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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 ...

To introduce this special issue, this paper gives a brief overview of the key themes in the Alcator C-Mod tokamak research program. These include energy, particle and momentum transport, wave-plasm...

An overview of the diagnostics installed on the Alcator C-Mod tokamak is presented. Approximately 25 diagnostic systems are being operated on C-Mod. The compact design of the machine and the cryostat enclosing the vacuum vessel and magnetic field coils make access challenging. Diagnostics are used to study four focus areas: transport, plasma boundary, waves, and macrostability. There is significant overlap between these topics, and they all contribute toward the burning plasma and advanced tokamak thrusts. Several advanced and novel diagnostics contribute to the investigation of C-Mod plasmas, e.g., electron cyclotron emission, phase-contrast imaging, gas puff imaging, probe measurements, and active magnetohydrodynamic antennas.

A high speed (250 kHz), 300 frame charge coupled device camera has been used to image turbulence in the Alcator C-Mod Tokamak. The camera system is described and some of its important characteristics are measured, including time response and uniformity over the field-of-view. The diagnostic has been used in two applications. One uses gas-puff imaging to illuminate the turbulence in the edge/scrape-off-layer region, where D2 gas puffs localize the emission in a plane perpendicular to the magnetic field when viewed by the camera system. The dynamics of the underlying turbulence around and outside the separatrix are detected in this manner. In a second diagnostic application, the light from an injected, ablating, high speed Li pellet is observed radially from the outer midplane, and fast poloidal motion of toroidal striations are seen in the Li+ light well inside the separatrix.

Research on the Alcator C-Mod tokamak [1] is focused on high particle- and power-density plasma regimes to understand particle and energy transport in the core, the dynamics of the H-mode pedestal, and scrape-off layer and divertor physics. The auxiliary heating is provided exclusively by RF waves, and both the physics and technology of RF heating and current drive are studied. The momentum which is manifested in strong toroidal rotation, in the absence of direct momentum input, has been shown to be transported in from the edge of the plasma following the L–H transition, with timescale comparable to that for energy transport. In discharges which develop internal transport barriers, the rotation slows first inside the barrier region, and then subsequently outside of the barrier foot. Heat pulse propagation studies using sawteeth indicate a very narrow region of strongly reduced energy transport, located near r/a = 0.5. Addition of on-axis ICRF heating arrests the buildup of density and impurities, leading to quasi-steady conditions. The quasi-coherent mode associated with enhanced D-Alpha (EDA) H-mode appears to be due to a resistive ballooning instability. As the pedestal pressure gradient and temperature are increased in EDA H-mode, small ELMs appear; detailed modelling indicates that these are due to intermediate n peeling–ballooning modes. Phase contrast imaging has been used to directly detect density fluctuations driven by ICRF waves in the core of the plasma, and mode conversion to an intermediate wavelength ion cyclotron wave has been observed for the first time. The bursty turbulent density fluctuations, observed to drive rapid cross-field particle transport in the edge plasma, appear to play a key role in the dynamics of the density limit. Preparations for quasi-steady-state advanced tokamak studies with lower hybrid current drive are well underway, and time dependent modelling indicates that regimes with high bootstrap fraction can be produced.

Simultaneous high resolution measurements of the soft x-ray emission at the top and outboard edge of the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are presented. Impurity density profiles are derived from these measurements. The impurity density varies considerably on a flux surface in the high confinement mode (H-mode) pedestal region. The soft x-ray and impurity density pedestals are not on the same flux surface at the top and outboard edge. The pedestal widths are consistently larger at the outboard edge in the Enhanced D-alpha (EDA) H-mode, suggesting a ballooning-like character of the EDA quasi-coherent mode. The usual separation of time scales between radial and parallel impurity transport does not hold in the H-mode pedestal region of Alcator C-Mod. Thus, one should not necessarily expect that the impurity density be constant on a flux surface in this region.

H-modes exhibiting improved confinement above the L-mode are achieved in Alcator C-Mod with ICRF and with ohmic heating alone without boronization. Both ELM-free and ELMy H-modes are obtained with total input power from 0.75 to 4.2 MW over a range of densities (0.8 to ) and toroidal fields (3 to 8 T). Type III ELMs are often observed to have coherent, high m and n precursor oscillations with frequencies of 100 - 160 kHz. The threshold power required to achieve the H-mode increases with density and toroidal field, in rough agreement with scalings derived from other tokamaks. The power densities and density times toroidal field products are an order of magnitude larger than in other tokamaks, in the range of values expected for ITER. The L - H and H - L transitions occur at approximately the same edge electron temperature. A low density limit to the H-mode is found at about . A high midplane neutral pressure limit of about 0.6 mTorr is also observed.

Direction reversals of intrinsic toroidal rotation have been observed in diverted Alcator C-Mod Ohmic L-mode plasmas following electron density ramps. For low density discharges, the core rotation is directed cocurrent, and reverses to countercurrent following an increase in the density above a certain threshold. Such reversals occur together with a decrease in density fluctuations with 2 cm(-1)≤k(θ)≤11 cm(-1) and frequencies above 70 kHz. There is a strong correlation between the reversal density and the density at which the Ohmic L-mode energy confinement changes from the linear to the saturated regime.

International audience; Intrinsic rotation has been observed in I-mode plasmas from the C-Mod tokamak, and is found to be similar to that in H mode, both in its edge origin and in the scaling with global pressure. Since both plasmas have similar edge ∇T, but completely different edge ∇n, it may be concluded that the drive of the intrinsic rotation is the edge ∇T rather than ∇P. Evidence suggests that the connection between gradients and rotation is the residual stress, and a scaling for the rotation from conversion of free energy to macroscopic flow is calculated.

The direct measurement of lower hybrid (LH) waves in plasma can provide information on the propagation and absorption of LH waves in tokamak plasma and validate numerical simulations. Here, we propose to utilize the scattering of probe waves (60 GHz) off LH waves (4.6 GHz) to diagnose LH waves in the scrape off layer. We develop an analytic model to describe the scattering process in weakly inhomogeneous plasma based on the three‐wave coupling problem. We then present first experimental results obtained by upgrading an ordinary‐mode reflectometer system in Alcator C‐Mod.

Plasma flow drive via ICRF mode conversion (MC) has been demonstrated on Alcator C‐Mod. The toroidal rotation in these D(3He) MC plasmas is typically more than twice above the empirically determined intrinsic rotation scaling in ICRF minority heated plasmas. In L‐mode plasmas at 3 MW ICRF power input, up to 90 km/s toroidal rotation and 2 km/s localized ( r/a∼0.4) poloidal rotation has been observed. The MC ion cyclotron wave (ICW) was detected by a phase contrast imaging system in heterodyne setup. Through TORIC 2‐D full wave simulation, and comparison with other experimental evidence, we hypothesize that the interaction between the MC ICW and the 3He ions may be the mechanism for the observed MC flow drive. TORIC simulation suggests that similar flow drive scenario may be realized on JET D(3He) plasmas. The promising scenarios on ITER are the inverted minority scenario (T)D and high field launch for T‐D‐(3He) plasma. In non‐radioactive phase, these correspond to (3He)‐H and 4He(3He) plasmas respectively.

The effects of a low-energy charge-exchange process between hydrogenlike ions and neutral hydrogen on x-ray spectra of He-like argon ions from the Alcator C tokamak are investigated. Dependences of charge-exchange cross sections on both the principal and angular momentum quantum numbers n and l are estimated from the observed spectra, taking cascade effects into account with the use of a collisional radiative model.

An instrument designed to measure k-filtered plasma fluctuations has been used to look at edge fluctuations in Alcator C-Mod tokamak plasmas. The instrument utilizes a sieve technique, whereby an optical image is split and filtered through two out-of-phase, striped transmission masks, and then imaged onto photodiode detectors. The difference signal is then digitized and analyzed to give the frequency characteristics of the fluctuations at a particular value of k, as determined by the wavelength of the mask stripes. Measurements have been made looking at the edge plasma with hydrogen Balmer-alpha filtering, and k both parallel and perpendicular to the magnetic field, with |k| in the range from 5 to 15 cm−1. Broad band (ν

ICRF mode conversion (MC) in H(3He, D) and D(H) plasmas have been studied in detail in Alcator C‐Mod. In H(3He, D) plasma, the mode converted ion cyclotron wave (MC ICW) was observed in tokamak plasmas for the first time using a phase contrast imaging system. The MC ICW was observed in the low field side of the ion‐ion hybrid layer, and generally had a wavelength in‐between the fast wave and the MC ion Bernstein wave (IBW). Localized mode conversion electron heating (MCEH) has been clearly observed for the first time in D(H) plasmas with moderate hydrogen concentration in Alcator C‐Mod. Both on‐ and off‐axis (high field side) MCEH have been studied. The MCEH profile was obtained from a break in slope analysis of Te signals in the presence of rf shut‐off. The experimental profiles were qualitatively in agreement with the predictions of the two‐dimensional full‐wave poloidal mode code TORIC. The electron heating contributions from MC ICW and MC IBW are examined from the TORIC simulations.

Measurements on impurity pellet ablation trails indicate that the emission from these clouds extends much farther along the magnetic field than across it. For carbon pellets on TFTR, the aspect ratio of this cigar‐shaped plume is of the order of 10. Strong visible line emission from one of the ionization states can be imaged with high time resolution, and since ions should be well confined along the field lines, it should be possible in this way to measure the radial profile of the pitch of the total internal magnetic field. For a pellet injected along a major radius in the plasma midplane, the appropriate viewing geometry consists of imaging the pellet along its flight from behind. A detector scheme capable of accomplishing the measurement, consisting of an array of 1‐D spot‐imaging photodiodes, is described. In order to make meaningful measurements with regard to the tokamak physics of interest, it is highly desirable to measure the field angle profile with a precision of ±10 mrad or better. In order to achieve this for a cigar with aspect ratio of 10, measurement of the mean vertical position for each slice of the cigar must be made with a precision of about 10% of the vertical thickness of the cigar on that slice.

Measurements of H- and He-like X-ray spectra of elements between Z = 16 and 26 (hv = 2 to 8 keV) are discussed and illustrated with recent results obtained for argon. Examples of instrumental development are presented together with the outlook for future experiments.

Short bursts of aluminum were injected into the Adiabatic Toroidal Compressor (ATC) Tokamak, and measured the time evolution of the radial distributions of highly ionized states of aluminum. The results are compared with a computer code describing neoclassical impurity diffusion and good agreement is found. (auth)

Resultats des premieres mesures radiales des transitions Rx, n=2 a n=1, des elements Z eleves dans des plasmas de deuterium de tokamaks. Les rapports des intensites des raies observees ne peuvent s'expliquer par des processus d'excitation par impact electronique et recombinaison dielectronique. La recombinaison radiative et de transfert de charge est proposee comme mecanisme de population additionnelle. Les raies emises dans la cascade suivant la capture constituent un diagnostic unique de la composante neutre des plasmas

This paper presents advances in the use of high resolution x-ray spectroscopy for the measurements of tokamak plasmas and the study of atomic processes. The measurements refer to H- and He- like spectra of Ar (λ = 3.7 to 4.2 A) obtained with a new crystal spectrometer at the Alcator C tokamak. With record count rates of up to 0.5 MHz, time resolved spectroscopy of individual plasma discharge was demonstrated at the 20 ms time scale. In particular, the time evolution of ion and electron temperatures, Ti(t) and Te(t), was determined and occurrences of transient plasma phenomena were recorded in discharges with deuterium pellet injection. X-ray line emission was for the first time observed along different chords through the plasma so as to determine the radial dependence of the principal plasma parameters Ti(r) and Te(r). These radial scan data are shown to be of interest for identifying manifestations of underlying atomic processes and ion transport effects. On the basis of the present results we assess the future use of x-ray spectroscopy diagnostics to obtain extended information on the temporal and spatial dependences of the main plasma parameters together with new information on transient phenomena as well as on the neutral content of tokamak plasmas.

A system for producing 300‐μsec bursts of 1017 metal atoms with 3‐eV average energy is described. It is shown that this system can be successfully used to introduce impurities into CTR‐oriented tokamaks for transport and confinement studies.

The He-like spectrum of 1s2 - 1snp transitions and satellites in Ar16+ have been measured from the plasma of the Alcator C tokamak. In the wavelength region of λ = 3.0 to 3.4 A, three groups of satellite lines 1s22σ - 1s2σ nl are observed which accompany each of the resonance lines emanating from the upper orbitals from n = 4 to n = 10. Results of new calculations are presented and used for identification of the observed spectra and for discussion of line intensities and wavelengths. All satellite lines belonging to the high n states observed (n = 4 to n = 10) have been identified in the calculations.

Short bursts of aluminum were injected into the Adiabatic Toroidal Compressor (ATC) Tokamak, and measured the time evolution of the radial distributions of highly ionized states of aluminum. The results are compared with a computer code describing neoclassical impurity diffusion and good agreement is found. (auth)

A system for producing 300‐μsec bursts of 1017 metal atoms with 3‐eV average energy is described. It is shown that this system can be successfully used to introduce impurities into CTR‐oriented tokamaks for transport and confinement studies.

An experiment measuring the scaling with changing plasma conditions of a parameter characteristic of the transport of aluminum injected into the ATC tokamak is discussed. This parameter is the time after injection at which the photon signal from Al XI reaches its maximum. It is found that the data are in agreement with the predictions of a computer code which uses neoclassical theory in the Pfirsch-Schluter regime. An approximate model describing impurity transport in ATC is also presented. This model is utilized to calculate the expected scaling with current of the aluminum transport assuming classical, neoclassical and pseudoclassical forms for the diffusion coefficient. The data are in agreement with both the neoclassical and pseudoclassical results from this model.

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.

Electron temperature measurements from line ratios of He‐ and H‐like argon in the Alcator C tokamak