The impact of plasma shaping on electron heat transport is investigated in TCV L-mode plasmas. The study is motivated by the observation of an increase in the energy confinement time with decreasing plasma triangularity which may not be explained by a change in the temperature gradient induced by changes in the geometry of the flux surfaces. The plasma triangularity is varied over a wide range, from positive to negative values, and various plasmas conditions are explored by changing the total electron cyclotron (EC) heating power and the plasma density. The mid-radius electron heat diffusivity is shown to significantly decrease with decreasing triangularity and, for similar plasma conditions, only half of the EC power is required at a triangularity of −0.4 compared with +0.4 to obtain the same temperature profile. Besides, the observed dependence of the electron heat diffusivity on the electron temperature, electron density and effective charge can be grouped in a unique dependence on the plasma effective collisionality. In summary, the electron heat transport level exhibits a continuous decrease with decreasing triangularity and increasing collisionality. Local gyro-fluid and global gyro-kinetic simulations predict that trapped electron modes are the most unstable modes in these EC heated plasmas with an effective collisionality ranging from 0.2 to 1. The modes stability dependence on the plasma triangularity is investigated.

Predicting intrinsic plasma rotation and its shear, which often help stabilize plasma instabilities affecting plasma performance, is important for prospective fusion grade devices. Although rotation in ITER-like scenarios has been extrapolated from measured experimental plasma rotation data, little is understood about the underlying mechanisms governing either the generation or dissipation of momentum in a tokamak plasma. This paper reports on studies of intrinsic toroidal and poloidal plasma rotation from charge exchange spectroscopy using a low power diagnostic beam on the TCV tokamak [Tonetti et al., in Proceedings of the Symposium on Fusion Technology (1991), p. 587] that drives negligible toroidal velocity. In TCV, plasma behavior can be separated by the core and edge regions. In limited configurations, the core rotates in the counter-current direction and can reverse to the co-current direction with a <10% increase in the plasma density. This is different for diverted configurations where the core rotates in the co-current direction reversing to the counter-current direction at higher plasma densities. For all these situations, core toroidal momentum is strongly transported by plasma sawteeth oscillations. In contrast, the toroidal edge rotation is close to stationary for limited discharges but evolves with plasma density for diverted configurations. Theoretical models that predict a change in momentum transport from turbulence have previously been suggested to provide a mechanism that might explain these phenomena. In this paper, mode activity that changes at the toroidal velocity reversal, is identified as a new possible candidate. In the absence of an available model that can explain these basic phenomena, this paper presents observations and, where possible, scaling of the rotation profiles with some of the major plasma parameters such as current, density and shape to guide the development of a physics model for use in improving the extrapolation of the rotation amplitude and profiles to future devices.

The question of plasma rotation in toroidal devices is receiving increasing attention due to its impact on the stability of resistive wall modes, neoclassical tearing modes and microinstabilities causing anomalous transport. TCV provides unique insight into the mechanisms generating and regulating plasma rotation, and the related interplay between particle, energy and momentum transport, by exploring Ohmic and EC heated discharges with minimum external momentum injection. These investigations use the diagnostic neutral beam, which induces a small rotation (~1 km/s, compared to ~20 km/s in other machines), due to a combination of quasi-orthogonal injection angle, and a modest average deposited power (20 kW). Toroidal measurements of bulk plasma rotation in the TCV tokamak, in the absence of applied external torque, are described for limited and diverted plasma configurations. Core toroidal rotation is seen to scale inversely with plasma current (Scarabosio et al) and to exhibit a direction inversion with increasing plasma density in limited regimes (Bortolon et al).These apparent changes in the internal torque balance of the plasma are seen to inverse for diverted configurations with rotation at the plasma edge now able to vary with the plasma parameters in contrast to near zero values for limited discharges. This complicated behaviour indicates the presence of several competing torques on the plasma. The paper will include results on the poloidal velocity profiles to complete the rotation measurements and will describe the limitations of the neoclassical model to predict rotation profiles and their dynamics, providing a clear benchmark for the understanding of momentum transport in Tokamak plasmas. References[1] Scarabosio et al, PPCF 48, 663-683 (2006) [2] Bortolon et al, PRL 97, 235003 (2006)

Transport analyses using first-principle turbulence codes and 112-D transport codes usually study radial transport properties between the tokamak plasma magnetic axis and a normalized minor radius around 0.8. In this region, heat transport shows significantly stiff properties resulting in temperature scalelength values (R∕LT) that are relatively independent of the level of the radial heat flux. We have studied experimentally in the tokamak à configuration variable [F. Hofmann et al., Plasma Phys. Controlled Fusion 36, B277 (1994)] the radial electron transport properties of the edge region, close to the last closed flux surface, namely, between ρV=V/Vedge=0.8 to 1. It is shown that electron transport is not stiff in this region and high R∕LTe values (∼20–40) can be attained even for L-mode confinement. We can define a “pedestal” location, already in L-mode regimes, where the transport characteristics change from constant logarithmic gradient, inside ρV = 0.8, to constant gradient between 0.8 and 1.0. In particular, we demonstrate, with well resolved Te and ne profiles, that the confinement improvement with plasma current Ip, with or without auxiliary heating, is due to this non-stiff edge region. This new result is used to explain the significant confinement improvement observed with negative triangularity, which could not be explained by theory to date. Preliminary local gyrokinetic simulations are now consistent with an edge, less stiff, region that is more sensitive to triangularity than further inside. We also show that increasing the electron cyclotron heating power increases the edge temperature inverse scalelength, in contrast to the value in the main plasma region. The dependence of confinement on density in ohmic plasmas is also studied and brings new insight in the understanding of the transition between linear and saturated confinement regimes, as well as of the density limit and appearance of a 2/1 tearing mode. The results presented in this paper provide an important new perspective with regards to radial transport in tokamak plasmas which goes beyond L-mode plasmas and explains some previous puzzling results. It is proposed that understanding the transport properties in this edge non-stiff region will also help in understanding the improved and high confinement edge properties.

The first toroidal rotation measurements in TCV ohmic L-mode plasmas with no external momentum injection are presented. The toroidal velocity profile of the fully stripped carbon species is measured by active Charge eXchange Recombination Spectroscopy with a temporal resolution of typically 90 ms and a spatial resolution of 2.5 cm, about 1/10 of the plasma radius. The observed carbon velocity is of the order of the deuterium diamagnetic drift velocity and up to 1/5 of the deuterium thermal velocity. It is directed opposite to plasma current in the electron diamagnetic toroidal drift direction. The profile reverses when reversing the plasma current. The angular velocity profile is flat, or hollow, inside the sawtooth inversion radius and decreases quasi linearly towards the plasma edge. By vertically shifting the plasma magnetic axis within the TCV vessel the plasma edge velocity profile was measured with high resolution. Such experiments confirm that, close to the limiter, the stationary rotation velocity is close to zero or somewhat positive (co-current directed). This suggests that the angular momentum is not driven from the plasma edge. The maximum carbon velocity scales as v φ,Max [km s −1 ] = −12.5T i /I p [eV/kA] for a significant range of densities and values of the edge safety factor. Comparison with neoclassical predictions show that the TCV plasma rotation is mainly driven by radial electric fields, with a negligible contribution from the toroidal electric fields. The neoclassical theory of small toroidal rotation quantitatively and qualitatively disagrees with the experimental observation. An alternative empirical equation for the angular momentum flux, able to reproduce the measured stationary profile outside the inversion radius, is proposed.

The first experimental evidence of parallel momentum transport generated by the up-down asymmetry of a toroidal plasma is reported. The experiments, conducted in the Tokamak à Configuration Variable, were motivated by the recent theoretical discovery of ion-scale turbulent momentum transport induced by an up-down asymmetry in the magnetic equilibrium. The toroidal rotation gradient is observed to depend on the asymmetry in the outer part of the plasma leading to a variation of the central rotation by a factor of 1.5-2. The direction of the effect and its magnitude are in agreement with theoretical predictions for the eight possible combinations of plasma asymmetry, current, and magnetic field.

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The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device’s unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with electron-cyclotron resonance heating (ECRH) was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1 MW neutral beam injector has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and nearly non-inductive H-mode discharges sustained by electron cyclotron and neutral beam current drive. In the H-mode, the pedestal pressure increases modestly with nitrogen seeding while fueling moves the density pedestal outwards, but the plasma stored energy is largely uncorrelated to either seeding or fueling. High fueling at high triangularity is key to accessing the attractive small edge-localized mode (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The geodesic acoustic mode, possibly coupled with avalanche events, has been linked with particle flow to the wall in diverted plasmas. Detachment, scrape-off layer transport, and turbulence were studied in L- and H-modes in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power ‘starvation’ reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in the H-mode is obtained with impurity seeding and has shown little dependence on flux expansion in standard single-null geometry. In the attached L-mode phase, increasing the outer connection length reduces the in–out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variable-configuration baffles and possibly divertor pumping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5 MW ECRH and 1 MW neutral beam injection heating will be added.