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Tokamak Energy Ltd, UK, is developing spherical tokamaks using high temperature superconductor magnets as a possible route to fusion power using relatively small devices. We present an overview of the development programme including details of the enabling technologies, the key modelling methods and results, and the remaining challenges on the path to compact fusion.

Experimental data on the transition from the low to high confinement mode in tokamaks is briefly reviewed, concentrating on those cases where the transition is made by a slow change in external parameters. The first H-modes, which occurred after the sudden application of neutral beam heating, appeared to result from a bifurcation of the edge transport. However, slow transitions produced, for example, by ohmic heating do not have the character of a bifurcation but appear to result from a slow and reversible change in the characteristics of the edge turbulence, which becomes increasingly intermittent as the high confinement mode is approached. The experimental results are interpreted in the light of various models of the transition process and of the type III or transition edge localized modes that accompany it. The evidence is mainly against the bifurcation hypothesis but nonlinear processes are clearly involved. The implications for the next generation of tokamaks intended to reach thermonuclear ignition are discussed

Monte Carlo modelling of plasma extinction by neutral particles requires energy and momentum sinks to be determined with sufficient statistical significance. We focus on this aspect using calculations of an idealized 1D plasma-gas interface with the Eirene code. A novel non-linear version including self-collisions amongst neutral particles is also applied to admit gas viscosity. It emerges that statistically significant momentum losses often cannot practicably be determined, even at modest plasma temperatures (∼ 10 eV), using conventional collision-estimator techniques. Gas viscosity effects also tend to inhibit extraction of plasma momentum and hinder extinction. Certain possibilities to improve numerical methods are mentioned.

A description of the various ELM types observed in single null divertor plasmas in COMPASS-D is given, including type I and type III ELMs in both ECRH and ohmically heated plasmas. Precursor oscillations to both type I and type III ELMs have been observed. The large variation in the precursor mode structure is discussed, as well as models of the effect on the plasma.

In reply to the Comment by Biel et al (2016 Nucl. Fusion 57 038001) on our recent papers Costley et al (2015 Nucl Fusion 55 033001) and Costley (2016 Nucl. Fusion 56 066003), we point out that the fusion triple product, nTτ E, and fusion power gain, Q fus, cannot be expressed solely in terms of independent engineering design variables such as major radius, R, and toroidal field, B; output performance variables such as normalised beta, β N, safety factor, q, and fusion power P fus, have to be invoked. Further, we show that the density limit has the effect of largely cancelling the size dependence in nTτ E and Q fus, which would otherwise be present, when these parameters are expressed in terms of P fus. Considerations of engineering aspects are also briefly discussed.

The COMPASS-D tokamak has recently obtained H-mode plasmas in a single-null X-point configuration with no additional heating. Clear H-mode signatures have been observed including reduction in Dalpha light and the unmistakable appearance of ELMs and ELM-free periods. Recycling and gas puff can play crucial roles in achieving and terminating the H-modes. The emergence of distinct ELMs, from the apparently-random fluctuations, of the background Dalpha light, is usually one of progression, taking some 10s of ms rather than a sudden discontinuity. The relatively slow evolution allows for detail examination of important phenomena, such as improvements to confinement and fluid rotation. The H-mode quality improves with density, unless impurity accumulation becomes important. Improvements to the energy and particle confinement are observed and are accompanied by positive increases in the radial electric field at approximately=4 cm inside the last closed flux surface (LCFS). The ohmic powers in discharges with clearly defined H-modes have not been less, so far, than the power obtained from power threshold scalings from larger machines.

The START (Small Tight Aspect Ratio Tokamak) experiment became operational in January 1991 at AEA Fusion Culham Laboratory. It is the only tokamak producing hot plasmas at aspect ratios as low as A=R/a approximately 1.3, and is providing valuable insight into the physical processes (such as toroidicity and trapping) involved at low aspect ratio. Three topics are discussed: equilibrium properties, including evidence of neo-classical and bootstrap effects; energy confinement studies; and MHD properties, including possible explanations for the apparent absence of the major disruption at low A.

Low‐aspect‐ratio tokamaks offer both the economic advantage of smaller size and a number of physics advantages which are not available at conventional aspect ratio. The Small Tight Aspect Ratio Tokamak (START) [Fusion Technology 1990, edited by B. E. Keen, M. Huguet, and R. Hemsworth (North‐Holland, Amsterdam, 1991), Vol. 1, p. 353] was conceived as a first substantial test of tokamak plasma behavior at low aspect ratio. It has achieved plasma currents up to 200 kA, peak densities of ∼2×1020 m−3 and central electron temperatures of ∼500 eV at an aspect ratio of 1.3–1.5. Central beta values of ∼13% have been measured and the volume‐averaged beta 〈β〉 can approach the Troyon limit. Plasmas are naturally elongated (κ≲2.0) and are vertically stable without feedback control. Major disruptions have not been observed at low aspect ratios (A≤2.0).

Experimental results from the COMPASS-C tokamak reveal a sharp threshold in amplitude above which externally applied static resonant magnetic perturbations (RMPs) induce stationary magnetic islands. Such islands (in particular, m=2, n=1 islands) give rise to a significant degradation in energy and particle confinement, suppression of the sawtooth oscillation and a large change in the impurity ion toroidal velocity. The observed threshold for inducing stationary (2,1) islands is consistent with a phenomenological resistive MHD model which takes into account plasma rotation (including poloidal flow damping) and externally applied resonant fields. Broadly similar results are found for applied fields other than m=2, n=1. Other results from RMP experiments are also discussed, such as the stabilization of rotating MHD activity, stimulated disruptions and extensions to the disruptive density limit. Finally, the likely effect of field errors on large tokamaks is briefly examined in the light of the COMPASS-C results

Low aspect ratio tokamaks may provide a low cost route to developing magnetic fusion energy and may offer simpler and more economic fusion reactors. The first test of this concept with hot plasmas has been conducted with the Small Tight Aspect Ratio Tokamak (START) experiment now in operation at Culham Laboratory. The novel method of plasma formation is described and observations of plasmas at aspect ratios as low as 1.3 are presented, indicating high temperature discharges with good global stability properties and typical tokamak characteristics. This represents a new and significant extension of the parameter range of the conventional tokamak

Summary form only given. We report on experimental and modelling studies of "detached " plasma operation in the UMIST Linear System (ULS) divertor simulator. The ULS is a device designed to study a range of edge plasma physics issues relevant to tokamak gas target divertors and is capable of producing steady-state plasmas with electron densities and temperatures in the range 10/sup 17/-10/sup 19/ m/sup -3/ and 2-15 eV respectively; this plasma is made to flow into a separate gas target chamber into which a variety of gases can be introduced. Previous studies of detached plasmas in the ULS have centred on the interaction between hydrogen plasma and low pressure (< 10 mTorr) neutral hydrogen gas and have identified a regime in which molecular activated recombination processes appear to be the dominant plasma loss mechanism (MG Rusbridge et al, Plas. Phys. Cont. Fus. 42, 588 (2000)). Here we report on further studies in which the upstream plasma parameters are varied such that three-body and radiative electron-ion recombination (EIR) of hydrogen plasmas can be dominant. Spectroscopic and Langmuir probes data have demonstrated the resulting highly non-equilibrium distribution of excited neutral states resulting from these recombining plasmas. Evidence of hysteresis is found in the transition between the two modes (EIR and MAR) of recombination. Initial modelling of the recombination region in the target chamber is being undertaken using simplified one-dimensional electron energy balance and continuity equations (Krasheninnikov et al, Phys. Plas. 4, 1644 (1997)). We determine the factors that govern the threshold between MAR and EIR dominant detached regimes in terms of the upstream plasma parameters. We discuss the significance of these results for future divertor simulator research.

It is generally accepted that the route to fusion power involves large devices of ITER scale or larger. However, we show, contrary to expectations, that for steady state tokamaks operating at fixed fractions of the density and beta limits, the fusion gain, Qfus, depends mainly on the absolute level of the fusion power and the energy confinement, and only weakly on the device size. Our investigations are carried out using a system code and also by analytical means. Further, we show that for the two qualitatively different global scalings that have been developed to fit the data contained in the ITER ELMy H-mode database, i.e. the normally used beta-dependent IPB98y2 scaling and the alternative beta-independent scalings, the power needed for high fusion performance differs substantially, typically by factors of three to four. Taken together, these two findings imply that lower power, smaller, and hence potentially lower cost, pilot plants and reactors than currently envisaged may be possible. The main parameters of a candidate low power (?180?MW), high Qfus (?5), relatively small (?1.35?m major radius) device are given.

The nature of the L- to H-mode transition is re-examined on the basis of the more recent experimental data, especially that obtained from Ohmically-heated plasmas where the transition is generally more gradual. This raises the question whether the transition is better explained as a gradual change in the characteristics of the turbulence at the plasma edge rather than by a sudden bifurcation in the transport process.

Magnetic feedback control has been used in the DITE tokamak to substantially reduce saturated {ital m}=2, {ital n}=1 instabilities in both Ohmic discharges and discharges with lower hybrid current drive (LHCD). Feedback has been used for the first time to significantly increase the disruptive density limit in a tokamak. LHCD on DITE stabilizes the sawtooth instability but generates a large {ital m}=1,2, {ital n}=1 instability. Both components of this mode have been controlled for the first time with feedback. Open loop experiments with LHCD show that mode locking occurs and allow detailed study of this phenomenon.

The main experimental results on confinement and transport processes in tokamaks are summarised. It is difficult to draw definite conclusions from measurements of the global energy containment time even in the quasi-steady phase of those ohmically heated discharges, which do not exhibit gross instability or appreciable effects due to runaway electrons. However, comparison of particle and energy confinement suggests that the loss process is mainly conduction rather than convection. In order to separate the various energy input and loss mechanisms the assumptions of classical ohmic heating and equipartition are made. The experimental evidence for these assumptions is presented. The dominant energy transport process is electron thermal conduction, for which a formalism is suggested for comparing experimental data with various theoretical models. Particle and iimpurity transport appears to require the neoclassical pinch tenn in addition to anomalous particle diffusion at a significantly lower rate than electron conduction. The contribution of the sawtooth instability to transport can be measured in principle and is only important in certain conditions. 1-D models of the discharge are quite successful provided empirical forms of the electron conduction and particle diffusion are used. Partial dat a exists on various related topics: plasma rotation, radial electri c fields, the boundary layer, the effects of toroidal field ripple and low frequency instabilities. This material is summarised.

A system for servo-control of the plasma position in a tokamak discharge is described. Measurements of the stability boundary of the servo-loop in the Cleo-Tokamak apparatus are presented and compared with a theoretical model. They show reasonable agreement. Using servo-control, the discharge time was extended from 110 ms to nearly 200 ms, and the plasma remained within ±10 mm of the centre of the vacuum vessel for up to 140 ms. The instability caused by a deliberately applied vertical field with positive gradient was stabilized.

The latest analysis of results from the Bundle Divertor and prospects for application to a tokamak reactor are presented. The upgraded version of the machine, DITE IB is described briefly. Initial results of a programme to study the causes of disruption at high density in ohmic and beam-heated discharges are also described.

The beam-driven current produced by neutral injection into the DITE tokamak has been measured. The variation of this current with electron density and gas current has also been investigated. Good agreement is found between experiment and a model in which the Fokker-Planck equation describes the fast-ion distribution and a magnetic field diffusion equation gives the time dependence of the electromagnetic fields.

Experimental results from a large number of tokamaks have been collected. It is shown that the measured energy confinement times are consistent in the sense that they may be represented (accurately) by simple empirical formulas. These formulas are used in predicting the Ohmically heated temperatures in future tokamaks. The predicted temperatures are similar to but slightly lower than those in present machines. Te0 is about 0.5 to 1 keV. This result emphasizes the point that substantial additional heating will be required for the attainment of ignition temperature.

The electric and magnetic fields to be expected within the ionosphere from a background source uniformly distributed across the sky are studied. The ray theory for an isotropic, horizontally stratified loss-free slowly varying ionosphere is first given, and it is shown that the mean-square moduli of the electric field components have n times their free-space value, and the mean-square moduli of the magnetic field components have n 3 times their free-space value where n is the local refractive index. The corrections to be expected when a full-wave theory is used are discussed. The ray theory for an anisotropic, loss-free ionosphere is then given and it is shown that the energy flux in a given direction is proportional to the Gaussian curvature at the associated point on the refractive index surface. For the ordinary wave, near the level of reflexion, the electric field is predominantly parallel to the earth’s magnetic field, and the small perpendicular component is nearly 100% circularly polarized. Results for the magnetic fields and for the fields of the extraordinary wave are also given and discussed.

Electron density measurement in ionosphere by technique based on E region plasma resonances

The admittance of a wire dipole of overall length 20 m was measured at a height of approximately 125 km in the night-time ionosphere. Measurements were obtained at 60 kHz and at a frequency swept over the range 1.4 MHz to 1.8 MHz so as to include the gyro frequency. The results are interpreted in terms of a theory due to Kaiser, and fair agreement is obtained. The electron density and collision frequency in the plasma surrounding the aerial are estimated.

Instabilities of the plasma current have been observed when the transform near the plasma boundary is rational, whose strength decreases with increasing mode number. The t=y2 instability is sufficiently strong to limit the current attainable. The instabilities do not seem to depend strongly on other plasma parameters such as n and Te which can vary 50% from shot to shot. The variation of instability strength with t could be MHD in character.

The author considers one of the practical problems presented by the work of Lowder and Thomassen (see abstr. A9731 of 1974), who give a theoretical treatment of the feedback stabilisation of magnetohydrodynamic instabilities.

This paper is concerned with the relations between measured noise voltages or currents at the terminals of short antennas and the temperature of the radiation field to which the antenna is exposed. The variations in these relations due to the presence of a very simple ionosphere surrounding the antenna is considered. The theory is applied to the experimental measurements of sky brightness at 3.8 Mc/s by Molozzi, Franklin and Tyas.

Measurements of edge broadband magnetic field, density and electric field fluctuations up to 300 kHz have been performed in DITE Ohmic and electron cyclotron resonance heated (ECRH) plasmas. Detailed characterization of the structure of the magnetic fluctuations has been obtained using measurements in different poloidal and toroidal locations and radial profiles in the scrape-off layer. The cross-field particle flux driven by electric field and density fluctuations has been measured and found to be comparable to the total particle outflow measured by Langmuir probes embedded in the limiters. In particular, the increase of particle transport during ECRH is completely accounted for by the increase of electric field and density fluctuations. Magnetic fluctuations also increase during ECRH but do not contribute significantly to particle transport and are probably simply a consequence of the perpendicular and parallel currents driven by pressure fluctuations. In the conditions of the experiment, the edge response is not due to direct interaction of the radiofrequency with the boundary plasma, but rather to the effects of the thermal wave propagating from the absorption zone.

This paper describes experiments which demonstrate that a change in the boundary conditions can change the behaviour of high βp, beam heated discharges in DITE. In particular data will be presented from discharges which clearly do not exhibit the soft βp limit.