The physics base for the ITER Physics Design Guidelines is reviewed in view of application to DEMO and areas are pointed out in which improvement is needed to arrive at a consistent set of DEMO Physics Design Guidelines. Amongst the proposed improvements, the area of power exhaust plays a crucial role since predictive capability of present-day models is low and this area is expected to play a major role in limiting DEMO designs due to the much larger value of Ptot/R in DEMO than in present-day devices and even ITER.

Analysis of MHD activity in pellet enhanced performance (PEP) pulses is used to determine the position of rational surfaces associated with the safety factor q. This gives evidence for negative shear in the central region of the plasma. The plasma equilibrium calculated from the measured q values yields a Shafranov shift in reasonable agreement with the experimental value of about 0.2 m. The corresponding current profile has two large off-axis maxima in agreement with the bootstrap current calculated from the electron temperature and density measurements. A transport simulation shows that the bootstrap current is driven by the steep density gradient, which results from improved confinement in the plasma core where the shear is negative. During the PEP phase (m,n)=(1,1) fast MHD events are correlated with collapses in the neutron rate. The dominant mode preceding these events usually is n=3, whereas the mode following them is dominantly n=2. Toroidal linear MHD stability calculations assuming a non-monotonic q-profile with an off-axis minimum decreasing from above 1 to below 1 describe this sequence of modes (n=3,1,2), but always give a larger growth rate for the n=1 mode than for the n=2 mode. This large growth rate is due to the high central poloidal beta of 1.5 observed in the PEP pulses. Finally, a rotating (m,n)=(1,1) mode is observed as a hot spot with a ballooning character on the low field side. The hot spot has some of the properties of a 'hot' island consistent with the presence of a region of negative shear

The geodesic acoustic mode (GAM) is a coherently oscillating zonal flow that may regulate turbulence in toroidal plasmas. Uniquely, the complete poloidal and toroidal structure of the magnetic component of the turbulence-driven GAM has been mapped in the TCV tokamak. Radially localized measurements of the fluctuating density, ECE radiative temperature and poloidal flow show that the GAM is a fully coherent, radially propagating wave. These observations are consistent with electrostatic, gyrokinetic simulations.

The achievable efficiency for external current drive through electron-cyclotron waves in a demonstration tokamak reactor is investigated. Two possible reactor designs, one for steady state and one for pulsed operation, are considered. Beam propagation, absorption and current drive are modelled employing the beam tracing technique and including momentum conservation in electron-electron collisions. It is found that for midplane injection the achievable current drive efficiency is limited by second-harmonic absorption at levels consistent with previous studies. Higher efficiencies can be achieved by injecting the beams from the top of the machine, exploiting wave absorption by more energetic (less collisional) electrons. Current drive efficiencies competitive with those usually obtained by neutral beam current drive are reported. These optimum efficiencies are found for frequencies around 230 GHz and 290 GHz for the steady-state and the pulsed DEMO, supposed to operate at a magnetic field B = 5.84 T and B = 7.45 T, respectively.

Here we present the first measurements by collective Thomson scattering of the evolution of fast-ion populations in a magnetically confined fusion plasma. 150 kW and 110 Ghz radiation from a gyrotron were scattered in the TEXTOR tokamak plasma with energetic ions generated by neutral beam injection and ion cyclotron resonance heating. The temporal behavior of the spatially resolved fast-ion velocity distribution is inferred from the received scattered radiation. The fast-ion dynamics at sawteeth and the slowdown after switch off of auxiliary heating is resolved in time. The latter is shown to be in close agreement with modeling results.

The effects of plasma shaping, in particular triangularity (δ), on temperature fluctuations in the tokamak à configuration variable have been studied using the correlation electron cyclotron emission system. It has been found that, in ohmic discharges with comparable density profiles, the relative fluctuation level measured at the edge is significantly reduced in plasmas with negative triangularity with respect to positive triangularity ones. Additionally, the critical temperature gradients for the onset of turbulence are observed to increase in the negative triangularity plasmas. An estimation of the correlation length of the fluctuating structure shows smaller structure size in the negative triangularity cases, which are known to be associated with improved confinement. Together, these observations suggest that changing triangularity from positive to negative strongly influences the nature of the turbulent fluctuations excited in the plasma.

Routine reaction to approaching disruptions in tokamaks is currently largely limited to machine protection by mitigating an ongoing disruption, which remains a basic requirement for ITER and DEMO [1]. Nevertheless, a mitigated disruption still generates stress to device. Additionally, in future fusion devices, high-performance discharge time itself will be very valuable. Instead of reacting only on generic features, occurring shortly before the disruption, the ultimate goal is to actively avoid approaching disruptions at an early stage, sustain the discharges whenever possible and restrict mitigated disruptions to major failures. Knowledge of the most relevant root causes and the corresponding chain of events leading to disruption, the disruption path, is a prerequisite. For each disruption path, physics-based sensors and adequate actuators must be defined and their limitations considered. Early reaction facilitates the efficiency of the actuators and enhances the probability of a full recovery. Thus, sensors that detect potential disruptions in time are to be identified. Once the entrance into a disruption path is detected, we propose a hierarchy of actions consisting of (I) recovery of the discharge to full performance or at least continuation with a less disruption-prone backup scenario, (II) complete avoidance of the disruption to sustain the discharge or at least delay it for a controlled termination and, (III), only as last resort, a disruption mitigation. Based on the understanding of disruption paths, a hierarchical and path-specific handling strategy must be developed. Such schemes, testable in present devices, could serve as guideline for ITER and DEMO operation. For some disruption paths, experiments have been performed at ASDEX Upgrade and TCV. Disruptions were provoked in TCV by impurity injection into ELMy H-mode discharges and in ASDEX Upgrade by forcing a density limit in H-mode discharges. The new approach proposed in this paper is discussed for these cases. For the H-mode density limit sensors used so far react too late. Thus a plasma-state boundary is proposed, that can serve as an adequate early sensor for avoiding density limit disruptions in H-modes and for recovery to full performance.

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.