The performance of TFTR as measured by the maximum achievable stored energy or fusion power is limited by disruptive instabilities rather than confinement. Techniques to enhance the MHD stability of the plasma by current and pressure profile modifications are being explored. Methods of increasing the peakedness or internal inductance, l_i of the current profile as well as broadening the current profile to create low or reversed shear in the core were tried. These new operational regimes have their own unique MHD activity.
Plasmas with enhanced l_i have been created at up to 2.3 MA through a novel technique of increasing the minor radius of the plasma at constant plasma current, i.e.\rm, ramping q(a). With this technique the l_i of high current plasmas has been increased from \approx 1.1 to \approx 1.4. These plasmas had substantially broader sawtooth inversion radii in the ohmic phase than normal supershots, consistent with a more peaked plasma current profile. It is expected that these plasmas will be stable at \beta_n \approx 15-25 % higher than similar supershots or with stored energy as high as \approx 7 - 8 MJ. To date, stable high l_i plasmas with up to 6 MJ of stored energy and 8.2MW of fusion power have been produced.
Disruptions at the \beta limit in reversed central shear (RS) plasmas have been studied. Plasmas with reversed central shear have disrupted with \beta_n in the range 0.4-1.8, although \beta_n \approx 2 has been reached in non-disrupting shots. Internal measurements of the structure of the disruption precursor suggest it is localized near or just outside the q_min surface. Ideal stabilty calculations are in reasonable agreement with the observed \beta limit as well as the structure of the precursor. The stability appears to be sensitive to the local shape of the pressure and current profiles in this region, and thus may not be well characterized by global parameters. Analysis and understanding of the stability of the RS plasma is also complicated by the observation of double tearing modes at or about q_min low order rational crossings, i.e. resistive as well as ideal stability must be considered. The double tearing modes can result in off-axis sawteeth and are well modeled by the MH3D code.
[6IB.02] High Confinement and High Density with Stationary Plasma Energy and Strong Edge Radiation Cooling in Textor-94
A.M. Messiaen (Laboratoire de Physique des Plasmas-Laboratorium voor Plasmafysica, Association "EURATOM-Belgian State", Ecole Royale Militaire-Koninklijke Militaire School, Brussels, Belgium)
A new discharge regime has been observed on the pumped limiter tokamak TEXTOR-94 in the presence of strong radiation cooling and for different scenarii of additional hearing. The radiated power fraction (up to 90%) is feedback controlled by the amount of Ne seeded in the edge. This regime meets many of the necessary conditions for a future fusion reactor. Energy confinement increases with increasing densities (reminiscent of the Z-mode obtained at ISX-B) and as good as ELM-free H-mode confinement (enhancement factor verus ITERH93-P up to 1.2) is obtained at high densities (up to 1.2 times the Greenwald limit) with peaked density profiles showing a peaking factor of about 2 and central density values around 10^14cm^-3. In experiments where the energy content of the discharges is kept constant with an energy feedback loop acting on the amount of ICRH power, stable and stationary discharges are obtained for intervals of more than 5s, i.e. 100 times the energy confinement time or about equal to the skin resistive time, even with the cylindrical q_\alpha as low as 2.8 \beta-values up to the \beta-limits of TEXTOR-94 are achieved (i.e. \beta _n \approx 2 of and \beta _p \approx 1.5) and the figure of merit for ignition margin f_H\/q_a in these discharges can be as high as 0.7. No detrimental effects of the seeded impurity on the reactivity of the plasma are observed. He removal in these discharges has also been investigated.
\beginitemize \item [1] Laboratoire de Physique des Plasmas-Laboratorium voor Plasmafysica, Association "EURATOM-Belgian State", Ecole Royale Militaire-Koninklijke Militaire School, Brussels, Belgium \item [2] Institut für Plasmaphysik, Forschungszentrum Jülich, GmbH, Association "EURATOM-KFA", Jülich, Germany \item [3] Fusion Energy Research Program, Mechanical Engineering Division, University of California at San Diego, La Jolla, USA \item [4] FOM Institüt voor Plasmafysica Rijnhuizen, Associatie "FOM-EURATOM", Nieuwegein, The Netherlands \item [*] Researcher at NFSR, Belgium \enditemize
[6IB.03] Beta Limits in Long-Pulse Tokamak Discharges
O. Sauter (CRPP-EPFL, Assoc. EURATOM-Confédération Suisse, Presently assigned to ITER-JCT San Diego)
The maximum normalized beta achieved in long pulse tokamak discharges falls significantly below both that observed in short pulse discharges and that predicted by the ideal MHD theory. Recent long-pulse experiments, in particular those aimed at simulating ITER-like discharges with low collisionality (\nu*), are limited by low (m,n) non-ideal MHD modes. The effect of the saturated modes is a reduction of the confinement time by 10%-20% (soft beta limit), depending on the island size and location.(Z. Chang et al., Nucl. Fus. 34, 1309 (1994).) If \beta_N is further increased, more modes are excited leading to a disruption (hard beta limit).
\par Detailed studies, using the reconstructed equilibria, show that soft and hard \beta limits of 20% up to as much as 50% below the ideal MHD limit are observed. Linear and non-linear resistive MHD cannot clearly explain the dependence of stability on \nu* and \beta. However more recent theories on neoclassical destabilization of tearing modes,(H. Wilson et al, Phys. Plasmas 3, 248 (1996) and references herein.) including the effects of a perturbed helical bootstrap current and polarization current, are succesful in explaining the behavior of the tearing mode beta limits in long-pulse discharges and recent TFTR results support the size of the saturated islands(Z. Chang et al, Phys. Rev. Lett. 74, 4663 (1995).). Also, a strong correlation is observed between the onset of these low m/n modes with sawteeth, ELMs, or fishbone events, consistent with the seed island required by the theory.
\par We will focus on a quantitative comparison between both the conventional resistive and neoclassical theories and the experimental results of ASDEX-U, COMPASS, DIIID, JT60U and TFTR, which have all observed these low (m,n) non-ideal modes. This will enable us to project the long-pulse beta limits of ITER-size tokamaks and also to discuss possible plasma control methods which would increase the soft \beta limit, decrease the seed perturbations and/or diminish the effects on confinement.
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