The spherical torus or spherical tokamak (ST) is a member of the tokamak family with its aspect ratio (A = R0/a) reduced to A ∼ 1.5, well below the normal tokamak operating range of A ≥ 2.5. As the aspect ratio is reduced, the ideal tokamak beta β (radio of plasma to magnetic pressure) stability limit increases rapidly, approximately as β ∼ 1/A. The plasma current it can sustain for a given edge safety factor q-95 also increases rapidly. Because of the above, as well as the natural elongation κ, which makes its plasma shape appear spherical, the ST configuration can yield exceptionally high tokamak performance in a compact geometry. Due to its compactness and high performance, the ST configuration has various near term applications, including a compact fusion neutron source with low tritium consumption, in addition to its longer term goal of an attractive fusion energy power source. Since the start of the two mega-ampere class ST facilities in 2000, the National Spherical Torus Experiment in the United St...

Recent progress on spherical torus research

Extensive data on energetic particle-driven (EPD) modes in the tight-aspect ratio MAST and START experiments with super-Alfvenic neutral beam injection is reviewed. The dependence of toroidal Alfven eigenmodes (TAEs) and frequency-sweeping 'chirping' modes on the thermal plasma β-value is investigated experimentally, and a decrease both in the mode amplitudes and in the number of unstable modes with increasing β is found, in line with theoretical expectations. 'Fishbone' modes in MAST are seen at higher β-values than TAEs or 'chirping' modes, and occasionally are found to trigger long-lasting modes, which may affect global MHD stability. In the light of the data analysis the possible importance of EPD instabilities in larger burning plasma spherical tokamaks, which would necessarily operate in high-β regimes, is discussed.

Beta-dependence of energetic particle-driven instabilities in spherical tokamaks

Different methods of plasma formation have been studied in spherical tokamaks (ST). In inductive methods, breakdown and formation of ST plasma are achieved through the application of an electric field produced by a rapid change of currents in poloidal field coils, in the central solenoid or by using internal electrodes. In non-inductive methods, breakdown and formation of ST plasma are achieved by the application of radio frequency (RF) power, neutral beam injection or other ionization and current drive techniques. A combination of these methods is often used. Inductive methods utilizing flux from the central solenoid have limited performance as there is limited space for the central solenoid in STs, so methods not using the central solenoid flux are of special interest. In this paper we discuss different methods developed and used on the START and MAST STs at Culham; in particular, direct induction and electron cyclotron resonance preionization assisted formation are discussed in detail. These experiments have been performed at different RF frequencies and results of these studies are compared with the theory. Other methods, for example, the merging–compression and the double-null merging, are mentioned and prospects of different formation methods for future STs are discussed.

http://iopscience.iop.org/article/10.1088/0029-5515/46/8/S02/pdf

Plasma formation in START and MAST spherical tokamaks

Improved core high-harmonic fast wave (HHFW) heating at longer wavelengths and during start-up and plasma current ramp-up has now been obtained by lowering the edge density with lithium wall conditioning, thereby moving the critical density for perpendicular fast-wave propagation away from the vessel wall. Lithium conditioning allowed significant HHFW core electron heating of deuterium neutral beam injection (NBI) fuelled H-mode plasmas to be observed for the first time. Large edge localized modes were observed immediately after the termination of rf power. Visible and infrared camera images show that fast wave interactions can deposit considerable rf energy on the outboard divertor. HHFW-generated parametric decay instabilities were observed to heat ions in the plasma edge and may be the cause for a measured drag on edge toroidal rotation during HHFW heating. A significant enhancement in neutron rate and fast-ion profile was measured in NBI-fuelled plasmas when HHFW heating was applied.

/pdf/advances-in-high-harmonic-fast-wave-physics-in-the-national-4okc6s35va.pdf

Advances in high-harmonic fast wave physics in the National Spherical Torus Experiment

In this paper, we review the thermal plasma confinement and transport properties observed and predicted in low aspect ratio tokamaks, or spherical tokamaks (STs), which can depart significantly from those observed at higher aspect ratio. In particular, thermal energy confinement scalings show a strong, near linear dependence of energy confinement time on toroidal magnetic field, while the dependence on plasma current is more modest, the opposite of what is seen at higher aspect ratio. STs have revealed a very strong improvement in normalized confinement with decreasing collisionality, much stronger than at higher aspect ratio, which bodes well for an ST-based fusion pilot plant should this trend continue at an even lower collisionality than has already been accessed. These differences arise because of fundamental differences in transport in STs due to the more extreme toroidicity (i.e. reduced region of bad curvature), and to the relatively larger Er×B shearing rates, both of which can suppress electrostatic drift wave instabilities at both ion and electron gyroradius scales. In addition, electromagnetic effects are much stronger in STs because they operate at high β T . Gyrokinetic (GK) studies, coupled with low- and high-k turbulence measurements, have shed light on the underlying physics controlling transport. At lower β T , both ion- and electron-scale electrostatic drift turbulence may be responsible for transport. At higher βT , microtearing, kinetic ballooning, and hybrid trapped electron/kinetic ballooning modes increasingly play a role, and they have a much stronger impact in the core of ST plasmas than at higher aspect ratio. Flow shear affects the balance between ion- and electron-scale modes. Non-linear GK simulations find regimes where the electron heat flux decreases with decreasing collisionality, consistent with the experimental global normalized confinement scaling. The ST is unique in that the relatively low toroidal magnetic field allows for localized measurements of electron-scale turbulence, and this coupled with turbulence measurements at ion-scales has facilitated detailed comparisons with GK simulations. These data have provided compelling evidence for the presence of ion temperature gradient and electron temperature gradient turbulence in some plasmas, and direct experimental support for the impact of experimental actuators like rotation shear, density gradient and magnetic shear on turbulence and transport.

Thermal confinement and transport in spherical tokamaks: a review

The main aim of the National Spherical Torus Experiment (NSTX) is to establish the fusion physics principles of the spherical torus (ST) concept. The NSTX device began plasma operations in February 1999 and the plasma current Ip was successfully brought up to the design value of 1 MA on 14 December 1999. The planned plasma shaping parameters, elongation κ = 1.6-2.2 and triangularity δ = 0.2-0.4, were achieved in inner wall limited, and single null and double null diverted configurations. The coaxial helicity injection (CHI) and high harmonic fast wave (HHFW) experiments were also initiated. CHI current of 27 kA produced up to 260 kA toroidal current without using an ohmic solenoid. With the injection of 2.3 MW of HHFW power, using 12 antennas connected to six transmitters, electrons were heated from a central temperature of 400 eV to 900 eV at a central density of 3.5 × 1013 cm-3, increasing the plasma energy to 59 kJ and the toroidal β, βT, to 10%. The NBI system commenced operation in September 2000. The initial results with two ion sources (PNBI = 2.8 MW) show good heating, producing a total plasma stored energy of 90 kJ corresponding to βT ≈ 18% at a plasma current of 1.1 MA.

/pdf/overview-of-the-initial-nstx-experimental-results-3imohns6di.pdf

Overview of the initial NSTX experimental results

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Overview of the initial NSTX experimental results