Plasma Physics Reports, Vol. 29, No. 11, 2003, pp. 993–995. Translated from Fizika Plazmy, Vol. 29, No. 11, 2003, pp. 1070–1072.Original Russian Text Copyright © 2003 by Shafranov.

Derek Robinson and Historical Experimentin Magnetic Fusion Research

It has been one year since Prof. Derek Robinson,prominent fusion research scientist, a leading figure inthe UK Fusion Research Program, and Director of theUKAEA Culham Science Centre, died at the age of 61.He was a key person in the historical experiment onlocal measurement of the electron temperature by theThompson scattering technique in the T-3 tokamak(Kurchatov Institute) in the second half of 1969. Thisexperiment convinced the international fusion commu-nity of the validity of information presented 35 yearsago at the 3rd IAEA Conference (Novosibirsk) by Aca-demician L.A. Artsimovich, the leader of the Sovietfusion research program, who reported that a quasi-steady plasma with an unprecedented (for that time)temperature of about 1 keV had been obtained in the T-3 tokamak. This implied that enhanced (Bohm) plasmadiffusion, which had been observed in Princeton stellar-ators and which did not allow one to increase theplasma temperature at a given heating power, was not auniversal phenomenon and, therefore, was not an insu-perable obstacle in creating a fusion reactor. The major-ity of participants at the Novosibirsk conference lis-tened to Artsimovich’s report skeptically. One of thosewho took the report seriously was Prof. R.S. Pease, theDirector of the Harwell Plasma Labortatory. He was thefirst Western scientist to recognize the critical mind andphysical intuition of Artsimovich a few years before theNovosibirsk conference, after he had found in Artsimo-vich’s book Controlled Thermonuclear Reactions, pub-lished in 1961, a valid interpretation of the results of theLivermore experiment (performed in late 1960) onrepeated plasma compression in an open confinementsystem (later, the authors of the experiment correctedtheir interpretation of the results obtained). Aware ofthe importance of increasing the plasma temperature infusion devices, Prof. Pease agreed with Artsimovich’sproposal on the necessity of independently checkingthe T-3 plasma parameters. At that time, Dr. Robinsonand his colleagues were involved in investigations ofthe relation between plasma turbulence and the profilesof the magnetic field and electron temperature in theZETA device. To measure the distribution of the plasmaelectron temperature, a diagnostic technique based onthe Thomson scattering of a ruby laser had been cre-ated. A group of English physicists headed by N.J. Pea-cock arrived at the Kurchatov Institute with this appa-ratus. At the height of the Cold War, it was no easy mat-ter to organize such a visit of British scientists andtechnicians, equipped with an experimental facility

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with a total mass of 5 t. Like Soviet scientists travellingabroad, their British colleagues would also obey certainrequirements. In particular, as Prof. Pease remembered,the visit of Dr. Robinson to the USSR would have beenimpossible had he not gotten married a year before.

Derek Robinson was born on May 27, 1941, in Dou-glas, on the Isle of Man. In 1965, he graduated fromManchester University and, as a talented student, wassent to work at the Atomic Centre in Harwell. There, heinvestigated plasma turbulence in the ZETA toroidaldevice, in which large-scale plasma instabilities relatedto the toroidal electrical current were stabilized (in con-trast to tokamaks) by a relatively weak toroidal mag-netic field, decreasing practically to zero at the plasmaedge. Dr. Robinson found that small-scale plasma tur-bulence was suppressed (the so-called “quiescent”regime) when the direction of the toroidal magneticfield outside of the main current channel was reversed,whereas the total toroidal magnetic flux was conserveddue to the high electric conductivity of the vacuum ves-sel. Systems of this type were called reversed fieldpinches (RFPs). Such systems are a remarkable exam-ple of plasma self-organization: the macroscopic stabil-ity of the plasma in an RFP is provided by supplyingpower to maintain the current pulse continuously gen-

Fig. 1. Derek Robinson. Report at the Dubna workshop,1969.

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erating a toroidal magnetic field with the radial profilethat is necessary for macroscopic plasma stability. Inaddition to experimental investigations, Dr. Robinsonperformed a number of theoretical studies on plasmastability in ZETA-type systems. In particular, he calcu-lated the maximum possible ratio of the plasma pres-sure to the magnetic field pressure and the plasma den-sity profiles stable against ideal magnetohydrodynamicperturbations. In experiment, he was the first to imple-ment a technique for measuring the electron tempera-ture by Thomson scattering. It is this technique withwhich the British scientists performed the above exper-iment to determine the electron temperature in the T-3tokamak. As V.V. Sannikov, a Soviet participant of thisexperiment, remembered, his British colleagues firstused a ruby laser in the free-generation mode with apulse duration of ∆t = 1 ms and a pulse energy of E =50 J. The use of such pulses seemed to be attractive dueto their large energy, since, in this case, the number ofscattered photons was expected to be fairly large. How-ever, the amount of radiation emitted by the plasmaitself over a time of 1 ms also turned out to be verylarge. Since the cross section for the scattering by elec-trons is very small, σ ≅ 6.6 × 10–25 cm2, the integral sig-

Fig. 2. During Dr. Robinson’s historical report. The personon the left is L.A. Artsimovich, the chairman of the session.

nal from spontaneous plasma emission exceeded thatfrom the scattered laser radiation. For this reason,Dr. Robinson employed a Q-switched laser generatinga giant pulse with a duration of ∆t ≈ 10–9 s and anenergy of E ≈ 3 J; i.e., the integration time wasdecreased by a factor of 105. Although, in this case, thelaser energy decreased by more than one order of mag-nitude, the signal-to-noise ratio increased to about 10.This made it possible to record the scattered signalswith an accuracy of 7–10%.

The results of this joint Soviet–British experimentwere reported by Dr. Robinson at the Second Interna-tional Workshop on Toroidal Systems, held in Dubna,near Moscow, in 1969 (the first workshop was held atPrinceton in 1965), and were then published in Nature(1969, vol. 224, p. 488).

This experiment convinced the international fusioncommunity of the fact that an electron temperature oneorder of magnitude higher than in other contemporarydevices had indeed been achieved in a tokamak. For thisreason, in the 1970s, studies on magnetic plasma con-finement all over the world began to switch over totokamaks. The subsequent continuous progress inincreasing the plasma parameters showed that the tur-bulent processes in a tokamak plasma aggravate plasma

Fig. 3. After the report by Dr. Robinson. The person on theright is L.A. Artsimovich; at the center is V.V. Sannikov, theSoviet participant of the joint experiment.

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DEREK ROBINSON AND HISTORICAL EXPERIMENT IN MAGNETIC FUSION RESEARCH 995

thermal insulation to a lesser extent than in RFPs,which are conceptually rather similar to tokamaks. Thisconvinced scientists of the possibility of achieving theplasma parameters required for a fusion reactor. In fact,the a transition from tokamaks with a small plasmaradius of a = 0.15 m to those with a radius of a ≅ 0.4 m,and then with a ≅ 1 m, resulted in experiments withplasma parameters at which the power released in D-Tthermonuclear reactions was comparable to the inputpower. The continuous progress in increasing theplasma parameters in these devices has finally led to theelaboration of an international project: the InternationalThermonuclear Experimental Reactor (ITER).

Dr. Robinson contributed greatly to all stages of thisprogress toward his final goal. He actively participatedin the JET (the largest all-European tokamak) and

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ITER programs and gave lectures on controlled fusionresearch in a number of developing countries. In addi-tion to his participation in designing large devices, hecontinued searching for alternative approaches to mag-netic plasma confinement. Thus, he was the first toimplement the model of a conceptually new, extremelycompact tokamak with an aspect ratio R/a only slightlylarger than unity.

The name of Derek Robinson, a remarkable scientistand a highly intelligent, considerate, and kind man, willremain forever in the history of controlled fusionresearch.

V. D. Shafranov