Core Transport Reduction in Tokamak Plasmas with Modified Magnetic Shear

Abstract

Spontaneous improvements of plasma confinement during auxiliary heating have been observed in many tokamaks when the q profile has been modified from its normal resistive equilibrium so that q > 1 and the magnetic shear is reduced or reversed in a region near the magnetic axis. The effects on the overall plasma confinement result from the formation in the plasma interior of transport barriers, regions where the thermal and particle transport coefficients are substantially reduced.These internal barriers are sometimes tied to unique magnetic surfaces, such as the surface where the shear reverses. The reduction in transport appears to result from the suppression of turbulence by sheared plasma flow, which has now been measured in TFTR. Extensions of the theory for turbulence suppression show that this underlying paradigm may also explain other regimes of improved core confinement. The excitement generated by these discoveries must be tempered by the realization that transport and stability to pressure-driven MHD instabilities are intimately linked in these plasmas through the bootstrap current and the effect of the resulting current profile on the transport. Thus the development of control tools and strategies is essential if these improved regimes of confinement are to be exploited to improve the prospects for fusion energy production.

Internal transport barriers in tokamak plasmas*

“…Correspondingly, Q eq DT between, for instance, 0.3 in DIII-D and 1.2 in JT-60U have been achieved transiently. In TFTR (Bell et al 1997) and JET (Gormezano et al 1998) ITBs have been produced using the relevant fuel mixture of deuterium and tritium for a burning fusion plasma, resulting in actual Q DT values of 0.4 in JET (Söldner et al 1999).…”

“…For completeness it should be mentioned that also in TFTR a few ITBs with a fuel mixture of deuterium and tritium have been attempted (Bell et al 1997). As it was found that the already high power threshold for accessing the ERS regime (Type-I) had increased further with the addition of tritium, the discharges tended to approach the β-limit soon after the confinement transition.…”

“…(CXRS) (Hellermann et al 1990, Hawkes et al 1994, Bell et al 1997, Bell et al 1999b,Meister et al 2001 and equilibrium reconstructions of the respective magnetic field components B θ and B φ :…”

Absolute high spectral resolution measurements of surface solar radiation for detection of water vapour continuum absorption

“…This follows the standard Michelson design and is a transportable instrument with a resolution of up to 0.0035 cm −1 . The instrument has previously been used as a mobile transfer standard in a series of intercomparison campaigns of solar FTIR measurements within the Network for the Detection of Stratospheric Change, as well as providing groundbased and aircraft-borne measurements of trace atmospheric species [9][10][11]. The experimental set-up as used during the CAVIAR campaign is shown in figure 1, with the FTIR instrument in the background of the photograph.…”

Design and Commissioning of a Dual Visible/Near-IR Echelle Spectrograph for the AEOS Telescope

“…Except for the slit wheel, all were commercial stepper motor stages purchased from NEAT (Kollmorgen). The slit wheel is a 12-position Geneva mechanism designed for precision positioning repeatability and driven by a Phytron motor (Bell et al 1998). The other mechanisms include a rotary stage for switching between the two cross-dispersers, a linear stage for focusing the spectrograph camera lens, and an X-Y stage for translating the CCD dewar (in case a target spectral feature happened to fall on the ≈ 0.25 mm gap between the two detectors of the mosaic).…”