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Tungsten will be an important element in nearly all future fusion reactors because of its presence in plasma facing components. This makes tungsten a good candidate for a diagnostic element for ion temperature and toroidal velocity measurement, and it makes understanding tungsten emissions important for tokamak power balance. The effect of tungsten on tokamak plasmas is investigated at the Alcator C-Mod tokamak using VUV, bolometry, and soft X-ray spectroscopy. Tungsten was present in Alcator C-Mod as a plasma facing component and through laser blow-off impurity injection. Quasi-continuum emission previously seen at other tokamaks has been identified. Theoretical predictions are presented of tungsten emission that could be expected in future Alcator C-Mod measurements. Furthermore, spectra of highly charged tungsten ions have been studied at the SuperEBIT electron beam ion trap. This emission could prove useful for spectroscopic diagnostics of future high-temperature fusion reactor plasmas.

The understanding and predictive capability of transport physics and plasma confinement is reviewed from the perspective of achieving reactor-scale burning plasmas in the ITER tokamak, for both core and edge plasma regions. Very considerable progress has been made in understanding, controlling and predicting tokamak transport across a wide variety of plasma conditions and regimes since the publication of the ITER Physics Basis (IPB) document (1999 Nucl. Fusion 39 2137-2664). Major areas of progress considered here follow. (1) Substantial improvement in the physics content, capability and reliability of transport simulation and modelling codes, leading to much increased theory/experiment interaction as these codes are increasingly used to interpret and predict experiment. (2) Remarkable progress has been made in developing and understanding regimes of improved core confinement. Internal transport barriers and other forms of reduced core transport are now routinely obtained in all the leading tokamak devices worldwide. (3) The importance of controlling the H-mode edge pedestal is now generally recognized. Substantial progress has been made in extending high confinement H-mode operation to the Greenwald density, the demonstration of Type I ELM mitigation and control techniques and systematic explanation of Type I ELM stability. Theory-based predictive capability has also shown progress by integrating the plasma and neutral transport with MHD stability. (4) Transport projections to ITER are now made using three complementary approaches: empirical or global scaling, theory-based transport modelling and dimensionless parameter scaling (previously, empirical scaling was the dominant approach). For the ITER base case or the reference scenario of conventional ELMy H-mode operation, all three techniques predict that ITER will have sufficient confinement to meet its design target of Q = 10 operation, within similar uncertainties.

The Rydberg series (1s2 to 1snp) up to n = 14 of helium-like argon (Z = 18) has been observed from Alcator C-Mod plasmas using a high-resolution X-ray spectrometer array. High-n satellites to these...

Global and local transport experiments in ohmic, L-mode and H-mode regimes on the Alcator C-Mod tokamak are summarized. For ohmic plasmas, earlier results derived for energy confinement scaling in ...

H-mode research on Alcator C-Mod is described, with a focus on the edge transport barrier (ETB). ETB pedestals are characterized using several diagnostics, leading to a thorough description of prof...

Plasma profiles and flows in the low- and high-field side scrape-off-layer (SOL) regions in Alcator C-Mod are found to be remarkably sensitive to magnetic separatrix topologies (upper-, lower- and double-null) and to impose topology-dependent flow boundary conditions on the confined plasma. Near-sonic plasma flows along magnetic field lines are observed in the high-field SOL, with magnitude and direction clearly dependent on X-point location. The principal drive mechanism for the flows is a strong ballooning-like poloidal transport asymmetry: parallel flows arise so as to re-symmetrize the resulting poloidal pressure variation in the SOL. Secondary flows involving a combination of toroidal rotation and Pfirsch–Schluter ion currents are also evident. As a result of the transport-driven parallel flows, the SOL exhibits a net co-current (counter-current) volume-averaged toroidal momentum when B × ∇B is towards (away from) the X-point. Depending on the discharge conditions, flow momentum can couple across the separatrix and affect the toroidal rotation of the confined plasma. This mechanism accounts for a positive (negative) increment in central plasma co-rotation seen in L-mode discharges when B × ∇B is towards (away from) the X-point. Experiments in ion-cyclotron range-of-frequency-heated discharges suggest that topology-dependent flow boundary conditions may also play a role in the sensitivity of the L–H power threshold to X-point location: in a set of otherwise similar discharges, the L–H transition is seen to be coincident with central rotation achieving roughly the same value, independent of magnetic topology. For discharges with B × ∇B pointing away from the X-point (i.e. with the SOL flow boundary condition impeding co-current rotation), the same characteristic rotation can only be achieved with higher input power.

Trace non-recycling impurities have been injected into Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] plasmas in order to determine impurity transport coefficients. Subsequent impurity emission has been observed with spatially scanning x-ray and Vacuum Ultra-Violet (VUV) spectrometer systems. Measured time-resolved brightness profiles of helium- and lithium-like transitions have been compared with those calculated from a transport code which includes impurity diffusion and convection in conjunction with an atomic physics package for individual line emission. During Low-Confinement-Mode (L-mode) plasmas, the transport can be characterized by pure diffusion, with coefficients ∼5000 cm2/s, reflecting the ∼20 ms decay in the x-ray and VUV line brightnesses. During High-Confinement-Modes (H-modes), the impurity confinement times are much longer, and the modelling requires that there be a strong inward convection (of order 1000 cm/s) near the plasma edge, with greatly reduced diffusion (o...

Using calculations of collisional excitation rates in He-like argon (Ar XVII) and intensity factors and other data for nearby dielectronic satellites, X-ray line spectra have been synthesized as a function of electron temperature and density. The Ar XVII excitation rates were obtained from interpolations of R-matrix calculations, and are likely to be the best available at present. The resulting spectra are compared with spectra observed from high-temperature plasmas in the Alcator C tokamak. The agreement between observed spectra and those calculated using measured values of temperature and density illustrates the accuracy of the calculations, though there are small discrepancies which are likely to be connected with wavelengths or intensity factors for the Ar XVI satellites. Thus, argon X-ray line spectra can serve as a means of diagnosing low-density, astrophysical (in particular solar-flare) plasmas for which there is no independent means of measuring temperatures.

H-mode experiments on Alcator C-Mod [I.H. Hutchinson, et al., Phys. Plasma 1 (1994) 1511] which exhibit an internal transport barrier (ITB), have been examined with gyrokinetic simulations, near the ITB onset time. Linear simulations support the picture of ion and electron temperature gradient (ITG, ETG) microturbulence driving high {chi}{sub i} and {chi}{sub e}, respectively, and that stable ITG correlates with reduced particle transport and improved ion thermal confinement on C-Mod. In the barrier region ITG is weakly unstable, with a critical temperature gradient higher than expected from standard models. Nonlinear calculations and the role of E x B shear suppression of turbulence outside the plasma core are discussed in light of recent profile measurements for the toroidal velocity. The gyrokinetic model benchmarks successfully against experiment in the plasma core.

Recent H-mode experiments on Alcator C-Mod [I.H. Hutchinson, et al., Phys. Plasmas 1 (1994) 1511] which exhibit an internal transport barrier (ITB), have been examined with flux tube geometry gyrokinetic simulations, using the massively parallel code GS2 [M. Kotschenreuther, G. Rewoldt, and W.M. Tang, Comput. Phys. Commun. 88 (1995) 128]. The simulations support the picture of ion/electron temperature gradient (ITG/ETG) microturbulence driving high xi/ xe and that suppressed ITG causes reduced particle transport and improved ci on C-Mod. Nonlinear calculations for C-Mod confirm initial linear simulations, which predicted ITG stability in the barrier region just before ITB formation, without invoking E x B shear suppression of turbulence. Nonlinear fluxes are compared to experiment, which both show low heat transport in the ITB and higher transport within and outside of the barrier region.

A mercuric iodide (HgI/sub 2/) detector has been used to obtain x-ray spectra in the energy range from 5 to 200 keV from the Alcator-C tokamak. During high-density Ohmic operation of Alcator-C, this detection system provides a reliable electron-temperature diagnostic. Nonthermal x-ray spectra above 20 keV are observed under certain conditions when a substantial amount of lower-hybrid rf (LHRF) power is injected. The magnitude of this nonthermal behavior strongly increases as the electron density is lowered and is more pronounced in deuterium plasmas than in hydrogen. Model electron distribution functions, which can give rise to the observed x-ray spectra, are discussed.

We report on initial results from lower hybrid wave heating experiments carried out on the MIT Alcator C and Versator II tokamaks. In the Alcator C experiments a 4 waveguide array, with internally brazed ceramic windows has been used to inject 160 kW of microwave power at 4.6 GHz into the plasma with n O ≤ 1 × 10 15 cm −3 , and B O ≤ 12 T. An RF power density of 8 kW/cm 2 has been transmitted into the plasma without RF breakdown. RF coupling studies show optimal coupling (R ≤ 10%) when the local density at the waveguide mouth is 25–50 times overdense. Initial heating experiments show an ion tail formation in hydrogen discharge peaking at a density of at B = 8.9 T, and bulk ion heating at a density of at B ≃ 11 T. Evidence of RF current enhancement has been observed at a density of n ≃ 3 × 10 13 cm −3 .

Internal transport barriers (ITBs) marked by steep density and pressure profiles and reduction of core transport are obtained in Alcator C-Mod. Transient single barriers are observed at the back-transition from H- to L-mode and also when pellet injection is accompanied by ion cyclotron resonance frequency (ICRF) power. Double barriers are induced with injection of off-axis ICRF power deposition. These also arise spontaneously in ohmic H-mode plasmas when the H-mode lasts for several energy confinement times. C-Mod provides a unique platform for studying such discharges: The ions and electrons are tightly coupled by collisions with Ti/Te = 1, and the plasma has no internal particle or momentum sources. ITB plasmas with average pressure greater than 1 atm have been obtained. To form an ITB, particle and thermal flux are reduced in the barrier region, allowing the neoclassical pinch to peak the density while maintaining the central temperature. Gyrokinetic simulation suggests that long-wavelength drift wave turbulence in the core is marginally stable at the ITB onset, but steepening of the density profile destabilizes trapped electron modes (TEMs) inside the barrier. The TEM ultimately drives sufficient outgoing particle flux to balance the inward pinch and halt further density rise, which allows control of particle and impurity peaking.

Trace nonrecycling impurities (scandium and CaF 2 ) have been injected into Alcator C-Mod plasmas in order to determine impurity transport coefficient profiles in a number of operating regimes. Recycling Ar has also been injected to characterize steady-state impurity density profiles. Subsequent impurity emission has been observed with spatially scanning X-ray and vacuum ultraviolet spectrometer systems, in addition to very high spatial resolution X-ray and bolometer arrays viewing the plasma edge. Measured time-resolved brightness profiles of helium-, lithium-, and beryllium-like transitions have been compared with those calculated from a transport code that includes impurity diffusion and convection, in conjunction with an atomic physics package for individual line emission. Similar modeling has been performed for the edge observations, which are unresolved in energy. The line time histories and the profile shapes put large constraints on the impurity diffusion coefficient and convection velocity profiles. In L-mode plasmas, impurity confinement times are short (∼20 ms), with diffusivities in the range of 0.5 m 2 /s, anomalously large compared to neoclassical values. During Enhanced D α (EDA) H-modes, the impurity confinement times are longer than in L-mode plasmas, and the modeling suggests that there exists inward convection (≤50 m/s) near the plasma edge, with greatly reduced diffusion (of order 0.1 m 2 /s), also in the region of the edge transport barrier. These edge values of the transport coefficients during EDA H-mode are qualitatively similar to the neoclassical values. In edge localized mode-free H-mode discharges, impurity accumulation occurs, dominated by large inward impurity convection in the pedestal region. A scaling of the impurity confinement time with H-factor reveals a very strong exponential dependence. In internal transport barrier discharges, there is significant impurity accumulation inside of the barrier foot, typically at r/a = 0.5. Steady-state impurity density profiles in L-mode plasmas have a large up-down asymmetry near the last closed flux surface. The impurity density enhancement, in the direction opposite to the ion B X VB drift, is consistent with modeling of neoclassical parallel impurity transport.

Spontaneous toroidal rotation of impurity ions has been observed in the core of Alcator C-Mod plasmas with no external momentum input. The magnitude of the rotation ranges from -60 km/s (countercur...