[en] The pressure profile and plasma shape, parameterized by elongation (κ), triangularity ((delta)), and squareness (ζ), strongly influence stability. In this study, ideal stability of single null and symmetric, double-null, advanced tokamak (AT) configurations is examined. All the various shapes are bounded by a common envelope and can be realized in the DIII-D tokamak. The calculated AT equilibria are characterized by P₀/(l_angle)P} ∼ 2.0-4.5, weak negative central shear, high q_min (>2.0), high bootstrap fraction, an H-mode pedestal, and varying shape parameters. The pressure profile is modeled by various polynomials together with a hyperbolic tangent pedestal, consistent with experimental observations. Stability is calculated with the DCON code and the resulting stability boundary is corroborated by GATO runs

[en] The Motional Stark Effect (MSE) diagnostic will be essential for the study of advanced scenarios on ITER and its design is currently underway. 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 |B| can be inferred, where |B| is the magnitude of the total magnetic field. The baseline 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 optimized designs for both techniques and will present a comparison of them and discuss the associated design trade-offs

[en] Recent DIII-D results demonstrate that the snowflake (SF) divertor geometry (cf. standard divertor) enables significant manipulation of divertor heat transport for heat spreading and reduction in attached and radiative divertor regimes, between and during edge localized modes (ELMs), while maintaining good H-mode confinement. Snowflake divertor configurations have been realized in the DIII-D tokamak for several seconds in H-mode discharges with heating power PNBI ⩽ 4-5 MW and a range of plasma currents Ip = 0.8-1.2 MA. In this work, inter-ELM transport and radiative SF divertor properties are studied. Significant impact of geometric properties on SOL and divertor plasma parameters, including increased poloidal magnetic flux expansion, divertor magnetic field line length and divertor volume, is confirmed. In the SF-minus configuration, heat deposition is affected by the geometry, and peak divertor heat fluxes are significantly reduced. In the SF-plus and near-exact SF configurations, divertor peak heat flux reduction and outer strike point heat flux profile broadening are observed. Inter-ELM sharing of power and particle fluxes between the main and additional snowflake divertor strike points has been demonstrated. The additional strike points typically receive up to 10-15% of total outer divertor power. Measurements of electron pressure and poloidal beta !p support the theoretically proposed churning mode that is driven by toroidal curvature and vertical pressure gradient in the weak poloidal field region. A comparison of the 4-4.5 MW NBI-heated H-mode plasmas with radiative SF divertor and the standard radiative divertor (both induced with additional gas puffing) shows a nearly complete power detachment and broader divertor radiated power distribution in the SF, as compared to a partial detachment and peaked localized radiation in the standard divertor. However, insignificant difference in the detachment onset w.r.t. density between the SF and the standard divertor was found. The results complement the initial SF divertor studies in the NSTX and DIII-D tokamaks and contribute to the physics basis of the SF divertor as a power exhaust concept for future tokamaks.

[en] Recent experiments on tokamaks around the world [1-5] have demonstrated discharges with moderately high performance in which the q-profile remains stationary, as measured by the motional Stark effect diagnostic, for periods up to several τ_R. Hybrid discharges are characterize by q_min ∼ 1, high β_N, and good confinement. These discharges have been termed hybrid because of their intermediate nature between that of an ordinary H-mode and advanced tokamak discharges. They form an attractive scenario for ITER as the normalized fusion performance (β_NH_89P/q₉₅²) is at or above that for the ITER baseline Q_fus = 10 scenario, even for q₉₅ as high as 4.6. The startup phase is thought to be crucial to the ultimate evolution of the hybrid discharge. An open question is how hybrid discharges achieve and maintain their stationary state during the initial startup phase. To investigate this aspect of hybrid discharges, we have used the CORSICA code to model the early stages of a discharge. Results clearly indicate that neoclassical current evolution alone is insufficient to account for the time evolution of the q-profile and that an addition of non-inductive current source must be incorporated into the model to reproduce the experimental time history. We include non-inductive neutral beam and bootstrap current sources in the model, and investigate the difference between simulations with these sources and the experimentally inferred q-profile. Further, we have made preliminary estimates of the spatial structure of the current needed to bring the simulation and experiment into agreement. This additional non-inductive source has not been tied to any physical mechanism as yet. We present these results and discuss the implications for hybrid startup on ITER

[en] The power-law series for the poloidal magnetic flux function, up to the third-order terms, is presented for the case where two nulls of the poloidal magnetic field are separated by a small distance, as in a snowflake divertor. Distinct from the earlier results, no assumptions about the field symmetry are made. Conditions for the realization of an exact snowflake are expressed in terms of the coefficients of the power series. It is shown that, by a proper choice of the coordinate frame in the poloidal plane, one can obtain efficient similarity solutions for the separatrices and flux surfaces in the divertor region: the whole variety of flux surface shapes can be characterized by a single dimensionless parameter. Transition from a snowflake-minus to a snowflake-plus configuration in the case of no particular symmetry is described. The effect of the finite toroidal current density in the divertor region is assessed for the case of no particular symmetry.

[en] The specific size and structure of the edge current profile has important effects on the magnetohydrodynamic stability and ultimate performance of many advanced tokamak (AT) operating modes. This is true for both bootstrap and externally driven currents that may be used to tailor the edge shear. Absent a direct local measurement of j(r), the best alternative is a determination of the poloidal field. Measurements of the precision (0.1^o--0.01^o in magnetic pitch angle and 1--10 ms) necessary to address issues of stability and control and provide constraints for EFIT are difficult to do in the region of interest (ρ=0.9--1.1). Using Zeeman polarization spectroscopy of the 2S--2P lithium resonance line emission from the DIII-D LIBEAM [D. M. Thomas, Rev. Sci. Instrum. 66, 806 (1995); D. M. Thomas, A. W. Hyatt, and M. P. Thomas, Rev. Sci. Instrum. 61, 340 (1990)] measurements of the various field components may be made to the necessary precision in exactly the region of interest to these studies. Because of the negligible Stark mixing of the relevant atomic levels, this method of determining j(r) is insensitive to the large local electric fields typically found in enhanced confinement (H mode) edges, and thus avoids an ambiguity common to motional Stark effect measurements of B. Key issues for utilizing this technique include good beam quality, an optimum viewing geometry, and a suitable optical prefilter to isolate the polarized emission line. A prospective diagnostic system for the DIII-D AT program will be described

[en] 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 |B| can be inferred, where |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.

[en] The advanced tokamak research program at DIII-D relies critically on the measurement of the current density profile. This was made possible by the development of a Motional Stark Effect (MSE) polarimeter that was first installed in 1992. Three major upgrades have since occurred, and improvements in our understanding of critical performance issues and calibration techniques are ongoing. In parallel with these improvements, we have drawn on our DIII-D experience to begin studies and design work for MSE on burning plasmas and ITER. This paper first reviews how Motional Stark Effect polarimetry (MSE) is used to determine the tokamak current profile. It uses the DIII-D MSE system as an example, and shows results from the latest upgrade that incorporates an array of channels from a new counter-Ip injected neutral beam. The various calibration techniques presently used are reviewed. High-leverage or unresolved issues affecting MSE performance and reliability in ITER are discussed. Next, we show a four-mirror collection optics design for the two ITER MSE views. Finally, we discuss measurements of the polarization properties of a few candidate mirrors for the ITER MSE

[en] The motional Stark effect (MSE) diagnostic on DIII-D has been expanded to take advantage of a change in the neutral beam geometry, adding 24 new MSE channels viewing a beam injected counter to the plasma current. When data from these channels are used with those from two older MSE arrays viewing a different beam, the overall radial resolution improves near the magnetic axis at least a factor of 2, and the uncertainty in calculations of vertical magnetic field and radial electric field decreases in the edge at least a factor of 4. The new design uses two optical systems mounted on the same vacuum port with a common shutter and shielding

[en] 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