Alcator C-Mod Experiments Research Articles

The role of edge fueling in determining the pedestal density in high neutral opacity Alcator C-Mod experiments

SOLPS-ITER modeling of EDA H-mode experiments on Alcator C-Mod find that the electron density pedestal structure is unaffected by increased gas fueling when approaching ITER-like opaqueness conditions. SOLPS-ITER simulations show a decrease in the neutral penetration depth with increasing pedestal density, similar to prior experiments (Hughes et al., 2006). Neutral density and penetration depth vary poloidally, and we show that the highest neutral densities at the separatrix are found closest to the gas puff locations both on the high field side and the low field side. The decrease in the neutral penetration depth with increasing pedestal density, as well as the decrease in the neutral density at the separatrix, are not just limited to the midplane, but persist at every poloidal location. We find, however, that when gas puffs of similar magnitude as those employed in experiments are added, a much larger increase in the electron density is observed over the whole modeled plasma radius. This does suggest that changes in transport will need to be included self-consistently. • SOLPS-ITER provides insight on the dynamics of neutrals in high opacity H-modes. • SOL neutral opacity can be quantified by radial neutral density e-folding length. • Density pedestal formation is mostly unaffected as pedestal opacity increases. increases. • High gas-puffing only moderately increases neutral penetration of the pedestal. • At high pedestal opacity, main chamber fueling dominates fueling through the X-point.

Influence of high magnetic field on access to stationary H-modes and pedestal characteristics in Alcator C-Mod

Recent Alcator C-Mod experiments have explored access to and characteristics of H-modes at magnetic fields approaching 8 T, the highest field achieved to date in a diverted tokamak. The H-modes originated from L-mode densities ranging from to , allowing insight into the density dependence of the H-mode power threshold at high magnetic field. This dependence is compared to predictions from the ITPA scaling law ([]), finding that the law is approximately accurate at 7.8 T. However, the law underpredicted the high density H-mode threshold at lower magnetic field in previous C-Mod experiments ([]), suggesting that the overall dependence of the threshold on magnetic field is weaker than predicted by the scaling law. The threshold data at 7.8 T also indicates that the onset of a low density branch at this magnetic field on C-Mod occurs below , which is lower than predicted by an existing model for low density branch onset. The H-modes achieved steady-state densities ranging from to , and higher transient densities, and had values of q95 from 3.3 to 6.0. This parameter range allowed the achievement of all three types of H-mode routinely observed at lower magnetic field on C-Mod: the stationary, ELM-suppressed Enhanced Dα (EDA) regime, seen at high densities and high values of q95; the nonstationary ELM-free regime, seen at lower densities and values of q95; and the ELMy regime, seen at low density, moderate q95, and specialized plasma shape. The parameter space in which these regimes occur at 7.8 T is consistent with lower magnetic field experience. Pressure pedestal height at 7.8 T is compared to EPED [, ] predictions, and a scaling law for EDA density pedestal height developed between 4.5 T and 6.0 T is updated to include fields from 2.7 T to 7.8 T. Overall, this analysis increases confidence in the use of low magnetic field experience to predict some elements of high magnetic field tokamak behavior.

Expanding the role of impurity spectroscopy for investigating the physics of high-Z dissipative divertors

New techniques that attempt to more fully exploit spectroscopic diagnostics in the divertor and pedestal region during highly dissipative scenarios are demonstrated using experimental results from recent low-Z seeding experiments on Alcator C-Mod, JET and ASDEX Upgrade. To exhaust power at high parallel heat flux, q∥ > 1 GW/m2, while minimizing erosion, reactors with solid, high-Z plasma facing components (PFCs) are expected to use extrinsic impurity seeding. Due to transport and atomic physics processes which impact impurity ionization balance, so-called ‘non-coronal’ effects, we do not accurately know and have yet to demonstrate the maximum q∥ which can be mitigated in a tokamak. Radiation enhancement for nitrogen is shown to arise primarily from changes in Li- and Be-like charge states on open field lines, but also through transport-driven enhancement of H- and He-like charge states in the pedestal region. Measurements are presented from nitrogen seeded H-mode and L-mode plasmas where emission from N1+ through N6+ are observed. Active charge exchange spectroscopy of partially ionized low-Z impurities in the plasma edge is explored to measure N5+ and N6+ within the confined plasma, while passive UV and visible spectroscopy is used to measure N1+-N4+ in the boundary. Examples from recent JET and Alcator C-Mod experiments which employ nitrogen seeding highlight how improving spectroscopic coverage can be used to gain empirical insight and provide more data to validate boundary simulations.

Multispecies density peaking in gyrokinetic turbulence simulations of low collisionality Alcator C-Mod plasmas

Peaked density profiles in low-collisionality AUG and JET H-mode plasmas are probably caused by a turbulently driven particle pinch, and Alcator C-Mod experiments confirmed that collisionality is a critical parameter. Density peaking in reactors could produce a number of important effects, some beneficial, such as enhanced fusion power and transport of fuel ions from the edge to the core, while others are undesirable, such as lower beta limits, reduced radiation from the plasma edge, and consequently higher divertor heat loads. Fundamental understanding of the pinch will enable planning to optimize these impacts. We show that density peaking is predicted by nonlinear gyrokinetic turbulence simulations based on measured profile data from low collisionality H-mode plasma in Alcator C-Mod. Multiple ion species are included to determine whether hydrogenic density peaking has an isotope dependence or is influenced by typical levels of low-Z impurities, and whether impurity density peaking depends on the species. We find that the deuterium density profile is slightly more peaked than that of hydrogen, and that experimentally relevant levels of boron have no appreciable effect on hydrogenic density peaking. The ratio of density at r/a = 0.44 to that at r/a = 0.74 is 1.2 for the majority D and minority H ions (and for electrons), and increases with impurity Z: 1.1 for helium, 1.15 for boron, 1.3 for neon, 1.4 for argon, and 1.5 for molybdenum. The ion temperature profile is varied to match better the predicted heat flux with the experimental transport analysis, but the resulting factor of two change in heat transport has only a weak effect on the predicted density peaking.

Alcator C-Mod experiments in support of the ITER baseline 15 MA scenario

Experiments on Alcator C-Mod from 2009–2012 have examined the rampup, flattop and rampdown phases of the proposed ITER 15 MA baseline scenario. Rampup studies show ICRF heating can significantly reduce the V-s requirement, and that an H-mode late in the ramp can reduce this further. ICRF modifications to li in L-mode are minimal, although the Te profile is peaked relative to ohmic in the plasma centre, and reduces the sawtooth onset times. Flattop plasmas targeting ITER baseline parameters have been sustained for 20τE or 8 − 13τCR, but only reach H98 ∼ 0.6 at n/nGr = 0.85, rising to 0.9 at n/nGr = 0.65. Rampdown studies show H-modes can be routinely sustained with ICRF power injection, avoiding an OH coil over-current associated with the H–L transition. In addition, faster current rampdowns are preferred to avoid an over-current when an H–L transition ultimately does occur. In the H-mode rampdown the density is found to drop with Ip, preserving the n/nGr ratio, so long as ICRF power is injected.

Time-domain simulation of nonlinear radiofrequency phenomena

Nonlinear effects associated with the physics of radiofrequency wave propagation through a plasma are investigated numerically in the time domain, using both fluid and particle-in-cell (PIC) methods. We find favorable comparisons between parametric decay instability scenarios observed on the Alcator C-MOD experiment [J. C. Rost, M. Porkolab, and R. L. Boivin, Phys. Plasmas 9, 1262 (2002)] and PIC models. The capability of fluid models to capture important nonlinear effects characteristic of wave-plasma interaction (frequency doubling, cyclotron resonant absorption) is also demonstrated.

Fullwave Simulations of Lower Hybrid Waves Coupled to 3D Fokker-Planck Solver: Comparison with Alcator C-Mod Experiment

Non-inductive lower hybrid current drive (LHCD) experiments have been carried out on the Alcator C-Mod tokamak and the hard x-ray (HXR) spectrum has been measured. An improved analysis technique of the experimental HXR data has been developed to more accurately evaluate the HXR flux for this type of discharge. We have simulated a similar LHCD discharge with the full wave code LHEAF (Lower Hybrid wavE Analysis based on FEM). This code, combined with the newly developed 3D Fokker-Planck (v∥, v⊥, r) and synthetic HXR diagnostic modules, to calculate the steady state electron distribution function in the plasma and the resulting HXR radiation spectrum. The simulated non-thermal x-ray proflle shave been found to be in good agreement with the measured experimental profile. In particular LHEAF simulations were able to reproduce the broad width of the measured HXR profile for a discharge with low n∥, which has been a long standing issue for LHCD simulations on Alcator C-Mod.

Edge turbulence in different density regimes in Alcator C-Mod experiment

Plasma edge turbulence of Alcator C-Mod tokamak is studied with a fast camera in different density regimes. The statistical properties of the fluctuations, as well as the behaviour of the blobs, are characterized in plasma discharges at different normalized densities, studying the link between the edge turbulence and the Greenwald limit. It is shown that approaching the Greenwald density limit, the edge velocity field measured with the cross-correlation technique changes and the strong fluctuations, which for standard discharges develop mainly outside the separatrix, extend also in the radial region inside the last closed flux surface. At the same time, the blobs cover a larger radial region, suggesting a strong impact of the edge turbulence and transport on the Greenwald limit.

Fluctuations and power spectra in edge plasmas

The high-frequency range of power spectra of turbulent fluctuating quantities measured at the edge of magnetized plasmas displays a variety of trends: from power laws with different spectral indices to exponential. We propose a model able to account for the whole phenomenology simply by tuning the distribution in the duration of the signal spikes. Comparisons with data from RFX-mod and Alcator C-Mod experiments are performed. An attempt to relate the statistics of the bursts with their generating mechanism is made.

Web based electronic logbook and experiment run database viewer for Alcator C-Mod

Since 1991, the scientists and engineers at the Alcator C-Mod experiment at MIT have been recording text entries about the experiments being performed in an electronic logbook. In addition, separate documents such as run plans, run summaries and experimental proposals have been created and stored in a variety of formats in computer files. This information has now been organized and made available via any modern web browser. The new web based interface permits the user to browse through all the logbook entries, run information and even view some key data traces of the experiment. Since this information is being catalogued by Internet search engines, these tools can also be used to quickly locate information. The web based logbook and run information interface provides some additional capabilities. Once logged into the web site, users can add, delete or modify logbook entries directly from their browser. The logbook window on their browser also provides dynamic updating when any new logbook entries are made. There is also live C-Mod operation status information with optional audio announcements available. The user can receive the same state change announcements such as “entering init” or “entering pulse” as they would if they were sitting in the C-Mod control room. This paper will describe the functionality of the web based logbook and how it was implemented.

Role of trapped electron mode turbulence in internal transport barrier control in the Alcator C-Mod Tokamak

Nonlinear gyrokinetic simulations of trapped electron mode (TEM) turbulence, within an internal particle transport barrier, are performed and compared with experimental data. The results provide a mechanism for transport barrier control with on-axis radio frequency heating, as demonstrated in Alcator C-Mod experiments [S. J. Wukitch et al., Phys. Plasmas 9, 2149 (2002)]. Off-axis heating produces an internal particle and energy transport barrier after the transition to enhanced Dα high confinement mode. The barrier foot reaches the half-radius, with a peak density 2.5 times the edge density. While the density profile peaks, the temperature profile remains relatively unaffected. The peaking and concomitant impurity accumulation are controlled by applying modest central heating power late in the discharge. Gyrokinetic turbulence simulations of the barrier formation phase, using the GS2 code [W. Dorland et al., Phys. Rev. Lett. 85, 5579 (2000)] show that toroidal ion temperature gradient driven modes are suppressed inside the barrier foot, but continue to dominate in the outer half-radius. As the density gradient steepens further, trapped electron modes are driven unstable. The onset of TEM turbulence produces an outflow that strongly increases with the density gradient, upon exceeding a new nonlinear critical density gradient, which significantly exceeds the linear critical density gradient. The TEM turbulent outflow ultimately balances the inward Ware pinch, leading to steady state. Moreover, the simulated turbulent particle diffusivity matches that inferred from particle balance using measured density profile data and the calculated Ware pinch. This turbulent diffusivity exhibits a strong unfavorable temperature dependence that allows control with central heating.

Microtearing modes and fast edge fluctuations in Alcator C-Mod plasmas

Magnetic fluctuations, observed in the Alcator C-Mod experiment, are modelledvia the instability of microtearing modes with ratio ofpoloidal to toroidal mode number m/n = 4. These modes are predominantly electromagnetic in nature, although the electrostatic component is non-vanishing, are driven by the steeptemperature gradient at the plasma edge and are shown to have a relativelybroad spectrum for typical experimental parameters.

Data acquisition system for Alcator C-Mod

The Alcator C-Mod experiment requires an efficient data handling system to acquire and distribute the large number of measurements recorded during each pulse of the tokamak. Over 2000 signals amounting to over 80 megabytes of data are stored and distributed to 40 workstations for display and analysis. With machine pulses occurring every 10 to 15 min, most of the information must be distributed within 2 to 3 min after the pulse to enable the experimentalist to adjust the control settings for the next pulse. In addition to the local control room data distribution requirements, the system must also provide data access to remote sites which monitor diagnostics installed on the Alcator C-Mod experiment. This article will describe the hardware and software used to accomplish data handling tasks.

Status of the neutron diagnostic experiment for Alcator C-Mod

The Alcator C-Mod experiment is expected to produce neutrons at a rate of up to 1 × 1016 n/s during its highest performance operation. The global neutron detection system must provide accurate measurement of this production over the full operating regime of the experiment, 1011–1016 n/s. This is accomplished using a series of uranium fission detectors enriched in U235 of staggered sensitivity. These are distributed in two moderator assemblies. A BF3 detector is added to each moderator to cover the lowest operational range. Preliminary calibration of the system has been accomplished using a 65.4 μg Cf252 source placed inside the Alcator C-Mod vacuum vessel. The system design and calibration is discussed herein.

MIT fusion γ-ray diagnostic development

In conjunction with γ-ray diagnostic development for the NOVA Upgrade and Alcator C-MOD experiments, we have assembled a comprehensive set of absolutely calibrated γ-ray sources and several γ-ray detectors. These tools will be used in characterizing and calibrating new γ-ray diagnostic systems as they are assembled. The γ-ray sources comprise both radioisotopes and nuclear reactions. The latter are generated in the MIT Cockcroft–Walton fusion-product generator. The detectors include several ‘‘standard’’ NaI(Tl) scintillators and a high-resolution germanium detector. This paper briefly discusses the diagnostics planned for NOVA Upgrade and Alcator C-MOD. In addition it describes the γ-ray sources and detector characterizations we have performed in the laboratory.

Neutral particle diagnostics for ALCATOR C-Mod

The ALCATOR C-Mod experiment will be equipped with two PPPL charge exchange neutral particle analyzers (CENAs), one of which views the plasma tangentially (Rtan/R0=1.05), whereas the second has a horizontally scannable sight line (0≤Rtan/R0≤0.51). The perpendicularly viewing CENA will be capable of analyzing neutrals up to 600 keV amu for up to three separate species simultaneously. Thus high-energy tails can be observed together with the bulk ion temperature. The operation of both analyzers will allow simultaneous measurements from both the perpendicular and tangential chords. The CENAs will be used to study the effect of ICRF heating on the ion energy distribution with emphasis on the high-energy tail. A Fokker–Planck code (FPPRF) [Hammett, Ph.D. thesis, Princeton (1986)] is used to assess the appropriate operating regime of the analyzer (n≤4×1020 m−3 for Ti=2 keV, for Maxwellian ion energy distribution). The experimental design and computer simulations will be detailed.

ICRF-edge plasma interaction studies at MIT

The Alcator C-Mod experiment will study the high power ICRF heating of elongated, diverted plasmas at high density in a compact, high field tokamak. ICRF heating will be carried out in the minority heating regime by fast wave excitation in a deuterium plasma with a 3He or H minority. Because of the compact size of the tokamak, high loading will be required to couple the power at acceptable voltages and there will be high power density at the antenna surface. High edge densities and steep edge gradients are expected, making the ICRF-edge plasma interactions a key issue. Several studies have been undertaken to better understand these effects. Measurements have been made of the short wavelength perturbations to the RF fields caused by various Faraday shield geometries. Magnetic field probes show a perturbation which is largest near the ends and between the shield elements where the RF magnetic field is mostly radial. Electric field probes show a perturbation which is larger in the center of the elements and close to each element. A test antenna is being installed in the Versator II tokamak to study ICRF-edge plasma effects. The RF will be excited in the IBW polarization with κ ‖ chosen for interaction with edge electrons. The properties of different Faraday shield coatings, including titanium carbide and boron nitride, will be explored. It will also be possible to bias the protection limiters of this antenna in order to study the effect of biasing on the plasma density in the vicinity of the Faraday shield and resultant impurity generation.