James Christian Reardon

Experimental investigation of transport phenomena in the scrape-off layer and divertor

Abstract Transport physics in the divertor and scrape-off layer of Alcator C-Mod is investigated for a wide range of plasma conditions. Parallel (∥) transport topics include: low recycling, high-recycling, and detached regimes, thermoelectric currents, asymmetric heat fluxes driven by thermoelectric currents, and reversed divertor flows. Perpendicular (⊥) transport topics include: expected and measured scalings of ⊥ gradients with local conditions, estimated χ⊥ profiles and scalings, divertor neutral retention effects, and L-mode/H-mode effects. Key results are: (i) classical ∥ transport is obeyed with ion-neutral momentum coupling effects, (ii) ⊥ heat transport is proportional to local gradients, (iii) χ⊥ αTe−0.6 n−0.6 L−0.7 in L-mode, insensitive to toroidal field, (iv) χ⊥ is dependent on divertor neutral retention, (v) H-mode transport barrier effects partially extend inside the SOL, (vi) inside/outside divertor asymmetries may be caused by a thermoelectric instability, and (vii) reversed ∥ flows depend on divertor asymmetries and their implicit ionization source imbalances.

Radiofrequency-heated enhanced confinement modes in the Alcator C-Mod tokamak

Enhanced confinement modes up to a toroidal field of BT=8 T have been studied with up to 3.5 MW of radiofrequency (rf) heating power in the ion cyclotron range of frequencies (ICRF) at 80 MHz. H-mode is observed when the edge temperature exceeds a threshold value. The high confinement mode (H-mode) with higher confinement enhancement factors (H) and longer duration became possible after boronization by reducing the radiated power from the main plasma. A quasi-steady state with high confinement (H=2.0), high normalized beta (βN=1.5), low radiated power fraction (Pradmain/Ploss=0.3), and low effective charge (Zeff=1.5) has been obtained in Enhanced Dα H-mode. This type of H-mode has enhanced levels of continuous Dα emission and very little or no edge localized mode (ELM) activity, and reduced core particle confinement time relative to ELM-free H-mode. The pellet enhanced performance (PEP) mode is obtained by combining core fueling with pellet injection and core heating. A highly peaked pressure profile with...

Comparison of detached and radiative divertor operation in Alcator C-Mod

The divertor of the Alcator C‐Mod tokamak [Phys. Plasmas 1, 1511 (1994)] routinely radiates a large fraction of the power entering the scrape‐off layer. This dissipative divertor operation occurs whether the divertor is detached or not, and large volumetric radiative emissivities, up to 60 MW m−3 in ion cyclotron range of frequency (ICRF) heated discharges, have been measured using bolometer arrays. An analysis of both Ohmic and ICRF‐heated discharges has demonstrated some of the relative merits of detached divertor operation versus high‐recycling divertor operation. An advantage of detached divertor operation is that the power flux to the divertor plates is decreased even further than its already low value. Some disadvantages are that volumetric losses outside the separatrix in the divertor region are decreased, the neutral compression ratio is decreased, and the penetration efficiency of impurities increases.

Electron heating via mode converted ion Bernstein waves in the Alcator C-Mod tokamak

Highly localized direct electron heating [full width at half-maximum (FWHM)≅0.2a] via mode converted ion Bernstein waves has been observed in the Alcator C-Mod Tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. Electron heating at or near the plasma center (r/a⩾0.3) has been observed in H(3He) discharges at B0=(6.0–6.5) T and ne(0)≅1.8×1020 m−3. [Here, the minority ion species is indicated parenthetically.] Off-axis heating (r/a⩾0.5) has also been observed in D(3He) plasmas at B0=7.9 T. The concentration of 3He in these experiments was in the range of n3He/ne≅(0.2–0.3) and the locations of the mode conversion layer and electron heating peak could be controlled by changing the 3He concentration or toroidal magnetic field (B0). The electron heating profiles were deduced using a rf modulation technique. Detailed comparisons with one-dimensional and toroidal full-wave models in the ion cyclotron range of frequencies have been carried out. One-dimensional full-wave code predictions were found to ...

Mode conversion electron heating in Alcator C-Mod: Theory and experiment

Localized electron heating [full width at half maximum of Δ(r/a)≈0.2] by mode converted ion Bernstein waves (IBW) has been observed in the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. These experiments were performed in D(3He) plasmas at high magnetic field (B0=7.9 T), high-plasma density (ne0⩾1.5×1020 m−3), and for 0.05⩽nHe-3/ne⩽0.30. Electron heating profiles of the mode converted IBW were measured using a break in slope analysis of the electron temperature versus time in the presence of rf (radio frequency) modulation. The peak position of electron heating was found to be well-correlated with 3He concentration, in agreement with the predictions of cold plasma theory. Recently, a toroidal full-wave ion cyclotron range of frequencies (ICRF) code TORIC [M. Brambilla, Nucl. Fusion 38, 1805 (1998)] was modified to include the effects of IBW electron Landau damping at (k⊥ρi)2≫1, This model was used in combination with a 1D (one-dimensional) integral wave equation code METS [...

Electron cyclotron discharge cleaning (ECDC) experiments on Alcator C-Mod

Abstract Experiments were performed on Alcator C-Mod with electron cyclotron resonance (ECR) plasmas to help determine their applicabilty to a fusion reactor. Strong radial inhomogeneity of the plasma density was measured, decreasing by a factor of ten a few centimeters inside the resonance location, but remaining approximately constant ( n e ≈10 16 m −3 ) outside the resonance location. Electron temperature remained mostly constant outside the resonance location, T e ≈10 eV ; ion temperature increased outside the resonance location from T i ≈2 eV to 10 eV. Toroidal asymmetries in ion saturation current density were observed, indicating local toroidal plasma flow. The ECR plasma was used to remove a diamond-like carbon coating from a stainless-steel sample. Removal rates peaked at 4.2±0.4 nm/h with the sample a few centimeters outside the resonance location. Removal rates decreased inside and further outside the resonance location. The plasma did not remove the carbon from the sample uniformly, possibly due to plasma flow. Yields were calculated ( Y ≈10 −3 ) to be lower than other published results for chemical sputtering of deuterium ions on carbon, possibly due to toroidally asymmetric plasma conditions.

Observations of edge ion heating during ICRH in Alcator C-Mod

Observations of edge ion tails during ICRF injection on Alcator C-Mod have been made using a toroidally and poloidally scanning charge-exchange neutral particle analyzer. The ion tails create a large flux of charge-exchange neutrals (hydrogen and deuterium), at suprathermal energies, with a short rise time (⩽0.2 ms, the instrumental time resolution), but are not associated with impurity generation or loss of heating efficiency. For most values of toroidal field, there is an RF power threshold of 500kW for edge tail production, but this decreases to <10 kW at certain fields. A dedicated experiment was performed to map the poloidal and pitch-angle dependence of the escaping energetic neutrals as a function of RF power at a toroidal field where the edge-heating power threshold is low. The ion tails generated in this experiment are a mix of ripple-trapped, banana-trapped, and passing particles. The transport and loss mechanisms for these classes of particles are used to infer the spatial dependence of the par...

Testing and Conditioning of the First Alcator C‐Mod ICRF Antenna

The first antenna for ICRF heating in the Alcator C‐Mod tokamak has been fabricated and testing and conditioning has begun. Operational flexibility and protection from disruptions during initial C‐Mod operation mandated that the first antenna be radially movable. Space limitations made it necessary to use a single current strap for the movable antenna. This antenna will be used to study the effects of antenna radial position and plasma shape on coupling, as well as power handling limits and impurity generation during the first phase of Alcator C‐Mod operation. During initial low power testing, the antenna characteristic impedance and phase velocity will be measured and the length of the external resonant loop driving circuit will be determined. At power levels up to 1 kW, the antenna rf diagnostics will be calibrated and tuning procedures established. Then vacuum conditioning and voltage handling studies will be carried out in preparation for mounting the antenna in Alcator C‐Mod.

Fast wave transmisssion measurements on the Alcator C-Mod tokamak

Abstract During fundamental minority fast-wave heating in D(H), the fraction T of power transmitted through the plasma core (measured by loop probes on the inner wall) decreases with increasing minority concentration, in agreement with the predictions of a simple analytic theory and a full wave code. T also varies synchronously with the sawtooth instability in the plasma center.

ICRF heating in Alcator C-Mod: Present status and future prospects

Alcator C-Mod, the high field, high density, diverted, compact tokamak in the world’s portfolio of high performance plasma fusion devices, is heated exclusively with ICRF auxiliary power. In this paper an overview of recent results is summarized, with particular attention given to the importance of RF operation and the flexibility afforded by different heating scenarios. Besides the routine minority heating operation, results in the mode conversion heating regime are also presented (mainly direct electron heating through mode converted ion Bernstein waves). Recent attempts at improving plasma performance by establishing internal transport barriers (ITBs) by various transient profile control techniques (the so-called Advanced Tokamak mode of operation) are also presented. Future improvements in performance afforded by the recent addition of a new 4-strap antenna and 4 MW of tunable (40–80 MHz) ICRF power are also discussed. Mode-conversion current drive (MCCD) and fast wave current drive (FWCD) will be amo...