ттвоШрй/А, and во is the boiling temperature,К the thermal conductivity, p the density and s thespecific heat. On this basis, the most suitable materialsfor the discharge wall are those having high values ofthe quantity Л. Unfortunately, most electricallyinsulating materials have a low value of A because oftheir poor thermal conductivity. The only exceptionsare a few oxides and, in particular, beryllia, but theseare very difficult to fabricate. The other materialshaving high values of Л are electrical conductors andof these the metals with high A are the most suitable.

The first metal tube was made in 1951 of aluminiumand had two gaps. In preliminary experiments satis-factory discharges of several thousand amperes wereobtained with about 100 volts per turn. At highercurrents and voltages serious arc breakdown occurredacross the gaps, causing a short-circuit of the dischargeand melting the metal at the gaps. The first step takento prevent arc breakdown was to increase the numberof gaps to reduce the gap voltage. A series of multi-gaptubes was developed, culminating in 1955 in a large

169

170 SESSION A-9 P/3 T. E. ALLIBONE et al.

i i-iFigure 1. 64-sector torus

64-gap torus with tube diameter 30 cm and torusdiameter 100 cm, in which currents up to 8 x 104 ampwere produced (see Fig. 1). At currents much less thanthe maximum and an applied voltage correspondingto only 10 volts per gap, appreciable arc breakdownstill occurred at some of the gaps. The discharge wasvery unstable and wriggle velocities up to 107 cmsec"1 were observed. The instability caused seriousbombardment of the tube wall and may well havecaused local enhancement of the gap voltages, but,when a high temperature plasma is adjacent to ametal surface, an arc can form independently ofapplied voltages.4

Since 1955 a considerable amount of work has beendone to understand the nature of arc breakdown inorder to find ways of preventing it. Investigationshave been made of the role of the surface condition inglow-to-arc transitions in the hope of finding asuitable material or surface treatment with whichtransitions did not occur. Success has been achieved ingreatly reducing the amount of arc breakdown onsmall test electrodes immersed in the plasma of a 1000-amp hydrogen ring discharge. The electrodes were'conditioned'' by allowing arc discharges to form ontheir surfaces under controlled conditions, a processsimilar in some respects to that widely used in themanufacture of certain types of electronic tubes. Withall the metals tried the probability of a ring dischargeresulting in arc formation, which was initially unity,was found to decrease with the number of discharges.For example, in the case of copper and up to 4000volts applied between test electrodes of 10 sq. cm area,the probability could be reduced from near unity toabout 0.01 in some 100 flashes. Careful chemicalcleaning or baking in vacuo reduced the number ofarcs required to condition a surface but no treatmentwas found to obviate the need for 'conditioning'.

Various metals have been compared in these tests

and, in particular, copper was found to arc less thanaluminium. Because of this and the fact that copperhas a larger value of Л, copper liners were used inSceptre III to cover the aluminium near the gaps. Thisenabled over 40,000 discharges with currents from50,000 to 200,000 amp to be passed without arcdamage seriously interrupting research. Similar opera-tions without the copper liners lead to serious damageof the aluminium after only several hundred discharges.

Considerable progress has been made in under-standing the dynamic behaviour of the arc spots inthe presence of a high-current discharge. The arcspots undergo retrograde motion due to the self-magnetic field of the discharge. This effect causes thearcs to travel along the discharge tube and enter thegaps, and it is in the confined space of the gaps thatserious arc damage occurs. Most of the damage is onthe anode side of a gap. Further details of this workare given in a separate paper.4

Study of the Discharge in a Metal Torus

Kruskal and Schwarzschild5 showed theoreticallythat a self-constricted discharge is inherently unstable.There are many modes of instability but probably themost important is that known as the ''wriggle"instability which causes any slight kinks in the dis-charge filament to grow in amplitude. It was firstobserved in the United Kingdom in 1953.6 If such aninstability develops in a metal tube the eddy currentsinduced in the walls produce magnetic fields whichtend to prevent the wriggle developing, and it wasoriginally hoped that this effect would control theinstability. However, in experiments with a glasstorus, surrounded by a metal sheath, high speedphotography showed that stability was retained forcurrent values only slightly in excess of the currentsgiving rise to instability in the absence of a metalsheath. At high currents the discharge appeared towriggle over the whole tube. A simple analysis showsthat eddy current forces will not balance the in-stability forces arising from short wave instabilitiesuntil the wriggle approaches very near the wall.

As so little was known of the properties and, inparticular, the wavelength of the wriggling dischargein a metal tube, it was agreed with AERE that a studyshould be made of the phenomenon, especially as thedecision had been taken to construct ZETA at AERE,in which some instability was expected to occur. Thedischarge tube used initially was a four-gap aluminiumtorus with tube bore 30 cm and mean diameter 105 cm.Coupling between the primary and the gas was aidedby a four-ton iron core and the primary was wound onthe core to reduce its stray magnetic field in thedischarge regions. The condenser bank, which wasdischarged through an eight turn primary by a spark-gap switch, had a maximum energy storage of 66,000joules. In practice, however, the energies used werelimited to about 6000 joules because, with greaterenergy inputs, arcing occurred at all the gaps, resultingin a short-circuit of the discharge. The current pulsetime was about 1 millisecond. A rotating mirror camera

THERMONUCLEAR RESEARCH AT A.E.I. 171

Focal point.

Figure 2. Racetrack torus with "Pepper-Pot" section.Internal bore of tubing, 12 in.

was used to photograph the discharge as seen througha slot running across the discharge tube, and byviewing the discharge from two directions at rightangles, a two dimensional, time-resolved record ofthe discharge was obtained. Current and voltageoscillograms were also recorded and the apparatusprovided time correlation between the streak photo-graphs and the electrical waveforms.

In order to obtain information regarding the thirddimension, parallel to the tube axis, and in particularto determine the wriggle wavelength, further experi-ments were carried out with two straight sections,60 cm long, inserted in the torus and making it race-track in shape. The sections were square in crosssection and one, known as the "Pepper-Pot", had anarray of holes perforated in two of its sides as shownin Fig. 2. The holes were 1.3 cm in diameter and werespaced 2.5 cm apart in both directions. They weredrilled in such a way that their axes converged on afocus at the position of the camera lens. Photographswere taken first with an image converter cameragiving single-frame microsecond exposures, andsecondly with a Courtney-Pratt lenticular platecamera giving multiple-frame photography withexposure and framing times both 10 microseconds.

Very little success has been achieved in photo-graphing hydrogen discharges because of the veryrapid discharge movement and the low light levelinvolved. Because of this most of the work has beendone with argon discharges. The currents studied havebeen in the range 5000-20,000 amp and the mainresults can be summarised as follows. At the lowelectric fields used (0.3 to 1.0 v cm"1), the dischargeforms as a constricted filament near the centre of thetube and no pinch effect oscillations occur.7 The dis-charge has no kinks at this stage but these soondevelop and the discharge filament becomes approxi-mately sinusoidal in shape. The sine wave grows in

amplitude with time and in addition moves along thetube with a velocity of the same order of magnitudeas the rate of growth in amplitude. Figure 3 shows twosample image converter photographs of the racetracksection taken during the development of the in-stability, and the graphs in Fig. 4 give the magnitudeof the wriggle growth velocity in the radial directionand its variation with the applied electric field fordifferent argon pressures. In addition to the develop-ment of the instability, the width of the dischargechannel is increasing during this period, and measure-ments of current and gas resistance suggest that thegas kinetic pressure is increasing more rapidly thanthe confining magnetic pressure.

For a given set of experimental conditions the timeof initial onset and rate of growth of instability werefound to repeat accurately from one pulse to another.The wriggle wavelength was about 40 cm in all cases

10 /¿sec after initiation of discharge

* 4 Ш- #* & * •f "î:i1 1#: Щ '&• ''$& W Ш --i'11 '•"'• ••& '•'•%' '£•'' ! >

20 /¿sec after initiation of discharge

Figure 3. Development of wriggling in a metal torus."Pepper-Pot" section, Argon discharge

Each photograph shows two images of the discharges. The lowerimage in each case is for the direct view of the discharge and theupper image is photographed via the mirror as shown in Figure 2

172 SESSION A-9 P/3 T. E. ALLIBONE et a/.

WRIGGLEGROWTHVELOCITY

(CM SEC'XIO"

.5 I.O 1.5 2.OAPPLIED ELECTRIC FIELD (VOLTS CMH)

2.5

Figure 4. Wriggle growth velocities for different pressures inArgon vs. applied electric field

and only the rate of growth varied with pressure andcurrent. In other words it was the same mode ofinstability appearing first each time and, for fixedconditions, the time of onset and position of theinstability did not vary. These results suggest that theparticular mode observed is due to departures fromperfect toroidal geometry in the torus rather than tofundamental properties of the discharge.

The wriggle amplitude continues to grow until thedischarge is apparently touching the tube wall. At thelower currents studied, the discharge thereafterwriggles in a confused and random manner throughoutthe remainder of the pulse and other modes ofinstability may well be present. At currents greaterthan about 1.2 x 104 amp, and after the discharge hasreached the walls, streak photographs show bars oflight extending across the tube width which last about10 microseconds and appear at random intervals. Atstill higher currents (greater than about 1.5 x 104 amp)this ''Barring" effect occurs for only a short period inthe pulse and is then followed by an abrupt andpermanent reduction in the light emitted by the gas,although the oscillograms show a marked simul-taneous increase in current. It is believed that theeffect is due to short-circuiting arcs having formed atall the gaps at this instant. All these effects—thegrowth of wriggling, the barring, the reduction inlight intensity and the sudden increase of current—are shown in Fig. 5 which shows simultaneous streakphotograph and current oscillogram records for anargon discharge.

Prior to the short-circuiting arcs occurring at all thegaps, local intermittent arcing can take place at somegaps independently of others, and the barringphenomenon (which will be referred to later, since itappears on all records of hydrogen discharges in ZETAand Sceptre) is believed to be associated with thisarcing. An arc produces a local burst of gas and metalvapour which will lead to a region of high gas pressuremoving along the tube.

Study of the Discharge ¡n Applied Magnetic FieldsBefore the observation of the discharge instability

in 1953, a few experiments had been carried out inwhich toroidal magnetic fields were applied parallel tothe discharge. These fields were intended to maintainthe containment of charged particles during theperiods around the current zeros in the case ofoscillatory discharge currents. Apart from a reductionin the constriction of the discharge and a lowering ofgas resistance no marked effects were observed. Since1953 many experiments employing magnetic fieldshave been performed with a view to stabilizing thedischarge channel.

Some of the earlier experiments with steadymagnetic fields have been described already.7 Inparticular, a toroidal magnetic field, Вф applied tocurrents of a few thousand amperes in a glass toruswas found to destroy the pinch effect. This occurredeven when the magnetic field was applied after thedischarge had constricted. In many cases the photo-graphs showed sharp-edged light and dark regionstravelling across the discharge tube. The velocity ofthese bands appeared independent of magnetic fieldand had the magnitude to be expected for an acoustictype magnetohydrodynamic wave.

In other work with an alternating toroidal magneticfield, Miles confirmed the results of Bickerton ofAERE that circulating currents induced in the вdirection suppressed wriggling. It was found, however,that the discharge sat near the outer wall of the torusfor part of a half cycle of the alternating magnetic fieldand at other times near the inner wall. In an analysisof this type of discharge, Liley showed that such aneffect could be explained by taking account of thegradient of Вф and the phase angle between Вф andthe 6 currents in the gas. This work was not pursued,

1TIMEMARKER CURRENT

VOLTAGE""

IOOJU SEC

Figure 5. Streak photograph and oscillograms of Argon dischargeshowing barring and major arcing

The streak photograph includes two ¡mages of the discharge. Forthe upper ¡mage the discharge has been observed parallel to theaxis of rotation of the torus, and the upper edge of this ¡magecorresponds to the outer wall of the torus. The lower image isobserved along a diameter ¡n the equatorial plane of the torus

THERMONUCLEAR RESEARCH AT A.E.I. 173

however, since the results showed that very largereactive powers were required in order to producetoroidal magnetic fields capable of controlling largegas currents.

In 1956 Bickerton showed that a stable dischargecould be obtained with a steady toroidal magneticfield when combined with a metal torus.8 Experi-ments, therefore, were carried out with a steadymagnetic field applied to the discharge in the foursector aluminium torus described in the previoussection. In this work, Вф was again found to increasethe gas current, the increase being as much as fivetimes in the case of low pressure hydrogen. The currentrange studied was from 5000 amp to about 30,000 amp.With argon, the streak photographs were similar tothose with glass tubes. The amount of constriction ofthe discharge when it first becomes visible is pro-gressively reduced by increasing Вф, and in all casesthe transverse wave motion has again been observedat this point. The discharge subsequently expands andfills the whole tube. With hydrogen at the lowercurrents, photographs show a diffuse discharge fillingthe whole tube with some diagonal streaks, suggestinga helical instability. At the higher currents thesediagonal streaks are observed only at the beginning ofthe pulse; at later times the streak photographs havea turbulent appearance with intermittent bars similarto those already mentioned. The effect of Вф is toincrease the number of bars and make them moremarked in appearance.

In 1957 the "Pepper-Pot" racetrack sections werefitted to this torus and, after the experiments withoutmagnetic field, coils were added to extend the studyto discharges with Вф. It was at this time that the firstresults were obtained with ZETA at AERE 9 and theseindicated: (i) that at high currents in a metal toruswith Вф, a stable discharge with trapped field wasobtained and (ii) that the amount of arcing in thepresence of such a discharge was very much reduced,compared with similar currents in the absence of Вф.These results gave encouragement to studying highercurrents in the racetrack torus, and it was found thatin spite of the tube's having only eight gaps, large gascurrents could be obtained in the presence of Вф

without arcs short-circuiting the discharge. Currentsof 105 amp were obtained with the full condenserenergy.

With argon, the appearance of the discharge ismarkedly different under these conditions (Fig. 6).The discharge forms initially over the whole tube,contracts to a minimum diameter and then expandsagain, and over this time the discharge has a moreclearly defined edge. After the discharge has expandedto the walls the streak photographs become confusedfor a time and then seem to indicate a sharp-edgedconstricted discharge with bars superimposed on it.The two phenomena are not independent since markedsideways perturbations occur in the sharp-edgedchannel at the time of each bar. These excursions ofthe channel, however, are limited in amplitude andthe channel does not touch the tube wall. The con-

Figure 6. Streak photograph of Argon dischargePressure 1.0x10~3 mm Hg; Вф, 80 gauss; current 68,000amps

As with Fig. 5 the photograph includes two images of the dis-charge. In this case the lower ¡mage is the one viewed parallel tothe axis of the torus, and its lower edge corresponds to the outer

wall of the torus

striction of the channel is less the higher the initialgas pressure and the higher Вф.

With hydrogen the photographs in most cases showonly the bars, but in a few cases a sharp-edged corehas been .observed. With colour films this core is blue,whereas the bars are mainly red. Some streak photo-graphy was carried out with hydrogen, using a lineof holes along the length of the "Pepper-Pot" and thisshowed that the bars were travelling along the tube.The direction of motion was that of the electron driftvelocity in the discharge for most of the pulse, butchanged to the opposite direction towards the end ofthe pulse. The velocity appeared independent of Вф

and gas current and was always in the range 106 to2 x 106 cm sec-1.

Following the work of Ramsden at AERE,9

measurements were made of the ion temperature fromthe Doppler broadening of oxygen (Ov) impurity lines,since these were from the most highly ionised atomsobserved in the discharge. Voigt profiles were used tocorrect the line contours for instrumental broadeningand, by the middle of October 1957, broadeningcorresponding to temperatures more than 106 °Kwere recorded.

EXPERIMENTS WITH SCEPTRE}

Following Bickerton's observation of stability in ametal tube with Вф, the work of Bickerton, Liley andothers led to the understanding of the part played bytrapped magnetic fields in such discharges. A detailedanalysis by Tayler10 gave the conditions necessary forstability which are now well known. As a resultseveral experiments were proposed at the end of 1956,at AERE and AEI, all incorporating this method ofstabilisation but with different heating mechanisms.Liley suggested that adequate heating of the gas could

% Sceptre: Stabilised Controlled (E) Pinch ThermonuclearReaction Experiment.

174 SESSION A-9 P/3 T. E. ALLIBONE et о/.

Figure 7. Sceptre III

be achieved by ohmic heating due to both the в andф gas currents, aided by the adiabatic compression ofthe gas during contraction.

Two small tubes (Sceptre I and Sceptre II) based onthis idea were in construction in 1957 when the largeracetrack torus yielded the promising results describedabove. It was decided to rebuild this large toruswithout the racetrack sections and its primary wasadapted to produce higher currents. This apparatuswas renamed Sceptre III.

Figure 7 shows a photograph and Fig. 8 a plandiagram of Sceptre III. The torus is made of alu-minium tubing with a bore of 30 cm and a wallthickness of 1.2 cm. The eight porcelain insulators atthe gaps are protected by eight pairs of interlaced

A TON IRON CORE,

BROILS

.PRIMARY WINDINGS

copper liners, and double vacuum gaskets made fromindium wire are used throughout. Quartz-coveredslits—one in the vertical plane and one horizontal—allow observation of the discharge. The primary hasbeen placed near the torus to reduce the leakageinductance which is now an important factor becauseof the low impedance of a stabilised discharge. Itconsists of four separate windings, of eight turns each,and is wound as uniformly as possible over the surfaceof the torus so that the local magnetic fields of theprimary current cancel in the discharge space.

The toroidal magnetic field is produced by 102layer-wound coils placed around the torus perimeterbeneath the primary, and at pumping and viewingports the field is maintained constant by compensatingcoils. The maximum Вф is 1000 gauss. The condenserbank with maximum energy 66,000 joules andmaximum voltage 30 kv is discharged through theprimary by means of a spark gap switch.

The gas used so far with Sceptre III has been almostexclusively deuterium and all the results given belowrefer to this gas. The pressure range, has been 4 x 10~4

to 4 x 10~3 mm Hg. Work has been done with theprimary connected to give transformer turns ratiosof 8:1, 16:1 and 24:1.

Experimental Results

Conditioning Effect

The behaviour of Sceptre III shows a conditioningeffect. Following a period with no discharges, andparticularly after the apparatus has been dismantledand re-assembled, the first few pulses show a high gasresistance^ with a high level of radiation from theimpurities oxygen and nitrogen, and no neutronemission is detected. With further discharges theimpurity level of oxygen and nitrogen and the gasresistance decrease, though the copper and aluminiumimpurity increases. The degree of ionisation of theoxygen atoms increases and the lines become broader.The current waveform changes in shape and, after atime, exhibit the characteristic changes of slopereferred to below. Simultaneously, neutrons aredetected and, with further discharges, their numberincreases. Lastly, the voltage observed across thegaps in the torus shows more fluctuations under theneutron producing conditions.

After a period of repeated discharges the variousparameters reach steady values and the results listedbelow refer to this ''conditioned" state.

Current and Voltage Oscillograms

In Fig. 9, peak gas current is plotted as a function ofmagnetic field for several condenser voltages with theeight turn primary and for one voltage with thesixteen turn primary. The length of the current pulse isproportional to the number of primary turns and thedecrease in peak current with the number of turns is

Figure 8. Line diagram of Sceptre III

§ In some cases the first two or three discharges showedvery high currents which were believed to be due to short-circuit arcs occurring at all the gaps.

THERMONUCLEAR RESEARCH AT A.E.I. 175

due to the increased damping that results. There is aslow increase of current with increase in pressure.

Figure 10 shows sample oscillograms for voltage,current and rate of change of current. The currentwaveforms have a characteristic shape. When thecurrent reaches a certain critical value there is a fairlyabrupt change in dl/dt and a corresponding changewhen the current returns to this value. The value ofthis current corresponds to a self-magnetic field BQ atthe discharge tube wall equal to the applied magneticfield Вф. The waveforms are consistent with the gashaving a kinetic pressure low compared with themagnetic pressure and with at least some of the dis-charge plasma filling the whole tube until the criticalcurrent is reached, then constricting and trappingmost of the Вф flux, and finally expanding to the wallswhen the current returns to this value. This behaviouris to be expected since, with trapped Вф, there is anoutward pressure of Вф

2/8тт which must be exceededby -Z?02/8TT before pinching can occur.

A considerable amount of information can beobtained from simple inductance measurements. First,before the discharge leaves the wall, and assuming thedischarge is stable, the leakage inductance of the gasis given b}̂ :

•Li = 4irR\)n{d/a)+yl4] (2)

where R is the major radius of the torus, 2a isthe width of the discharge and d is the distance of theprimary turns from the centre of the discharge. Thefactor y, associated with the magnetic field withinthe discharge, will be zero for an infinitely thin skincurrent and will rise to unity for a uniform currentdensity across the discharge. Measurement of L± willyield therefore a measure of у since the other com-ponents can be calculated. Unfortunately, the methodis rather inaccurate since the у component is only asmall fraction of the total inductance.

Secondly, when the current exceeds the criticalvalue at which the discharge leaves the wall, assumingВф to be completely trapped and the kinetic pressureto be small compared with the BQ magnetic pressure,the rate of change of inductance leads to an effectiveincrease, of 4TTR, in the inductance of the discharge.This follows from (a) the circuit equation,

dl(3)

where / is the measured gas current, Rg the ' 'effective"gas resistance and V the volts per turn, and (b) fromthe discharge pressure balance relationship,

Be = — = Вф = a -g Вфо, (4)

where 2b is the diameter of the discharge tube, Вф0

the applied field and a the fraction of Вф flux trapped,and where a is assumed near unity.

This yields\dl __ _}_аа_l~dt~ adt'

2OO

I 6O

kiloGrnps

120

8 0

4O

8:1 Primary16-1 Primary 25 KV

2OKV25 KV

15 KV

5 KV

О 2OO 4OO 6OO 8OO ЮООз Ъф Gauss

Figure 9. Peak current vs. Вф for discharges in Deuterium

The results with 8:1 primary are for 0.4x10~3 mm Hg; thosewith 16:1 for 1.4x10-3 mm Hg

VOLTS/tURN

CURRENT I

О 5ООM sec.

Figure 10. Sample oscillograms from Sceptre 111

and hence

dL\ dL\ da __ 4TTR dl

~dT = HalÜ ~~ ~Т"Ш'

Equation (3) then becomes

where ¿2 =

(5)

(6)

(7)

176 SESSION A-9 P/3 T. E. ALLIBONE et al.

A measurement of the change of dl¡it at the criticalcurrent yields the change of effective inductance. Ifthe measured increase in inductance is ¡34irR, then ¿8should equal unity if the above assumptions arecorrect.

Lastly, from Eqs. (2) and (4) we have

L% has been measured at peak current from the ratio(dV/dt)¡(d4ldt2) (on the assumption dR/dt ~ 0) and,since all other quantities are known in Eq. (8), a valuecan be obtained for a.

Table 1 shows a sample set of experimental resultsobtained for V/I, at peak current, and the quantitiesa, ¡3 and у for the eight turn primary. For this set,

Table 1. Results from Inductance Measurements

Condenser voltage 20 kv, pressure 4 x 10~4 mm HgV/I measured at dl/dt = 0

. , dV I ¿PI ,dl .a measured from -тг/-т^ at —- = 0at I at* dt

J3 measured from change of slope when discharge returnsto the wall

у measured at / = 0 (end of first half cycle)8 = skin depth as a fraction of discharge tube radius

(gauss)

2004006008001000

{ohms)

0.0220.0130.0090.0050.006

- a

0.92

0.951.30

/3

1.11.21.20.8

V

0.580.680.560.530.32

8

0.450.580.430.390.17

the kinetic pressure is known to be appreciably lessthan the magnetic pressure and the simple resultsjust derived can be used. For kinetic pressures com-parable with the magnetic pressures, as with thesixteen turn primary, a more exact analysis must beapplied. The values of ce indicate that the discharge istrapping most of the Вф magnetic field. The values of/? are near unity and confirm the assumptions made inthe abovcT analysis. The values of у are the leastaccurate of the three since they involve measuring asmall increment in the inductance. The values of skindepth corresponding to these values of у are shown inthe sixth column of the table as fractions of the dis-charge tube radius. The inductance and hence у havenot been measured at the beginning of the first halfcycle because the gas resistance is unknown and likelyto be large at this point. The high value of the ratioV/(dI/dt) before the discharge leaves the wall indicateseither a high gas resistance or high inductance.

The measurements of V/I at peak / cannot bedirectly taken to be a measure of gas resistance. Thisis because resistance associated with eddy currentsin the torus walls and arcs must be accounted for inthe ratio V/I, while any component of dl/dt whichis not in phase with the current will also be included inthis ratio.

Streak Photography

The predominant feature of the streak photographsis the "bars" which are observed in all cases withdeuterium and hydrogen discharges. During the"conditioning" of the discharge a sharp-edged core isalso observed which is blue in colour, whereas thebars are mainly red.11 With the decrease in impuritycontent which results from 'conditioning', this corebecomes invisible. Its continued presence is indicatedby the more sensitive detection of the light fromhighly ionised impurity atoms described below.

Spectroscopic Measurements

The spatial and temporal variation of the intensityof impurities Unes has been studied by combining aphotomultiplier with a monochromator. This workindicates that the oxygen O v light comes mainly froma narrow core at the centre of the discharge, with adiameter from ^ to ^ of the tube diameter. On theother hand, O I V has minima at the centre of the dis-charge and is otherwise fairly uniform across the tube.

At a condenser voltage of 25 kv and a pressure of1.4xlO~3 mm Hg, the intensity distributions havebeen obtained for different Вф. At 250 gauss the wave-forms exhibit considerable fluctuations, suggestingsome instability of this central core, and in thisparticular case the O v intensity is a maximum at aradius of about 4 cm and is less at the centre. Athigher fields the waveforms show only small fluctua-tions and the intensity has a sharp maximum in thecentre. The oscillograms indicate that the O v lightappears shortly after the change of slope on thecurrent waveforms and has a maximum intensity atabout peak currents; at all times it comes mainly fromthe central core of the discharge.

Measurements have also been made of the Dopplerbroadening of the O v lines using a Hilger mediumquartz spectrograph. In one case the line profile waschecked to be Gaussian down to -£ъ of the peakintensity. This shape is consistent with the oxygenions having a Maxwellian distribution of velocities andthe temperature indicated by the line broadening. It

1-*- Neutron "Temperature"— Ion "Temperature"

TemperatureIOe oK

4

O ICO 2OO 3OO 4OO SOO бОО 7OO 8OO 900 ЮООЪр Gauss

Figure 11. Variation of apparent ion and neutron temperatureswith applied magnetic field Вф

THERMONUCLEAR RESEARCH AT A.E.I. 177

2.O

1.7

О ЮО 2OO ЗОО 4ОО 5ОО 6ОО 7ОО 8ОО 9ОО ЮОО3 Вф, (GAUSS)

Figure 12. Neutron yield as a function of axial magnetic field at25 kv condenser voltage, 16:1 turns ratio, and various deuterium

pressures (1/¿ = 10~3 mm Hg)

is not proof of the temperature, however, since arandom instability could have velocities which, whenaveraged over time, give a Maxwellian distribution.

On the assumption that the broadening is wholly dueto thermal motion, the ion "temperatures" have beencalculated. The line breadth measurements comefrom photographic exposures for the whole durationof the current pulse and for many discharges. The"temperatures" therefore are to some extent averagevalues, but the time variation of Ov intensity weightsthe average in favour of temperature at peak currents.Figure 11 shows a graph of ion "temperature" againstapplied magnetic field Вф. With the eight-turn primarythe ion "temperatures" were found to decrease withincreasing condenser voltage over the range studied(11-20 kv) but with sixteen turns it increases withincreasing voltage.

Nuclear Measurements

Neutrons and protons from the D(d,n) and D(d,p)reactions respectively have been observed from thedischarge, and X-rays of energies up to 300 kev arealso present, mainly in an intense pulse at the begin-ning of the current cycle. The neutrons appear shortlyafter the change of slope on the current waveformand are emitted in a continuous manner throughoutthe rest of the current pulse and also part of the wayinto the second half cycle. Figure 12 shows examplesof the variation of neutron yield with the appliedmagnetic - field Вф for the sixteen-turn primary.Neutrons have been observed over the pressure4 x 10~4 to 4 x 10~3 mm Hg, there being a peak yieldat 1.4 x 10~3 mm Hg. With the eight-turn primary theneutron yield was less by a factor of about 3; withthe 24-turn primary the yields were about the sameas for sixteen turns but the optimum was at a higherpressure, namely 3 x 10~3 mm Hg. Figure 13 shows

two examples of the variation of neutron yield withtime; in each case it is for one flash of the discharge.The neutron yield is seen to continue in each casebeyond the current zero into the second half cycle ofthe current. Some delayed counts are expected to bedue to capture gamma rays in the scintillationcounter, but the number of observed counts at thecurrent zero and afterwards is too great to be explainedin this way. There is a real neutron emission at thesetimes.

Nuclear plates, shielded by thin metal foils to reducefogging by light and very soft X-ray, have been exposedto the discharge: proton tracks, of length corre-sponding to an energy of 3 Mev, have been observed.The energy spectra of the protons observed at 30°,90° and 150° to the direction of current flow in thedischarge have been measured. Significant differencesin the mean proton energies have been observed. (Aprovisional value for the difference between theenergies'at 30° and 150° is 0.17 + 0.04 Mev.) Theseindicate that at least some of the protons are due tonuclear reactions involving deuterons acceleratedpreferentially in the direction of current flow. Thestatistical accuracy of the results is not yet highenough to exclude the possibility that some of theprotons may be of thermonuclear origin. The protonenergy measurements confirm that these arise fromthe D(d,p) reaction and since their yield is in agree-ment with the measured neutron yield, the origin ofthe neutrons is also confirmed as being the D(d,n)reaction. From the direction of the proton tracks itcan be deduced that these originate near the centre ofthe torus rather than from the walls.

Four indium activation counters have been posi-tioned around the outer walls of the torus at roughly90° intervals, and the induced activities measured.These were constant within the statistical accuracy ofthe results, which is 10%, indicating that the neutronyield did not vary appreciably round the torus.

4 0

NeutronYieldArbitrary 30-

2 0 -

10-

iv-o'1=0(30)

100 2OO 3ÓO 400 500 600 700 800 9ÓO IÓOO3 Time after Initiation ¡i sees

Figure 13. Time resolved neutron yields for 25 kv and 30 kvcondenser voltages

178 SESSION A-9 P/3 T. E. ALLIBONE et al.

SCEPTRE THEORY

Before drawing conclusions from the experimentalresults, it is of interest to summarize the theory ofohmic and adiabatic heating, as developed by Liley,since this has served as a yardstick with which tocompare experimental results. In this theory it hasbeen necessary to make many simplifying assumptionsand these can be listed as follows:

(1) complete trapping of the applied magnetic fieldup to the time of peak current;

(2) the kinetic pressure of the gas is small comparedwith the magnetic pressure;

(3) the skin depth is a constant fraction of thedischarge radius;

(4) the exchange time, for energy transfer betweenelectrons and ions, is short compared with thecurrent pulse, so that their respective temperaturesare always equal;

(5) the temperature is constant across the discharge;(6) the rate of change of current, after the discharge

leaves the wall and up to the time of peak current, isconstant; and

(7) all heat losses are negligible.

The basic equations are then:

2nR j t fb

62

rp, (9)

(10)

(И)

(12)

where N is the total number of electrons and ions perunit length of the discharge tube, Ie the total currentcirculating in the в direction, Re the corresponding gasresistance, n the number of electrons and ions perunit volume, and v the number of primary turns.Equation ,(12) is used only to give the period of thecurrent pulse. The leakage inductance used was thatfor Sceptre where d/b is equal to 1.5.

With the various assumptions listed above, thetemperature reached at peak current is found to berelatively independent of compression over the range1.2 < b/a < 2.5 and is given in degrees Kelvin by

T = (13)

where / is the energy in joules per centimetre of thedischarge tube length, p is the initial pressure inmicrons of mercury, EQ is the initial applied electricfield in volts per cm, y' is a factor proportional to skindepth and is unity when the skin depth is half the tuberadius. It has been taken as unity in the presentcalculation. The theory indicates that the temperatureshould rise fairly rapidly after the discharge leaves thewall and then more slowly up to peak current.

Since the choice of tube size is an important factorin the cost and speed of pursuing our programme ofthermonuclear research, we must consider further thequantity рЬ*Ео. Subject to the assumption of uniformtemperature distributions, criteria have been derivedfor adequate field trapping and for sufficiently shortelectron-ion energy exchange times. These criteriainvolve the parameters/, №EQ and №p; the variablesp, b and Eo do not appear as independent quantities.Therefore, from Eq. (13), the maximum tempera-ture is obtained by making both b2Eo and №p each assmall as these criteria permit. It is important to notethat, in this sense, T is independent of b—any changein this variable requiring corresponding changes in pand EQ to ensure maximum temperature.

CONCLUSIONS

It is possible to calculate the deuterium temperaturewhich would be necessary to produce the observedneutrons by thermonuclear reactions. Examples ofthese "temperatures" are shown in Fig. 11, where theyare compared with the corresponding ion "tempera-ture" from Doppler broadening.

When the first results were obtained with SceptreIII, the good agreement between the Dopplerbroadening "temperatures", neutron "temperatures"and theoretical "temperatures" was strong evidencethat the true temperatures were those indicated andthat the theory indicated the method of heating.11

The more recent experimental results, given above,and further theoretical considerations have brought tolight several serious discrepancies which cast doubton these conclusions. The main discrepancies can besummarized as follows.

(1) Although no electron temperatures have beenmeasured yet, what evidence there is indicates anelectron temperature less than the measured ion"temperatures". The weakness of the O^1 Unes relativeto O v and O I V suggests that the electron temperatureis not greater than 10e °K. The apparent high gasresistance also suggests an electron temperature lessthan this value.

(2) There is a marked discrepancy in many casesbetween the ion "temperature" and the "temperature"deduced from the neutron yield, and the nuclear plateexperiments indicate that at least some of the protons,and consequently some of the neutrons, are of non-thermonuclear origin.

(3) In some cases, if all the gas which is initiallypresent in the discharge is at the temperatureindicated, the kinetic pressure would exceed themagnetic, pressure Вв

2/8тг and the discharge shouldnot be constricted. The current waveforms, however,give evidence that the discharge is constricted.

The following conclusions can be drawn from theresults.

First, it is reasonable to conclude that both theDoppler broadening and neutron yields give upperlimits for the true deuterium temperature. From thisit follows that in some cases all of the neutrons

THERMONUCLEAR RESEARCH AT A.E.I. 179

observed are produced by non-thermonuclear pro-cesses. For example, in Fig. 11 at 25 kv and 600 gauss,the Doppler broadening indicates a temperature of1 x 10e °K at which the thermonuclear reaction rate isundetectable, but nevertheless a large yield wasobtained. Second, the evidence indicates an electrontemperature appreciably greater than 105 °K but lessthan 10e °K. The deuterium ion temperature may begreater or less than the electron temperature.

Finally, the inductive and capacitive energy remain-ing at peak current, as estimatedfrom the oscillograms,shows that over half of the initial condenser energyhas been dissipated in the gas by that time. Sincefor an initial pressure of 1.4 x 10~3 mm Hg this energyis sufficient to raise all the original particles to thetemperature 3 x 107 °K, it must be concluded thatmost of the energy is being lost from the dischargeor shared with impurities.

Despite these many uncertainties, however, there isone outstanding achievement indicated by the results,and particularly by the study of O v light, and this isthat a stable discharge was achieved for severalhundred microseconds.

ACKNOWLEDGEMENTS

The authors wish to point out that this paperdescribes the work of a team of scientists andengineers whom it was their pleasure to direct. Thenuclear physics measurements were made by membersof the Nuclear Physics Section of this Laboratory.

The authors would also like to acknowledge theclose co-operation with the Atomic Energy ResearchEstablishment, Harwell, and, since 1956, the stimulusof exchange of information with the United States.Most of the work was carried out under a contractwith the Atomic Energy Authority.

REFERENCES

1. A. A. Ware, Phil. Trans. Roy Soc. London, 243A, No.863, 197 (1951).

2. S. W. Cousins and A. A. Ware, Proc. Phys. Soc, 64B,159 (1951).

3. W. H. Bennett, Phys. Rev., 45, 890 (1934).4. J. L. Craston et al., The Role of Materials in Controlled

Thermonuclear Research, P/34, Vol. 15, these Proceedings.5. M. Kruskal and M. Schwarzschild, Proc. Roy. Soc, 223A,

348 (1954).6. R. Carruthers and P. A. Davenport, Proc. Phys. Soc,

70B, 49 (1957).

7. R. F. Hemmings, H. T. Miles and A. A. Ware, Proceedingsof the Third International Conference on IonizationPhenomena in Gases, P/464, Venice (June 1957).

8. R. J. Bickerton, Proceedings of the Third InternationalConference on Ionization Phenomena in Gases, P/101,Venice (June 1957).

9. P. С Thonemann et al., Nature, 181, 217 (1958).

10. R. J. Tayler, Proc. Phys. Soc, 70B, 1049 (1957) andProceedings of the Third International Conference onIonization Phenomena in Gases, P/1067, Venice (1957).

11. N. L. Allen et al., Nature, 181, 222 (1958).

Mr. Ware presented Paper P¡3, above, at theConference and added the following remarks:

With reference to the spectroscopic study of lightfrom highly ionised oxygen, Fig. 14 shows intensityprofiles for the Ov light at different times during thedischarge pulse. There are some high frequencyfluctuations on the intensity oscillograms, but the lightin the outer regions of the tube is always small com-pared with the intensity in the centre of the tube.This suggests that gross instabilities have been over-come for the duration of the pulse.

Measurements of Doppler broadening have recentlybeen made, viewing the discharge in the tangentialdirection (parallel to ф) as well as the radial direction.

The Une broadening was found to be the same forboth directions. The results also indicated a shift ofthe broadened Une when viewed tangentially. The O v

particles have a directed velocity parallel to the posi-tive gas current from 106 to 2 x 10e cm/sec.

Magnetic measurements have been made with asmall probe inserted into the discharge. Figure 15shows typical Be and Вф profiles for peak current andthe conditions shown. The Be profile shows that thedischarge has an edge outside which there is eitherzero or a small negative current density. The Вф

profile shows that the appUed magnetic field is trappedby the discharge, and in addition extra Вф is producedwithin the discharge so that Вф outside the discharge

Table 2. Proton Energy Measurements in Sceptre

Вф field, Peak gas Deuterium Nominal angle Number of Relative mean Energy shiftgauss current, ha pressure, и of observation proton tracks proton energy, M ev £450—£135°

500

1000

100

140

1.8

1.4

45°90°135°

45°90°135°

56

40

20442145

3.07 ±0.03

2.91+0.03

3.07 ±0.023.00 + 0.032.84 ±0.02

0.16 ±0.04

0.23 ±0.03

180 SESSION A-9 P/3 T. E. ALLIBONE et al.

3.14

400 ju sec.

3SO.U КС

/ 3 0 0 a мс

Figure 14. OV light Intensity distributions at various times afterinitiation of the discharge

24001

1600

800

800

•Wall

Pressure: 1.4 ¡i of deuteriumCondenser Icílovoltage: 20Turns ratio; 16: 1Applied В .. 700 gauss

I

14 12 10/8

Radius, cm

•Centre

Figure 15. Magnetic field distribution at peak current

is negative. The oscillograms show substantialfluctuations.

With reference to the measurement of protonenergies, Fig. 16 shows histograms of the protonenergies for Вф fields of 500 and 1000 gauss. The dis-charge conditions and detailed results are tabulated

Bg - 5OO GAUSS

135°NUMBER 24OF 22TRACKS 2 O

18.16I4j12IО8-642-\

2-93MCV 3-O9Mev

I 'i45°

—\. 1-6 27 2-8 2-9 3O 31 3 2 33RELATIVE PROTON ENERGY (Mev)

B0-IOOO GAUSS

34

135° 2-84 Mev 3O7Mev

NUMBER 4 O iOFTRACKS

2O

I O

I LJ

4f1.16b

2-5 2 6 2-7 2-8 2 9 3O 31 3-2 3 3 3-4RELATIVE PROTON ENERGY ( M e v )

Figure 16. Proton energies at nominal angles of 45° and 135° topositive gas current flow; (а) Вф = 500 gauss, (b) Вф = 1000 gauss

in Table 2. In this work the protons studied werethose emitted at angles of 45°, 90° and 135° withrespect to the positive gas current.