We have made and installed a multipulse Nd:YAG Thomson scattering system for measuring electron temperature and density profiles in the throat of the divertor of the Alcator C-Mod machine. The observing head is located in the vacuum vessel in a re-entrant chamber. It is optically fast and very compact. A system for providing feedback to maintain the laser beam alignment is a part of the observing head assembly. The head is designed to minimize eddy currents, and have a very rigid adjustable mounting to resist the forces generated by the eddy currents during plasma disruptions. A four wavelength polychromator using 25 element avalanche photodiode arrays for spatial resolution has been designed and built for this system. Two of these polychromators and a single observing head will provide 50 spatial resolution elements.

For application to a ruby laser Thomson scattering system, we have developed a laser triggered intensified charge coupled device (CCD) with 80 mm aperture, two stages of intensification, and 80 ns gating. To improve the dynamic range, the CCD is cooled and read out slowly (1 s). To obtain a good extinction ratio (>1.1×107), the zoom electrode of the first intensifier is gated using a ∼10 kV laser triggered spark gap. The stability of this spark gap has been greatly improved by frequency doubling the laser trigger light.

It is the purpose of this report to present a detailed description of the TV Thomson scattering system on the JFT-2M tokamak, produced through the collaboration of the Princeton Plasma Physics Laboratory (PPPL), and the JFT-2M group of the Japan Atomic Energy Research Institute (JAERI).

A technique is described for maintaining the highly precise alignment needed for accurate density profile measurements with the TFTR Thomson scattering system. The method uses the transverse coherence of the fiber bundles in the collection optics to encode the alignment information in the spatial distribution of the scattered light signal. A scheme for decoding this information is demonstrated and the limitations of this technique are discussed.

Magnetohydrodynamic (MHD) activity within three zones (core, half-radius, and edge) of TFTR [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. 1, p. 51] tokamak plasmas are discussed. Near the core of the plasma column, sawteeth are often observed. Two types of sawteeth are studied in detail; one with complete, and the other with incomplete, magnetic reconnection. Their characteristics are determined by the shape of the q profile. Near the half-radius the m/n=3/2 and 2/1 resistive ballooning modes are found to correlate with a beta collapse. The pressure and the pressure gradient at the mode rational surface are found to play an important role in stability. MHD activity is also studied at the plasma edge during limiter H modes. The edge localized modes (ELM’s) are found to have a precursor mode with a frequency between 50–200 kHz and a mode number m/n=1/0. The mode does not show a ballooning structure. While these instabilities have been studied on many other machines, on TFTR the studies have been extended to high pressure (plasma pressure greater than 4×105 Pa) and low collisionality [vi*(a/2)<0.002, ve*(a/2)<0.01].

The PBX-M Thomson scattering system is reviewed after its first 9 months of operation. The system measures Te(R) and ne(R) at 55 radial points on the horizontal midplane with a spatial resolution of ≤1.1 cm. The scattered light is collected by a Bouwers concentric mirror system and imaged onto 12-m-long fiber bundles. A composite entrance slit is used to optimize the system performance. The synchronization between the scattered light and detector gate is done via a laser-triggered spark gap. A penetration in the indentation coil allows the (ruby) laser beam to be dumped away from the plasma chamber. Other aspects of the system will be discussed.

The H-mode transition is accompanied by the development of an electron temperature pedestal of about 0.3 keV and a dramatic steepening of the density gradient inside the separatrix. A change of density profile from peaked to flat is obtained for both H- mode and ohmically heated plasmas, after an internal disruption. Hollow ne profiels are found in low Bt discharges (low q). These phenomena indicate a reduction of the electron energy transport at the edge of the plateau region.

A system is under construction which will make use of the laser and collection optics of an existing multichannel Thomson scattering system on the tokamak fusion test reactor (TFTR). A scanning sampling mechanism will divert signal from a 10-cm segment of the radial-beam path to a spectrometer–detector system which is optimized for low temperatures and densities. In the absence of stray laser light and background plasma light the system will be capable of measuring temperatures down to 3–5 eV at 1–3×1011 cm−3 minimum densities.

The characteristics of plasma operation on the axisymmetric inner toroidal limiter in TFTR are described. After conditioning, plasmas with low metal content and low zeff are obtained with this limiter. There is no substantial increase in zeff with total input power during neutral beam injection. Compared to operation on the outer blade limiter, additional gas is required to fuel plasmas on the inner limiter. Injection of D pellets increased the plasma density substantially and produced energy confinement times up to 0.5 s in ohmically heated plasmas. The four neutral beam lines have injected up to 13.5 MW total power into the plasma for 0.5 s with up to 90 kV accelerating voltage. The scaling of the plasma stored energy was studied as a function of the input power, plasma current and plasma density. In the range 1.4 to 2.2 MA, the overall and incremental confinement times for both the total and thermal stored energies increase with plasma current at fixed density. There appears to be a weak negative scaling of the total stored energy with density at high injection power.

Thomson scattering systems capable of providing snapshot profiles of electron temperature and density with high spatial resolution (>50 points) have become routine diagnostics on the Princeton large torus (PLT), Princeton divertor experiment (PDX), and tokamak fusion test reactor (TFTR) tokamaks. The design parameters of these systems are compared. Particular attention is given to describing those new components and techniques which have contributed most to improved data quality and reliability in the ten-year evolution of these systems. Examples of recent TFTR Te(R) and ne(R) profiles are presented.

Recent experiments on TFTR have extended the operating regime of TFTR in both ohmic- and neutral-beam-heated discharges. The TFTR tokamak has reached its original machine design specifications (I/sub p/ = 2.5 MA and B/sub T/ = 5.2 T). Initial neutral-beam-heating experiments used up to 6.3 MW of deuterium beams. With the recent installation of two additional beamlines, the power has been increased up to 11 MW. A deuterium pellet injector was used to increase the central density to 2.5 x 10/sup 20/ m/sup -3/ in high current discharges. At the opposite extreme, by operating at low plasma current (I/sub p/ approx. 0.8 MA) and low density (anti n/sub e/ approx. 1 x 10/sup 19/ m/sup -3/), high ion temperatures (9 +- 2 keV) and rotation speeds (7 x 10/sup 5/ m/s) have been achieved during injection. In addition, plasma compression experiments have demonstrated acceleration of beam ions from 82 keV to 150 keV, in accord with expectations. The wide operating range of TFTR, together with an extensive set of diagnostics and a flexible control system, has facilitated transport and scaling studies of both ohmic- and neutral-beam-heated discharges. The results of these confinement studies are presented.

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.

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: BT increased to 4.0 T, Ip to 2.0 MA, injection power (Pb) to 5 MW with H° or D° beams, ne to 5 × 1019 m−3 and Zeff reduced to 1.4. With ohmic heating (OH) alone, the previously established scaling for gross energy confinement time (τE ∝ ne) has been confirmed at higher Ip and BT, and the maximum τE of 0.4 sec has been achieved. With NBI at Pb, substantially (by factor > 2) higher than POH, excellent power and particle accountability have been established. This suggests 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. τE is observed to scale approximately as Ip Pb−0.5 (independent of ne), consistent with previous L-mode scalings. With NBI we have achieved the maximum τE of 0.2 s and the maximum Ti (o) of 4.4 keV in the normal operating regime, and even higher Ti(o) in the energetic-ion regime with low-ne and low Ip operation.

The TFTR Thomson scattering system has been operational since January 1984. The diagnostic uses two ruby lasers and two spectrometer-detector systems to provide profiles at two times in a discharge. The two scattering lines consist of 76 spatial channels which span the 200-cm vacuum vessel along a major radius. The detectors are gated, intensified CCD arrays with single photoelectron sensitivity. Te(R) and Ne(R) profiles are presented.