A high-density and high-confinement tokamak plasma regime for fusion energy.

The tokamak approach, utilizing a toroidal magnetic field configuration to confine a hot plasma, is one of the most promising designs for developing reactors that can exploit nuclear fusion to generate electrical energy1,2. To reach the goal of an economical reactor, most tokamak reactor designs3-10 simultaneously require reaching a plasma line-averaged density above an empirical limit-the so-called Greenwald density11-and attaining an energy confinement quality better than the standard high-confinement mode12,13. However, such an operating regime has never been verified in experiments. In addition, a long-standing challenge in the high-confinement mode has been the compatibility between a high-performance core and avoiding large, transient edge perturbations that can cause very high heat loads on the plasma-facing-components in tokamaks. Here we report the demonstration of stable tokamak plasmas with a line-averaged density approximately 20% above the Greenwald density and an energy confinement quality of approximately 50% better than the standard high-confinement mode, which was realized by taking advantage of the enhanced suppression of turbulent transport granted by high density-gradients in the high-poloidal-beta scenario14,15. Furthermore, our experimental results show an integration of very low edge transient perturbations with the high normalized density and confinement core. The operating regime we report supports some critical requirements in many fusion reactor designs all over the world and opens a potential avenue to an operating point for producing economically attractive fusion energy.

Spectrally resolved polarization angle across the motional Stark effect spectrum.

A new diagnostic technique has been developed that couples a spectrometer and an image-intensified camera into the traditional motional Stark effect (MSE) system on DIII-D. The image-intensified camera syncs with the photo-elastic modulators to spectrally resolve the Stokes parameters across the Stark multiplet. Polarization dependent phase shift, likely from a plasma facing mirror, leads to the spectropolarimeter measuring a variation in the polarization angle across the MSE spectrum of ∼8°.

Four-dimensional calibration turntable of the motional Stark effect diagnostic on EAST.

The motional Stark effect (MSE) diagnostic is applied to measure the safety factor q and current density profile of a tokamak device, which are important parameters in realizing the high-performance and long-pulse steady state of a tokamak. A single-channel MSE diagnostic based on dual photoelastic modulators, whose sightline meets with the neutral beam injection at a major radius of R = 2.12 m, has been built for the D window of the Experimental Advanced Superconducting Tokamak (EAST). According to the requirements of MSE diagnostic polarimetric calibration, a high-precision four-dimensional calibration turntable, driven by four stepping motors and controlled by software running on the computer, was designed for EAST. The turntable allows us to rapidly calibrate the MSE diagnostic in a series of positions and angles during EAST maintenance. The turntable can move in four dimensions of translation, yaw, pitch, and roll of the polarizer and can create linearly polarized light at any given angle with accuracy of ∼0.05° for the MSE system offline calibration. The experimental results of the MSE diagnostic calibration in the laboratory show that the turntable has the advantages of high positioning accuracy, flexible spatial movement, and convenient control and fully meets the calibration requirements of an MSE diagnosis system.

Asymmetries in the motional Stark effect emission on the DIII-D tokamak.

Spectrometer measurements and filter upgrades to a motional Stark effect polarimeter measuring the outer half-radius of the DIII-D tokamak helped to identify asymmetries in the polarization angle of Stark-split emission. The measured polarization angle of the π components differs and is not orthogonal to the σ component. These differences persist over a range of densities and with low levels of background light. It is suggested that the difference in the polarization angle between components is from a change in the ellipticity of the emitted light across the Stark components coupled with imperfect polarization preservation from an in-vessel mirror.

Pedestal magnetic field measurements using a motional Stark effect polarimeter.

Temperature-controlled, 0.15 nm interference filters were installed on an edge-viewing system of the motional Stark effect (MSE) polarimeter on the DIII-D tokamak. The upgraded system provides a factor of two reduction in the bandpass compared to the previous design, and linear control of the bandpass, which is unaltered by wavelength tuning. With the new system, there is a reduced dependence of the inferred polarization angle on the filter wavelength calibration. Recent measurements from the calibrated edge-viewing system show increased agreement with other MSE arrays.

Magnetohydrodynamic interference with the edge pedestal motional Stark effect diagnostic on DIII-D.

Accurate measurement of internal magnetic field direction using motional Stark effect (MSE) polarimetry in the edge pedestal is desired for nearly all tokamak scenario work. A newly installed 500 kHz 32-channel digitizer on the MSE diagnostic of DIII-D allows full spectral information of the polarimeter signal to be recovered for the first time. Fourier analysis of this data has revealed magnetohydrodynamic (MHD) fluctuations in the plasma edge pedestal at ρ ≥ 0.92. By correlating edge localized mode fluctuations seen on lock-in amplifier outputs with MSE spectrograms, it has been shown that edge pedestal tearing mode fluctuations cause interference with MSE second harmonic instrument frequencies. This interference results in unrecoverable errors in the real-time polarization angle measurement that are more than an order of magnitude larger than typical polarimeter uncertainties. These errors can cause as much as a 38% difference in local q. By using a redundant measure of the linear polarization found at the fourth harmonic photo-elastic modulator (PEM) frequency, MHD interference can be avoided. However, because of poorer signal-to-noise the fourth harmonic signal computed polarization angle shows no improvement over the MHD polluted second harmonics. MHD interference could be avoided in future edge pedestal tokamak polarimeters by utilizing PEMs with higher fundamental frequencies and a greater separation between their frequencies.

Measurements of the internal magnetic field using the B-Stark motional Stark effect diagnostic on DIII-D (inivited).

Results are presented from the B-Stark diagnostic installed on the DIII-D tokamak. This diagnostic provides measurements of the magnitude and direction of the internal magnetic field. The B-Stark system is a version of a motional Stark effect (MSE) diagnostic based on the relative line intensities and spacing of the Stark split D(α) emission from injected neutral beams. This technique may have advantages over MSE polarimetry based diagnostics in future devices, such as the ITER. The B-Stark diagnostic technique and calibration procedures are discussed. The system is shown to provide accurate measurements of B(θ)/B(T) and ∣B∣ over a range of plasma conditions. Measurements have been made with toroidal fields in the range of 1.2-2.1 T, plasma currents in the range 0.5-2.0 MA, densities between 1.7 and 9.0×10(19) m(-3), and neutral beam voltages between 50 and 81 keV. The viewing direction and polarization dependent transmission properties of the collection optics are found using an in situ beam into gas calibration. These results are compared to values found from plasma equilibrium reconstructions and the MSE polarimetry system on DIII-D.

A digital lock-in upgrade of the motional Stark effect diagnostic on DIII-D.

The use of lock-in amplifiers for phase sensitive detection of motional Stark effect (MSE) diagnostic signals is of critical importance to real-time internal current profile measurements in tokamak plasmas. A digital lock-in (DLI) upgrade utilizing field programable gate array firmware has been installed on the MSE system of the DIII-D tokamak for the eventual replacement of largely obsolete analog units. While the new digital system has shown a small reduction in electronic noise over the analog, the main advantages are reduced cost, hardware simplicity, compact size, and phase tracking during plasma operations. DLI recovery of MSE polarization angles was accomplished through use of reference processing to produce only photoelastic modulator (PEM) second harmonic frequencies and electronic signal processing to maximize the fidelity of the recovered signal. A simplified discrete analytical solution was found that accurately describes the new DLI hardware. The DLI algorithm was found to cause a prohibitively large oscillating artifact atop the demodulated signal. The artifact was caused by the accumulator interval not containing an exact integer number of PEM multiplier periods. Successful MSE measurements require the minimization of this oscillating artifact amplitude. The analytical solution was used to select an appropriate accumulator interval that both reduces the artifact and maintains the greatest temporal resolution possible. Sample EFIT equilibria reconstructions and corresponding safety factor profiles showed very close agreement between the analog and digital lock-ins.

Magnetic-flux pumping in high-performance, stationary plasmas with tearing modes.

Analysis of the change in the magnetic field pitch angles during edge localized mode events in high performance, stationary plasmas on the DIII-D tokamak shows rapid (<1 ms) broadening of the current density profile, but only when a m/n=3/2 tearing mode is present. This observation of poloidal magnetic-flux pumping explains an important feature of this scenario, which is the anomalous broadening of the current density profile that beneficially maintains the safety factor above unity and forestalls the sawtooth instability.

Measurements of the internal magnetic field on DIII-D using intensity and spacing of the motional Stark multiplet.

We describe a version of a motional Stark effect (MSE) diagnostic based on the relative line intensities and spacing of Stark split D(alpha) emission from the neutral beams. This system, named B-Stark, has been recently installed on the DIII-D tokamak. To find the magnetic pitch angle, we use the ratio of the intensities of the pi(3) and sigma(1) lines. These lines originate from the same upper level and so are not dependent on the level populations. In future devices, such as ITER, this technique may have advantages over diagnostics based on MSE polarimetry. We have done an optimization of the viewing direction for the available ports on DIII-D to choose the installation location. With this placement, we have a near optimal viewing angle of 59.6 degrees from the vertical direction. All hardware has been installed for one chord, and we have been routinely taking data since January 2007. We fit the spectra using a simple Stark model in which the upper level populations of the D(alpha) transition are treated as free variables. The magnitude and direction of the magnetic field obtained using this diagnostic technique compare well with measurements from MSE polarimetry and EFIT.

Overview of equilibrium reconstruction on DIII-D using new measurements from an expanded motional Stark effect diagnostic.

Motional Stark effect (MSE) measurements constrain equilibrium reconstruction of DIII-D tokamak plasmas using the equilibrium code EFIT. In 2007, two new MSE arrays were brought online, bringing the system to three core arrays, two edge arrays, and 64 total channels. We present the first EFIT reconstructions using this expanded system. Safety factor and E(R) profiles produced by fitting to data from the two new arrays and one of the other three agree well with independent measurements. Comparison of the data from the three arrays that view the core shows that one of the older arrays is inconsistent with the other two unless the measured calibration factors for this array are adjusted. The required adjustments depend on the toroidal field and plasma current direction, and on still other uncertain factors that change as the plasma evolves. We discuss possible sources of calibration error for this array.

Optimization of the optical design of the ITER MSE diagnostic.

The motional Stark effect (MSE) diagnostic will be essential for the study of advanced scenarios on ITER and its design is currently underway with initial emphasis on the optical design. Optical performance, as measured by photon throughput and minimization of polarization aberrations, will be critical to the success of the diagnostic. Consequently, the initial design work has been focused heavily on this area. In order meet the ITER MSE diagnostic design requirements, two approaches for the measurement are under consideration. The first is based on standard polarimeter techniques to measure the polarization of the emitted light, whereas the second measures the Stark splitting from which absolute value(B) can be inferred, where absolute value(B) is the magnitude of the total magnetic field. The base line design of the optical system is centered on the first approach. Emphasis in this case is placed on minimizing the polarization aberrations of the optical relay system. Motivation for the second method results from concern that the optical properties of the plasma-facing mirror, particularly its diattenuation and retardance, will degrade with plasma exposure. The second approach, while less sensitive to aberrations induced by plasma exposure effects, requires greater optical throughput in order to measure the complete Stark spectrum. We have developed an optimized optical design applicable to both measurement techniques. A summary of the design is presented and design issues are discussed.

Intense geodesic acousticlike modes driven by suprathermal ions in a tokamak plasma.

Intense axisymmetric oscillations driven by suprathermal ions injected in the direction counter to the toroidal plasma current are observed in the DIII-D tokamak. The modes appear at nearly half the ideal geodesic acoustic mode frequency, in plasmas with comparable electron and ion temperatures and elevated magnetic safety factor (q_{min}>or=2). Strong bursting and frequency chirping are observed, concomitant with large (10%-15%) drops in the neutron emission. Large electron density fluctuations (n[over ]_{e}/n_{e} approximately 1.5%) are observed with no detectable electron temperature fluctuations, confirming a dominant compressional contribution to the pressure perturbation as predicted by kinetic theory. The observed mode frequency is consistent with a recent theoretical prediction for the energetic-particle-driven geodesic acoustic mode.

Increasing the magnetic helicity content of a plasma by pulsing a magnetized source.

By operating a magnetized coaxial gun in a pulsed mode it is possible to produce large voltage pulses of duration approximately 500 mus while reaching a few kV, giving a discrete input of helicity into a spheromak. In the sustained spheromak physics experiment (SSPX), it is observed that pulsing serves to nearly double the stored magnetic energy and double the temperature. We discuss these results by comparison with 3D MHD simulations of the same phenomenon.

New mode of operating a magnetized coaxial plasma gun for injecting magnetic helicity into a spheromak.

By operating a magnetized coaxial plasma gun continuously with just sufficient current to enable plasma ejection, large gun-voltage spikes (approximately 1 kV) are produced, giving the highest sustained voltage approximately 500 V and highest sustained helicity injection rate observed in the Sustained Spheromak Physics Experiment. The spheromak magnetic field increases monotonically with time, exhibiting the lowest fluctuation levels observed during formation of any spheromak (B/B>/=2%). The results suggest an important mechanism for field generation by helicity injection, namely, the merging of helicity-carrying filaments.

The relationship of intercompartmental excitation transfer rate constants to those of an underlying physical model.

In studies on photosynthetic systems it is common practice to interpret the results of time-resolved fluorescence experiments on the basis of compartmental, or target, models. Each compartment represents a group of molecules with similar fluorescence characteristics. In cases of practical interest, the members of each compartment are spatially contiguous and make up part of an overall energy-transferring system. Since a rate constant describing the overall transfer between compartments is not that of any pair of molecules in the system, this question naturally rises: what do we learn about the microscopic structure from these data? In this note we introduce 'compartment melting', a smooth mathematical connection between the compartmental and microscopic levels. We then show, on the basis of model calculations on finite lattices in one, two, and three dimensions, that average microscopic rates at the interfaces between compartments may be estimated from observed intercompartmental rates. The estimate involves a modest number of structural assumptions about the system. As examples of the method, which is applicable mainly to systems containing homogeneous pigment pools, some recent chlorophyll-protein antenna studies are analyzed.