[en] The spectrum of turbulent density fluctuations at long poloidal wavelengths in the edge plasma of the DIII-D tokamak peaks at nonzero radial wave number. The associated electric-potential fluctuations cause sheared tilde ExB flows primarily in the poloidal direction. These zonal flows have been predicted by theory and are believed to regulate the overall level of turbulence and anomalous transport. This study provides the first indirect experimental identification of zonal flows

[en] The Advanced Tokamak scenario, one of the modes of operation being considered for ITER, relies on the attainment of a high bootstrap current fraction. These scenarios are typically characterized by an internal transport barrier (ITB). An internal feedback loop then governs the current profile, which strongly affects the confinement and thus the properties and location of the high-gradient region, where in turn the bootstrap current component is localized. The bootstrap current fraction can reach 100% only if the bootstrap current profile can be exactly and stably aligned with the high-gradient region it engenders. Recent work on the TCV tokamak has shown that such an alignment is indeed possible. We have produced discharges in which the current is entirely self-generated by the plasma in conditions of intense electron cyclotron resonance heating (ECRH, up to 2.7 MW), by employing two different methods. In one scenario, high-power, second-harmonic ECRH waves are launched with no current drive component, during the initial plasma current ramp-up. A strong ITB is generated by the transient negative central magnetic shear that develops in the current penetration phase. The Ohmic flux swing is zeroed immediately after breakdown, cutting off the external plasma current source. The plasma can then evolve spontaneously towards a stationary and quiescent state, characterized by a narrow ITB with a confinement enhancement of 2.5 - 3 over L-mode. The discharge remains stationary over the time scale of a TCV pulse (1 - 2 s), which is significantly longer than a typical resistive current redistribution time (∼ 150 - 300 ms) and orders of magnitude longer than the confinement time (∼ 3 - 6 ms). Following a different approach, we have also succeeded in achieving a 100% bootstrap fraction by annulling the total EC-driven current in pre-existing stationary conditions. Standard stationary non-inductive eITBs are first generated by off-axis co-ECCD; counter-ECCD is then added gradually until the total driven current density is nominally zero everywhere. In some cases a quasi-stationary state is indeed established. In this case the barrier remains broad with a standard confinement enhancement factor of the order of 4 - 5. As a result, the discharge is not truly quiescent, because of significant MHD activity causing oscillations and jumps both in the confinement and in the total current. (author)

[en] A novel CO₂ laser phase contrast imaging diagnostic has been developed for the DIII-D tokamak, where it is being employed to investigate density fluctuations at the outer edge of the plasma. This system generates 16-point, 1-D images of a 7.6 cm wide region in the radial direction, and is characterized by long wavelength (7.6 cm) and high frequency (100 MHz) capability, as well as excellent sensitivity (rvec n approx-gt 10⁹ cm^-3). The effects of vertical line integration have been studied in detail, both analytically and numerically with actual flux surface geometries generated by the EFITD magnetic equilibrium code. It is shown that in the present configuration the measurement is mostly sensitive to radial wave vectors. Experimental results on fluctuation suppression at the L- to H-mode transition and on the L-mode wave number spectrum are discussed briefly. Finally, future plans for extending the measurement to the core of the plasma and for investigating externally launched fast waves are presented

[en] Electron cyclotron current drive (ECCD) is an important prospective tool for tailoring the current profile in next-step devices. To fill the remaining gaps between ECCD theory and experiment, especially in the efficiency and localisation of current drive, a better understanding of the physics of suprathermal electrons appears necessary. On TCV, the fast electron population is diagnosed by a multichordal, spectrometric hard X-ray camera and by a high field side ECE radiometer. The main modeling tool is the quasi-linear Fokker-Planck code CQL3D, which is equipped with a radial particle transport model. Systematic studies of fast electron dynamics have been performed in TCV by square-wave modulation of the electron cyclotron power, followed by coherent averaging, in an attempt to identify the roles of collisional relaxation and radial diffusion in the dynamics of the suprathermal population. The temporal evolution on turn-on and turn-off is found to involve multiple and disparate time scales. The role of radial diffusion has been evidenced by CQL3D modeling, which is able to reproduce the experimental current drive efficiency only if a modest diffusivity is included. (author)

[en] An extensive study of digital pulse processing methods is presented. Existing methods, both traditional and more recent, are compared with original advanced techniques within an appropriate modeling and benchmarking framework. This comprehensive approach ensures general applicability to the broad field of pulse processing, even though the focus lies on hard X-ray spectrometers operated at high count rates. In this regime, pile-up is the main issue and the individual pulse shape characteristics play a minor role, although they remain important for the algorithm parameter optimization. The digital implementation of double-differentiating analog filters and trapezoidal FIR filter methods results in excellent performance that is second only to that of optimum digital FIR filters. Several more complex methods involving increased computational effort are found not to meet the expectations. -- Highlights: • Digital pulse processing methods are studied using a general benchmarking framework. • The results are generally applicable in the field of pulse processing. • The focus lies on hard X-ray spectrometers operated at high count rates. • The best performance is obtained with optimum finite impulse response (FIR) filters. • Second are double-differentiating analog and trapezoidal FIR filter methods

[en] Measurements of electron cyclotron emission from the high field side of the TCV tokamak have been made on plasmas heated by second and third harmonic X-mode Electron Cyclotron Heating (ECH) and Electron Cyclotron Current Drive (ECCD). Suprathermal Electron Cyclotron Emission (ECE), up to a factor of 6 in excess of thermal emission, is detected in the presence of second harmonic X-mode (X2) ECCD and of third harmonic X-mode (X3) ECH. The measured ECE spectra are modelled using a bi-Maxwellian describing the bulk and the suprathermal electron populations. Suprathermal temperatures between 10-50keV and densities in the range 1. 10¹7 -6. 10¹8m^-3 are obtained, and correspond to 3 -15 bulk temperatures and 1% -20% bulk densities. Good agreement between ECE suprathermal temperatures and energetic photon temperatures, measured by a hard X-ray camera, is found. For optically thin X3 Low Field Side (LFS) injection in presence of X2 CO-ECCD, the suprathermal population partly explains the discrepancy between global and first pass absorption measurements. (author)

[en] Full text: The Tokamak configuration variable (TCV) is equipped with very high electron cyclotron resonance heating (ECRH) power in relation to its plasma volume. Therefore a significant suprathermal electron population is produced. Additionally, MHD events, like magnetic reconnection during sawtooth crashes, also accelerate electrons within a small time scale to very high energies. Collisions of these suprathermal electrons with ions in the plasma lead to hard X-ray Bremsstrahlung emission in the region of 5 to several hundred keV. This radiation is measured by two diagnostics based on CdTe detectors, the Hard X-ray Tomographic Spectrometer (HXRS) and the Tangential X-ray Detector Array (TXDA). The HXRS, currently under design, will ultimately consist of several cameras, each with 24 lines of sight in a fan design. This will enable a tomographic inversion in the poloidal plane to determine the emission locally on a square pixel grid. The spectroscopic grade CdTe detectors and fast electronics provide a sufficient energy resolution to permit quantitative analysis of the suprathermal electron population. The TXDA, on the other hand, has no energy resolution because its detectors are operated in current mode to detect fast events like sawtooth crashes at a typical acquisition frequency of 250 kHz. Due to its tangential view with respect to the plasma current the diagnostic benefits from the increased emission in the forward cone of the electrons. The lines of sight can be moved to different positions depending on the area of interest. Additionally two HXRS cameras will also be rotatable to enable spatial and energy resolution by tangential lines of sight in both the co- and counter-current directions. Finally the Photo Multiplier Tube for hard-X rays (PMTX) diagnostic, by means of a scintillator, monitors the runaway electron population and provides a useful complement to the other diagnostics at the high end of the spectrum. (author)

[en] Full text: Electron internal transport barriers (eITBs) are generated in the TCV tokamak with strong electron cyclotron resonance heating (ECRH) in a variety of conditions, ranging from steady-state fully non-inductive scenarios to stationary discharges with a finite inductive component, and finally to transient current ramps without current drive. The confinement improvement over L-mode ranges from 3 to 6; the bootstrap current fraction is invariably large and is above 70% in the highest confinement cases, with good current-profile alignment permitting the attainment of steady state. Barriers are observed both in the electron temperature and density profiles, with a strong correlation both in location and in steepness; in particular, the density gradient being one-half as steep as the temperature gradient suggests a possible transition from an anomalous to a neoclassical regime. The dominant role of the current profile in the formation and properties of eITBs has been conclusively proven in a TCV experiment exploiting the large current-drive efficiency of the Ohmic transformer: small current perturbations accompanied by negligible energy transfer dramatically alter the confinement. The crucial element in the formation of the barrier is the appearance of a central region of negative magnetic shear, with the barrier strength improving with increasingly steep shear. This connection has also been corroborated by transport modeling assisted by gyro-fluid simulations. Rational safety-factor (q) values do not appear to play a role in the barrier formation, at least in the q range 1.3-2.3, as evidenced by the smooth dependence of the confinement enhancement on the loop voltage over a broad eITB database. MHD mode activity is however influenced by rational q values and results in a complex, sometimes cyclic, dynamical evolution. (author)

[en] This report describes the development of a CO₂ laser interferometer system, the engineering, design and installation of the hardware, and the selection of materials specific to the requirements of a CO₂ laser diagnostic. A brief description of system operation is included. A phase contrast interferometer diagnostic has been designed and installed on the DIII-D tokamak to enhance studies of the physical characteristics of plasma turbulence, and specifically to analyze plasma density fluctuations in the boundary region of the plasma. A 20 watt CO₂ laser beam, operating at the 10.6 micron wavelength, is expanded to a diameter of 76 mm and directed through a series of mirrors which provide for entry of the beam into the vessel at a point 70 cm above the midplane at the 285 degree toroidal location. After being reflected from a mirror inside the vessel, the beam is directed downward so that it passes through the edge of the plasma immediately in front of a four-strap fast wave current drive rf antenna. The laser beam is then reflected by a second internal mirror and exits the vessel 70 cm below the midplane (also at 285 degrees) returning to an optical table through a final series of external steering mirrors

[en] Highlights: • Algorithm vertical stabilisation using a linear parametrisation of the current density. • Experimentally derived model of the vacuum vessel to account for vessel currents. • Real-time contouring algorithm for flux surface averaged 1.5 D transport equations. • Full real time implementation coded in SIMULINK runs in less than 200 μs. • Applications: shape control, safety factor profile control, coupling with RAPTOR. - Abstract: Equilibrium reconstruction consists in identifying, from experimental measurements, a distribution of the plasma current density that satisfies the pressure balance constraint. The LIUQE code adopts a computationally efficient method to solve this problem, based on an iterative solution of the Poisson equation coupled with a linear parametrisation of the plasma current density. This algorithm is unstable against vertical gross motion of the plasma column for elongated shapes and its application to highly shaped plasmas on TCV requires a particular treatment of this instability. TCV's continuous vacuum vessel has a low resistance designed to enhance passive stabilisation of the vertical position. The eddy currents in the vacuum vessel have a sizeable influence on the equilibrium reconstruction and must be taken into account. A real time version of LIUQE has been implemented on TCV's distributed digital control system with a cycle time shorter than 200 μs for a full spatial grid of 28 by 65, using all 133 experimental measurements and including the flux surface average of quantities necessary for the real time solution of 1.5 D transport equations. This performance was achieved through a thoughtful choice of numerical methods and code optimisation techniques at every step of the algorithm, and was coded in MATLAB and SIMULINK for the off-line and real time version respectively