The TFTR tokamak has reached its original machine design specifications (Ip=2.5 MA and BT=5.2 T). Recently, the D degrees neutral beam heating power has been increased to 6.3 MW. By operating at low plasma current (Ip approximately=0.8 MA) and low density (ne approximately=1*1019 m-3), high ion temperatures (9+or-2 keV) and rotation speeds (7*105 m/s) have been achieved during injection. At the opposite extreme, pellet injection into high current plasmas has been used to increase the line-average density to 8*1019 m-3 and the central density to 1.6*1020 m-3. This wide range of operating conditions has enabled the authors to conduct scaling studies of the global energy confinement time in both ohmically and beam heated discharges as well as more detailed transport studies of the profile dependence.

/pdf/confinement-studies-of-neutral-beam-heated-discharges-in-mjprq7pbed.pdf

Confinement studies of neutral beam heated discharges in TFTR

Quantitative spectroscopic measurements of Zeff, impurity densities and radiated power losses have been made for ohmically heated and neutral beam heated TFTR discharges at a plasma current of 2.2 MA and a toroidal field of 4.7 T. Variations in these quantities with line average plasma density (e) and beam power up to 5.6 MW are presented for discharges on a movable graphite limiter. A detailed discussion of the use of an impurity transport model to infer absolute impurity densities and radiative losses from line intensity and visible continuum measurements is given. These discharges were dominated by low-Z impurities, with carbon having a considerably higher density than oxygen, except in high e Ohmic discharges where the densities of carbon and oxygen were comparable. Metallic impurity concentrations and radiative losses were small, resulting in hollow radiated power profiles and fractions of the input power radiated being 30–50% for Ohmic heating and 30% or less for beam heating. Spectroscopic estimates of the radiated power were in good agreement with bolometrically measured values. Because of an increase in the carbon density, Zeff rose from 2.0 to 2.8 as the beam power increased from 0 to 5.6 MW, pointing to a potentially serious dilution of the neutron producing plasma ions with increasing beam power. Both the low-Z and the metallic impurity concentrations were approximately constant with minor radius, indicating no central impurity accumulation in these discharges.

Spectroscopic study of impurity behaviour in neutral beam heated and ohmically heated TFTR discharges

It is argued that transport models with sufficiently small central (relative to edge) thermal conductivity, so as to allow sawteeth at the highest q, can easily explain 'profile consistency' in tokamaks. This includes the observed increase in temperature profile peakedness T(0)/T with increasing q and the relatively small variation of the peakedness with variation in heating profile. In fact, it is proven that if q(0) = 1 and the Spitzer relation between current and temperature profiles holds, then the peakedness is bounded by q2/3 T(0)/T q. A distinction is made between the central sawtooth mechanism which 'tops' the profiles and possible q dependent edge mechanisms which 'shrink' profiles. The latter are not precluded from contributing to profile consistency and may be required to understand confinement time scaling.

Comments on profile consistency

A Doppler‐shift spectroscopy system has been installed on the TFTR neutral beam injection system to measure species composition during both conditioning and injection pulses using short and long pulse ion sources. Two intensified vidicon detectors and two spectrometers are utilized in a system capable of resolving data from up to 12 ion sources simultaneously. By imaging the light from six ion sources onto one detector, a cost‐effective system has been achieved. Fiber optics are used to locate the diagnostic in an area remote from the hazards of the tokamak test cell allowing continuous access, and eliminating the need for radiation shielding of electronic components. Automatic hardware arming and interactive data analysis allow beam composition to be computed between tokamak shots for use in analyzing plasma heating experiments. Measurements have been made using lines of sight into both the neutralizer and the drift duct. Analysis of the data from the drift duct is both simpler and more accurate since only neutral particles are present in the beam at this location. Comparison of the data taken at these two locations reveals the presence of partially accelerated particles possessing an estimated 1/e half‐angle divergence of 15° and accounting for up to 30% of the extracted power from the short pulse ion sources. Operation with long pulse ion sources indicates a higher atomic composition, fewer partially accelerated particles, and somewhat reduced neutralization line densities.

/pdf/multiple-track-doppler-shift-spectroscopy-system-for-tftr-3bbn154594.pdf

Multiple track Doppler-shift spectroscopy system for TFTR neutral beam injectors

The Tokamak Fusion Test Reactor (TFTR) at Princeton University has been designed 1) to reach power break-even, Q approx. = 1 (Q = fusion power/heating power), in neutral-beam-injected, two-energy-component D-T plasmas, where the density confinement time requirement is less stringent than that for maxwellian, one-energy-component plasmas, and 2) to provide the physics understanding of reactor-like plasmas required for the development of tokamak reactors. TFTR has now been in operation for more than two years. Nearly 15,000 individual high power plasma discharges have been produced. Deuterium plasmas with ohmic heating currents up to 2.5 MA, deuterium neutral beam injection (NBI) with beam powers up to 6.3 MW (28 MW projected for 1986) and 80 KeV energy (120 KeV projected), deuterium pellet injection and adiabatic deuterium plasma compression have been investigated. The neutron source strength of a fusion plasma is a direct indicator of the fusion power generated. However, its measurement is complicated for a number of reasons. 1) The neutron source is extended and time varying. 2) It is surrounded by large, fixed scatterers near both source and detectors. 3) The dynamic range of interest in present experiments covers some ten orders of magnitude. In Section II the authors discuss brieflymore » the plasma, its properties, the detectors, their useful ranges and limitations and their calibration. Section II contains some of the neutron measurement results obtained so far.« less

Tokamak Fusion Test Reactor Neutron Source Strength Measurements Instrumentation and Results

The paper describes the present (end of February 1985) status of the plasma confinement studies in the TFTR tokamak with emphasis on those with neutral beam injection (NBI). Recent improvements in the device capabilities have substantially extended operating parameters: B /sub T/ increased to 4.0 T, I /sub p/ to 2.0 MA, injection power (P /sub b/ ) to /sup 5/ MW with H/sup 0/ or D/sup 0/ beams, n /sub e/ to 5 x 10/sup 19/ m/sup -3/ and Z /sub eff/ reduced to 1.4. With ohmic heating (OH) alone, the previously established scaling for gross energy confinement time (T /sub E/ /sup 0/ n /sub e/ q) has been confirmed at higher I /sub p/ and B /sub T/ , and the maximum T /sub E/ of 0.4 sec has been achieved. With NBI at P /sub b/ substantially (by factor > 2) higher than P /sub OH/ , excellent power and particle accountability have been established. This suggest that the less-than-expected increase in stored energy with NBI is not due to problems of power delivery, but due to problems of confinement deterioration. T /sub E/ is observed to scale approximately as I /sub p/ P /sub b/more » /sup 0.5/ (independent of n /sub e/ ), consistent with previous L-mode scalings. With NBI we have achieved the maximum T /sub E/ of 0.2 s and the maximum T /sub i/ (o) of 4.4 keV in the normal operating regime, and even higher T /sub i/ (o) in the energetic-ion regime with low-n /sub e/ and low I /sub p/ operation.« less

/pdf/confinement-studies-in-tftr-17nz6hq51n.pdf

Confinement studies in TFTR

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Confinement studies in TFTR