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A 693 GHz, eight-channel, poloidal high-k (k refers to wavenumber) collective scattering system is under development for the National Spherical Torus Experiment-Upgrade device. It will replace the previous 280 GHz, five-channel, tangential scattering system to study high-k electron density fluctuations, thereby providing a measurement of the kθ-spectrum of both electron temperature gradient and ion temperature gradient modes. A tool is under development to calculate the wavenumber that exists in the presence of strong magnetic pitch angles. We use this tool to motivate a new receiver optical design for significantly improved performance, details of which are presented herein.

The mission of the low aspect ratio spherical tokamak NSTX-U is to advance the physics basis and technical solutions required for optimizing the configuration of next-step steady-state tokamak fusion devices. NSTX-U will ultimately operate at up to 2 MA of plasma current and 1 T toroidal field on axis for 5 seconds, and has available up to 15 MW of Neutral Beam Injection (NBI) power at different tangency radii and 6 MW of High Harmonic Fast Wave (HHFW) heating. With these capabilities NSTX-U will develop the physics understanding and control tools to ramp-up and sustain high performance fully non-inductive plasmas with large bootstrap fraction and enhanced confinement enabled via the low aspect ratio, high beta configuration. With its unique capabilities, NSTX-U research also supports ITER and other critical fusion development needs. Super-Alfvénic ions in beam-heated NSTX-U plasmas access energetic particle parameter space that is relevant for both -heated conventional and low aspect ratio burning plasmas. NSTX-U can also generate very large target heat fluxes to test conventional and innovative plasma exhaust and plasma facing component (PFC) solutions. This paper summarizes recent analysis, theory and modelling progress to advance the tokamak physics basis in the areas of macrostability and 3D fields, energetic particle stability and fast ion transport, thermal transport and pedestal structure, boundary and plasma material interaction, RF heating, scenario optimization and real-time control.

An 8-channel, high

To shed some light on the development of the fast m/n = 1/1 precursor to the sawtooth crash and its influence on plasma transport properties in the vicinity of the q = 1 surface, series of dedicated experiments have been conducted on the Tore Supra and TEXTOR tokamaks. It has been concluded that, before a crash, the hot core gets displaced with respect to the magnetic axis, drifts outwards by as much as 8–10 cm and may change its shape. Observation of the magnetic reconnection process has been made by means of electron cyclotron emission diagnostics. The heat pulse is seen far outside the inversion radius. The colder plasma develops a magnetic island on the former magnetic axis, after the hot core expulsion. Different kinds of behaviour of the m = 1 precursor before the crash, with respect to the displacement of the hot core and the duration of the oscillating phase, have been observed. An ideal kink model alone cannot be used for explanation; therefore, resistive effects play an important role in the mode development. Possible mechanisms that lead an m = 1 mode to such behaviour, and their links to the change in the central q-profile, are discussed. Results have been discussed in the light of various theoretical models of the sawtooth.

Enhanced physical understanding of the nature of the Edge Localized Mode (ELM) instability in the H-mode regime of toroidal plasmas requires advanced diagnostic tools to visualize their dynamics. A high spatial and temporal resolution 2-D Electron Cyclotron Emission Imaging (ECEI) system [1] was utilized successfully characterize the real time dynamics of ELMs [2] including the growth, saturation and bursting process of this instability during two campaigns (2010 and 2011) of the Korean Superconducting Tokamak Advanced Research (KSTAR) device. The measured ELMs are validated with the synthetic images based on numerical simulations.

A simple device has been developed to deposit elemental lithium onto plasma facing components in the National Spherical Torus Experiment. Deposition is accomplished by dropping lithium powder into the plasma column. Once introduced, lithium particles quickly become entrained in scrape-off layer flow as an evaporating aerosol. Particles are delivered through a small central aperture in a computer-controlled resonating piezoelectric disk on which the powder is supported. The device has been used to deposit lithium both during discharges as well as prior to plasma breakdown. Clear improvements to plasma performance have been demonstrated. The use of this apparatus provides flexibility in the amount and timing of lithium deposition and, therefore, may benefit future fusion research devices.

Electron gyroscale fluctuation measurements in National Spherical Torus Experiment H-mode plasmas with large toroidal rotation reveal fluctuations consistent with electron temperature gradient (ETG) turbulence. Large toroidal rotation in National Spherical Torus Experiment plasmas with neutral beam injection generates ExB flow shear rates comparable to ETG linear growth rates. Enhanced fluctuations occur when the electron temperature gradient is marginally stable with respect to the ETG linear critical gradient. Fluctuation amplitudes decrease when the ExB flow shear rate exceeds ETG linear growth rates. The observations indicate that ExB flow shear can be an effective suppression mechanism for ETG turbulence.

Observations of magneto-hydro-dynamic (MHD) induced redistribution or loss of energetic ions measured using the vertically scanning capability of the Neutral Particle Analyzer diagnostic on the National Spherical Torus Experiment (NSTX) are presented along with TRANSP and ORBIT code analysis of the results. Although redistribution or loss of energetic ions due to bursting fishbone-like and low-frequency (f ~ 10 kHz) kinktype MHD activity has been reported previously, the primary goal of this work is to study redistribution or loss due to continuous Alfvenic (f ~ 20 – 150 kHz) modes, a topic that heretofore has not been investigated in detail for NSTX plasmas. Initial indications are that the former drive energetic ion loss whereas the continuous Alfvenic modes only cause redistribution and the energetic ions remain confined.

The detection and control of instabilities in a tokamak is one of the exciting challenges in fusion research on the way to a reactor. Thanks to a combination of an innovative 2D temperature imaging technique (ECEI), a versatile ECRH/ECCD system and a unique possibility to externally induce tearing modes in the plasma, TEXTOR is able to make pioneering contributions in this field. This paper focuses on two meso-scale phenomena in tokamaks: m = 2 tearing modes and magnetic structures in the stochastic boundary. In these cases the 2D-ECEI diagnostic can resolve features not attainable before. In addition the possibility to use the diagnostic for fluctuation measurements is addressed.

Small-scale structures with high poloidal mode numbers (m=10-20) have been observed in the TEXTOR tokamak plasma with pulsed radar reflectometry and an electron cyclotron emission diagnostic, in conjunction with large 2/1 and 1/1 islands. The small islands have a peaked density profile, similar to that of the simultaneously observed large-scale 2/1 islands. This together with the observation that high-frequency density and temperature fluctuations are very pronounced near the X points of the large islands hints to a strongly perturbed magnetic topology around the X points.

Despite suppression of {rho}{sub i}-scale turbulent fluctuations, electron thermal transport remains anomalous in NSTX. For this reason, a microwave scattering system will be deployed to directly observe the w and k spectra of {rho}{sub e}-scale turbulent fluctuations and characterize the effect on electron thermal transport. The scattering system will employ a Gaussian probe beam produced by a high power 280 GHz microwave source. A five-channel heterodyne detection system will measure radial turbulent spectra in the range |k{sub r}| = 0-20 cm{sup -1}. Inboard and outboard launch configurations cover most of the normalized minor radius. Improved spatial localization of measurements is achieved with low aspect ratio and high magnetic shear configurations. This paper will address the global design of the scattering system, such as choice of frequency, size, launching system, and detection system.

To date, reliable measurements of electron temperature fluctuations in major tokamak experiments have remained elusive. Since electron heat transport in tokamaks is anomalously large, and appears to be driven by turbulent fluctuations, measuring spatially resolved electron and density temperature fluctuations is key to understanding both transport and the resultant turbulent confinement. Reflectometry has shown itself to be extremely sensitive to density fluctuations, and is a proven technique for making accurate electron density profiles. Electron cyclotron emission (ECE) imaging is a method whereby 2-D images of large amplitude coherent temperature fluctuations as well as 2-D electron temperature profiles of tokamak plasmas may be obtained. In this paper, we discuss the present status of each of these UC Davis diagnostic research areas, with particular emphasis on how the diagnostics may best be implemented on ITER.

An eight channel electron cyclotron emission imaging (ECEI) diagnostic system is being developed for use on the helically symmetric experiment (HSX) stellarator. The system utilizes a linear Schottky diode mixer/receiver array, coupled with low cost conventional ECE radiometer IF boards under development at U.C. Davis, to measure the second harmonic x-mode radiation from the plasma. The array is fed with a fixed local oscillator, with single sideband operation enabled via a dichroic plate high pass filter whose cutoff frequency equals that of the local oscillator. A multithrow, high speed (

One of the problems encountered in the application of reflectometry to fusion plasmas is that the transmit antenna must be aligned such that the reflected wave form is successfully collected by a suitably arranged receive antenna. This task is made even more difficult in the case of shaped plasmas, or when plasma motion is such that the incident wave is no longer normal to the plasma surface. One would ideally like to have antennas which may be steered in real time to compensate for changes in plasma shape and/or position. A program has been initiated at U.C. Davis to address this problem, and microwave beam steerers suitable for electronically scanned reflectometers have been developed. The approach taken has been to develop phased antenna array (PAA) concepts which operate over broad bandwidths. Laboratory test results of a proof-of-principle PAA system will be presented, along with a description of ongoing research activities by U.C. Davis in this field.

Moderate pulse reflectometry requires the efficient (low loss) gating/switching (∼100–300 ps rise times) of millimeter wave pulses with a high on/off contrast ratio. To satisfy this need, high speed switches utilizing beam control array technology are under development at U.C. Davis. A quasioptical planar grid switch array comprised of hundreds of Schottky varactor diodes embedded within an overmoded waveguide, is capable of generating the requisite 100–300 ps switching times with Watt-level power handling capabilities. A proof-of-principle, high speed V-band switch array has been constructed with excellent agreement between theory and experiment. Less than 1.6 dB insertion loss and >16 dB on/off contrast ratio are measured at 61.5 GHz, with rise and fall times of 7 GHz (contrast ratio >15 dB) has also been measured. A discussion will also be presented on designs at frequencies up to 170 GHz.

Monolithic nonlinear transmission lines can be employed as ultrashort pulse (1–5 ps rise/fall time) generators to extend ultrashort pulse reflectometry (USPR) to high field tokamaks such as TFTR, Alcator C-MOD, and ITER. Multiple barrier varactor devices have been successfully employed as high power and high speed devices in this application. Monolithic NLTLs can also be employed to simplify moderate pulse reflectometry systems, in which NLTLs may be employed as broadband frequency multipliers. Theoretical and experimental investigations will be presented.

Strong damping (depletion) of the driver wave (electron plasma wave) after electron acceleration has been observed in the {ital v}{sub {ital p}}{times}B acceleration scheme in microwave-plasma interaction experiments. This depletion can be controlled by changing the rise time of the rf pump wave. Time- and space-resolved observations of high-energy electrons reveal the possible existence of different acceleration mechanisms near the resonance absorption layer.