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Fast particle driven m=1/n=1 modes in the form of fishbones frequently initiate neoclassical tearing modes (NTMs) on Axisymmetric Divertor Experiment Upgrade [W. Koppendorfer et al., 14th International Conference on Plasma Physics and Controlled Fusion (International Atomic Energy Agency, Vienna, 1993), Vol. I, p. 127] and other neutral-beam-injection heated tokamaks, but disappear after the NTMs have reached a quasistationary state. A burst of other magnetohydrodynamic phenomena, described in this paper, appear instead, similar in repetition rate and time signature of frequency and amplitude. They are associated with the growth of the m=2/n=2 satellite of the m=3/n=2 neoclassical tearing mode, and a transient increase (termed FJ=frequency jump event) in the rotation frequency of the coupled mode system. They lead to a thermal crash, originating at q=3/2, propagating inwards up to the q=1 surface. With a lower repetition rate (more characteristic of sawteeth) similar events occur, where a large 1/1 component also appears and the fast crash propagates to the plasma center frequency jump with sawtooth character (FJST). The analogies to the fishbone case suggest that this phenomenon is caused by the interaction with fast particles, which transiently impress their precession frequency on the pre-existing magnetic perturbation of the NTM.

After many years of fusion research, the conditions needed for a D–T fusion reactor have been approached on the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)]. For the first time the unique phenomena present in a D–T plasma are now being studied in a laboratory plasma.The first magnetic fusion experiments to study plasmas using nearly equal concentrations of deuterium and tritium have been carried out on TFTR. At present the maximum fusion power of 10.7 MW, using 39.5 MW of neutral‐beam heating, in a supershot discharge and 6.7 MW in a high‐βp discharge following a current rampdown. The fusion power density in a core of the plasma is ≊2.8 MW m−3, exceeding that expected in the International Thermonuclear Experimental Reactor (ITER) [Plasma Physics and Controlled Nuclear Fusion Research (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 239] at 1500 MW total fusion power. The energy confinement time, τE, is observed to increase in D–T, relative to D plasmas, by 20% and the ni(0) Ti(0) τE product by 55%. The improvement in thermal confinement is caused primarily by a decrease in ion heat conductivity in both supershot and limiter‐H‐mode discharges. Extensive lithium pellet injection increased the confinement time to 0.27 s and enabled higher current operation in both supershot and high‐βp discharges. Ion cyclotron range of frequencies (ICRF) heating of a D–T plasma, using the second harmonic of tritium, has been demonstrated. First measurements of the confined alpha particles have been performed and found to be in good agreement with TRANSP [Nucl. Fusion 34, 1247 (1994)] simulations. Initial measurements of the alpha ash profile have been compared with simulations using particle transport coefficients from He gas puffing experiments. The loss of alpha particles to a detector at the bottom of the vessel is well described by the first‐orbit loss mechanism. No loss due to alpha‐particle‐driven instabilities has yet been observed. D–T experiments on TFTR will continue to explore the assumptions of the ITER design and to examine some of the physics issues associated with an advanced tokamak reactor.

Application of Ion Bernstein Wave Heating (IBWH) into the Princeton Beta Experiment‐Modification (PBX‐M) [Phys. Fluids B 2, 1271 (1990)] tokamak stabilizes sawtooth oscillations and generates peaked density profiles. A transport barrier, spatially correlated with the IBWH power deposition profile, is observed in the core of IBWH‐assisted neutral beam injection (NBI) discharges. A precursor to the fully developed barrier is seen in the soft x‐ray data during edge localized mode (ELM) activity. Sustained IBWH operation is conducive to a regime where the barrier supports large ∇ne, ∇Te, ∇νφ, and ∇Ti, delimiting the confinement zone. This regime is reminiscent of the H(high) mode, but with a confinement zone moved inward. The core region has better than H‐mode confinement while the peripheral region is L(low)‐mode‐like. The peaked profile enhances NBI core deposition and increases nuclear reactivity. An increase in central Ti results from χi reduction (compared to the H mode) and better beam penetration. Boot...

The experimental program of the Princeton Beta Experiment‐Modified (PBX‐M) tokamak [Phys. Fluids B2, 1271 (1990)] is directed toward tailoring plasma profiles to achieve greater stability and confinement and to gain access to the second stability region. Modification of the current density profile has been achieved with lower‐hybrid current drive (LHCD), leading to a regime free of global magnetohydrodynamic modes, while raising the value of q(0) above unity. The diffusion of the fast electrons produced by LHCD has been examined using two‐dimensional hard x‐ray imaging. Ion Bernstein waves (IBW) have been used for ion heating: a preliminary analysis shows that ion heating was spatially localized and in agreement with theoretical calculations. Divertor biasing has modified the electric field inside the last closed surface, resulting in the formation of a transport barrier, which in turn has reduced the threshold power of neutral beam injection (NBI) for H‐mode transition by 25%.

The highly indented plasmas of the PBX‐M tokamak experiment [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1989), Vol. 1, p. 97] have reached plasma regimes of both high volume‐averaged beta (βt), and high‐beta poloidal (βp), and show evidence of the suppression of external surface modes by the passive stabilizing system. Values of βt up to 4.0 I/aB (% MA/m T) with Ti(0)≊4 keV have been obtained. A magnetohydrodynamic analysis of plasmas with βp=2.0 indicates that these plasmas are near the threshold of the second stability regime. A value of βt of 6.8% has been reached with Ti(0)>5 keV and an indentation of 28%. Control of plasma shape is accomplished with a feedback system that uses a moment expansion about a single equilibrium and is augmented by time‐dependent waveforms to redefine plasma shape. Diagnostics to measure the safety factor q have been developed and used to make accurate measurements of q(r) and to verify changes made in q(0).

In tokamak plasmas, a key element in the understanding of magnetohydrodynamic (MHD) instabilities is the determination of their toroidal and poloidal mode numbers. This is usually accomplished by correlating measurements of radiation, particles, or magnetic oscillations at separate positions toroidally and poloidally around the torus. It is possible, however, to obtain the same information with arrays of soft x-ray detectors viewing the horizontal midplane and the poloidal cross section of the tokamak from a single location. The perturbation due to an MHD mode on a toroidal shell will modulate the soft x-ray emission along sightlines intersecting it. The number of maxima, and the angular separation between the channels detecting them, have a straightforward geometrical relationship to the mode number. This approach thus avoids the need for complex inversion of line-integrated signals. The method has already been used to determine poloidal mode numbers on the Princeton beta experiment-modification tokamak,...

An important issue for future tokamaks is real‐time plasma control for the avoidance of magnetohydrodynamic instabilities and other applications that require detailed plasma profile and fluctuation data. Although measurements from diagnostics providing this information require significantly more processing than magnetic flux data, recent advancements could make them practical for adjusting operational settings for plasma heating and current drive systems as well as field coil currents. On the Princeton Beta Experiment‐Modification (PBX‐M), the lower hybrid current drive phasing can be varied during a plasma shot using digitally programmable ferrite phase shifters, and neural beam functions can be fully computer controlled. PBX‐M diagnostics that may be used for control purposes include motional Stark‐effect polarimetry for magnetic field pitch angle profiles, soft x‐ray arrays for plasma position control and the separation of βp from li, hard x‐ray detectors for energetic electron distributions, a multich...

The efficiency of plasma current drive with lower hybrid waves is reflected in the distribution of suprathermal electrons formed during this process. Past measurements of the bremsstrahlung radiation from these electrons depended on arrays of hard x‐ray detectors at different toroidal and poloidal locations around tokamaks [Nucl. Fusion 25, 1515 (1985)]. To avoid the space requirements and the complex correlation and inversion procedures associated with such systems, a single 23‐cm‐diam hard x‐ray image intensifier tube is being installed for two‐dimensional imaging on PBX‐M. It will be optically coupled to an image processor already in place for a soft x‐ray pinhole camera [Rev. Sci. Instrum. 59, 1831 (1988)], and analysis algorithms similar to those already developed for this diagnostic will be used.

A detector setup with three LN‐cooled Si(Li) diodes is used to measure soft x‐ray spectra (0.8–20 keV) emitted from the inside walls of the PDX vessel during the plasma discharge. The setup is part of a pulse‐height‐analysis system, which is used to measure the plasma and wall radiation simultaneously at five different radial positions. The wall and the plasma radiation are measured under different plasma conditions (e.g., OH and neutral beam heating). The wall radiation is very much increased during the neutral beam heating, with an enhancement factor of at least 10 over the OH wall radiation. Since we measure the plasma and the wall radiation at the same time, the measurements allow the conclusion that the wall radiation can be attributed essentially to fluorescence (line radiation, e.g., Ti‐Kα) and scattering (continuum part of the wall radiation spectrum). The fluorescence and the scattering are both caused by soft x‐ray radiation flux coming from the plasma. There seems to be no need to invoke other,...

A bent quartz‐crystal spectrometer of the Johann type with a spectral resolution of λ/Δλ= 10000–25000 is used on the Tokamak Fusion Test Reactor (TFTR) to determine central plasma parameters from the spectra of helium‐like and lithium‐like metal impurity ions (Ti, Cr, Fe, and Ni). The spectra are observed along a central radial chord and are recorded by a position‐sensitive multiwire proportional counter with a spatial resolution of 250 μ. Standard delay‐line time‐difference readout is employed. The data are histogrammed and stored in 64 K of memory providing 128 time groups of 512‐channel spectra. The central ion temperature and the toroidal plasma rotation are inferred from the Doppler broadening and Doppler shift of the Kα lines. The central electron temperature, the distribution of ionization states, and dielectronic recombination rates are obtained from satellite‐to‐resonance line ratios. The performance of the spectrometer is demonstrated by measurements of the Ti xxi Kα radiation.

A mathematical model of a single‐arm x‐ray pulse‐height analyzer (PHA) system is developed. Given an assumption on the electron temperature and density profiles, a maximum likelihood technique is applied to calculate the peak electron temperature and enhancement factor of the plasma. This method is currently being used in the analysis of x‐ray data from the tokamak fusion test reactor (TFTR); sample results are presented.

A Bragg x‐ray spectrometer of high spectral resolution (λ/Δλ=10 000–20 000) which accommodates three crystals and position‐sensitive detectors in the Johann configuration has been installed in the diagnostic basement of the tokamak fusion test reactor (TFTR) for the measurement of radial ion temperature profiles. The ion temperature is derived from the Doppler broadening of Kα‐resonance lines of metal impurity ions, e.g., Ti, Cr, Fe, and Ni, in the helium‐like and hydrogen‐like charge states. The x‐ray diffraction plane is almost perpendicular to the magnetic axis, but slightly tilted by an angle of 3.8°, which makes it possible to measure poloidal and toroidal plasma rotation velocities of vΘ>5×103 m/s and vΦ>1×105 m/s, from the Doppler shift of spectral lines. Results obtained from the observation of TiXXl Kα‐line spectra with a 220‐silicon crystal of a 2d spacing of 3.8400 A and a curvature radius of 11.05 m are reported.

The x‐ray pulse‐height analysis (PHA) diagnostic uses a liquid‐nitrogen cooled array of 5 Si(Li) and one HpGe detectors to do time‐resolved (5–100 ms) x‐ray spectroscopy of the central horizontal chord of the Tokamak Fusion Test Reactor (TFTR) plasmas in the 1–50‐keV range. Central electron temperature Te and concentration of medium and low‐Z impurities are derived from the spectra. Remotely selectable absorber‐foil arrays provide selection of the energy range. Fixed and movable aperture arrays allow approximate equalization of count rates in different energy bands and extend dynamic range. Amplifier pulse shapes are approximately triangular. Main amplifier peaking time is 4 μs yielding 230‐eV FWHM at 5.9 keV. Pileup inspection times are selectable at 0.13, 0.4, or 0.9 μs. Throughput is up to 42 kHz. The PHA has been used to study temperature and impurities over a wide range of TFTR operational parameters. Dramatic variations in metal impurities with density, plasma current, and major radius have been obs...

The soft x‐ray continuum radiation in tokamak fusion test reactor (TFTR) low‐density neutral‐beam discharges can be much lower than its theoretical value obtained by assuming a corona equilibrium. This reduced continuum radiation is caused by an ionization equilibrium shift toward lower states, which strongly changes the value of the average recombination coefficient of metallic impurities γ, even for only slight changes in the average charge Z. The primary agent for this shift is the charge exchange between the highly ionized impurity ions and the neutral hydrogen, rather than impurity transport, because the central density of the neutral hydrogen is strongly enhanced at lower plasma densities with intense beam injection. In the extreme case of low‐density, high neutral‐beam power TFTR operation (energetic ion mode) the reduction in γ can be as much as one‐half to two‐thirds. We calculate the parametric dependence of γ and Z for Ti, Cr, Fe, and Ni impurities on neutral density (equivalent to beam power), electron temperature, and electron density. These values are obtained by using either a one‐dimensional impurity transport code or a zero‐dimensional code with a finite particle confinement time. As an example, we show the variation of γ and Z in different TFTR discharges.

Because soft‐x‐ray pulse‐height‐analysis spectra contain chordal information, the electron temperature and the radiation intensity (enhancement factor) measurements do not represent the local values. The correction factors for the electron temperature and the enhancement factor as a function of the temperature and density profile parameters and the energy are obtained. The spectrum distortion due to pulse pileup effects is also evaluated. A set of curves is given from which the distortion of the spectrum can be obtained if the electron temperature, the Be filter thickness, and the electronic parameters of the acquisition system are known.

An interpretation of the x‐ray bremsstrahlung emission by suprathermal electrons perpendicular to a magnetic field is given in terms of the parallel and perpendicular temperature of a three‐temperature distribution function. The slope (i.e., the temperature) of the distribution can be determined relatively well. Factor‐of‐two uncertainties remain for the number of electrons.

Metal impurity concentrations are measured by the pulse‐height‐analyzer (PHA) diagnostic from Kα x‐ray peak intensities by use of an averaged excitation rate 〈σv〉. Low‐Z impurity concentrations are inferred from the continuum enhancement (relative to a pure plasma) minus the enhancement due to metals. Since the PHA does not resolve lines from different charge states, 〈σv〉 is a weighted sum of rates; coronal equilibrium is usually assumed. The 〈σv〉 used earlier omitted the intercombination and forbidden lines from the dominant heliumlike state. The result was an overestimate of metals and an underestimate of low‐Z impurities in cases where metals were significant. Improved values of 〈σv〉 using recent calculations for H‐, He‐, and Li‐like Fe range from 10% to 50% larger than the earlier rates and yield metal concentrations in better agreement with those from vacuum ultraviolet (VUV) spectroscopy.

A detector setup with three LN‐cooled Si(Li) diodes is used to measure soft x‐ray spectra (0.8–20 keV) emitted from the inside walls of the PDX vessel during the plasma discharge. The setup is part of a pulse‐height‐analysis system, which is used to measure the plasma and wall radiation simultaneously at five different radial positions. The wall and the plasma radiation are measured under different plasma conditions (e.g., OH and neutral beam heating). The wall radiation is very much increased during the neutral beam heating, with an enhancement factor of at least 10 over the OH wall radiation. Since we measure the plasma and the wall radiation at the same time, the measurements allow the conclusion that the wall radiation can be attributed essentially to fluorescence (line radiation, e.g., Ti‐Kα) and scattering (continuum part of the wall radiation spectrum). The fluorescence and the scattering are both caused by soft x‐ray radiation flux coming from the plasma. There seems to be no need to invoke other,...

The synchrotron radiation emitted parallel and perpendicular to the magnetic axis of a “hot electron” plasma (Te = 80 keV) confined in a 50‐kG magnetic mirror field has been measured in the 4−0.2mm wavelength region of the electromagnetic spectrum by means of a spectrometer system capable of determining the absolute intensity of the synchrotron radiation. The experimental spectrum agrees with the theoretical spectrum based on single‐particle emission and a two‐dimensional Maxwellian distribution for the plasma electrons. The electron temperature and density predicted by the measured spectrum agrees with other methods of measurement of these quantities.

Resonance between rotating neoclassical tearing mode (NTM) and precessing trapped energetic ions has been considered. Resonant ions experience large radial excursions of their banana guiding centers leading to the loss of a considerable fraction of fast ions deposited in the resonance region. The resulting toroidal torque accelerates the magnetic island rotating in the direction of the ion diamagnetic drift. The torque balance equation has been coupled with the modified Rutherford equation, retaining the contributions from the bootstrap current and the ion polarization current. It has been shown that, due to the resonant interaction with energetic ions, this autonomous system of equations exhibits a Hopf bifurcation for some critical ratio of the momentum damping rate to the resistive diffusion rate. The resulting stable limit cycle resembles the frequency jump events observed during NTM activity in the Axisymmetric Divertor Experiment Upgrade (ASDEX Upgrade) tokamak [S. Sesnic et al., Phys. Plasmas 7, 93...