B. Welch

Characterization of enhanced Dα high-confinement modes in Alcator C-Mod

Regimes of high-confinement mode have been studied in the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. Plasmas with no edge localized modes (ELM-free) have been compared in detail to a new regime, enhanced Dα (EDA). EDA discharges have only slightly lower energy confinement than comparable ELM-free ones, but show markedly reduced impurity confinement. Thus EDA discharges do not accumulate impurities and typically have a lower fraction of radiated power. The edge gradients in EDA seem to be relaxed by a continuous process rather than an intermittent one as is the case for standard ELMy discharges and thus do not present the first wall with large periodic heat loads. This process is probably related to fluctuations seen in the plasma edge. EDA plasmas are more likely at low plasma current (q>3.7), for moderate plasma shaping, (triangularity ∼0.35–0.55), and for high neutral pressures. As observed in soft x-ray emission, the pedestal width is found to scale with the same parameters that determine the EDA/ELM-free boundary.

Scaling and transport analysis of divertor conditions on the Alcator C-Mod tokamak

Detailed measurements and transport analysis of divertor conditions in Alcator C‐Mod [Phys. Plasmas 1, 1511 (1994)] are presented for a range of line‐averaged densities, 0.7<ne<2.2×1020 m−3. Three parallel heat transport regimes are evident in the scrape‐off layer: sheath‐limited conduction, high‐recycling divertor, and detached divertor, which can coexist in the same discharge. Local cross‐field pressure gradients are found to scale simply with a local electron temperature. This scaling is consistent with classical electron parallel conduction being balanced by anomalous cross‐field transport (χ⊥∼0.2 m2 s−1) proportional to the local pressure gradient. A 60%–80% of divertor power is radiated in attached discharges, approaching 100% in detached discharges. Detachment occurs when the heat flux to the plate is low and the plasma pressure is high (Te∼5 eV). High neutral pressures in the divertor are nearly always present (1–20 mTorr), sufficient to remove parallel momentum via ion–neutral collisions.

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 ...

High confinement dissipative divertor operation on Alcator C-Mod

Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] has operated a High-confinement-mode (H-mode) plasma together with a dissipative divertor and low core Zeff. The initially attached plasma is characterized by steady-state enhancement factor, HITER89P [P. N. Yushmanov et al., Nucl. Fusion 30, 1999 (1990)], of 1.9, central Zeff of 1.1, and a radiative fraction of ∼50%. Feedback control of a nitrogen gas puff is used to increase radiative losses in both the core/edge and divertor plasmas in almost equal amounts. Simultaneously, the core plasma maintains HITER89P of 1.6 and Zeff of 1.4 in this nearly 100% radiative state. The power and particle flux to the divertor plates have been reduced to very low levels while the core plasma is relatively unchanged by the dissipative nature of the divertor.

Impurity transport in the divertor of the Alcator C-Mod tokamak

Abstract Argon has been injected into the private flux zone of the Alcator C-Mod divertor and its transport into the confined plasma studied. The injected gas pulse lasts for about 200 ms, but the impurity concentration increases to an equilibrium level and remains there for ∼ 200 ms. It is clear that the argon is acting as a recycling impurity. Nevertheless it has been observed that less than 0.5% of the injected atoms reach the confined plasma and that the percentage decreases with increasing plasma density. The good impurity screening is discussed in terms of the ionisation mean free path in the divertor.

Impurity sources at the first wall of Alcator C-Mod and their effect on the central plasma

In this paper we present data concerning impurity sources and transport in a tokamak with a divertor and molybdenum first wall. A spectrometer, viewing visible and UV light, has been used to take time dependent spectra of C, O and Mo from both the divertor and the inner limiter surfaces. In the divertor the sputtering threshold for Mo is not normally reached (T e ≤ 25 eV). Chord measurements from four filtered photo-diode arrays have enabled us to invert the measured brightness profiles tomographically and obtain 2D emissivity information for H α and CIII. The total C flux sputtered at the divertor target can be twice as high as from the inner wall surface, increasing with density to about 2 × 10 18 s −1 . However, modeling the SOL, we find that the penetration efficiency for C is roughly 10 times smaller for the divertor than for the inner wall. Oxygen fluxes are generally observed to be 1/3 of the C flux

ICRF heating in the Alcator C‐Mod tokamak

ICRF heating experiments have been carried out in the Alcator C‐Mod tokamak at power levels up to 3.5 MW. Features of Alcator C‐Mod include high density operation, molybdenum plasma facing components, and a closed divertor configuration. The heating is accomplished with two two‐strap antennas each run with dipole phasing at 80 MHz in deuterium plasmas with a hydrogen minority resonant at 5.3 T. Plasmas with Ti=4 keV and Te=5 keV at ne=1×1020 m−3 have been produced with 3.5 MW of heating power. The heating has been shown to be strongest with a low minority concentration (1–5%) and the resonance on axis. For ne≳2×1020 m−3, the central Zeff remains below 1.5 with more than 2.5 MW of applied rf power. L‐mode confinement scaling is observed both for ohmic and ICRF‐heated plasmas with some deterioration at higher densities. H‐mode transitions have been produced with the threshold for ELM‐free H‐modes at or below the ASDEX/DIII‐D scaling. Enhanced confinement with strongly peaked density profiles has been achi...

ICRF heated enhanced performance modes and mode conversion electron heating in alcator C-mod

D(H) minority heating experiments were performed at BT=5.3 T with up to 3.5 MW of RF power. The highest stored energy of 130 kJ was achieved in an ELM‐free H‐mode plasma. The H‐mode power threshold is roughly consistent with the ITER scaling, P/S=0.044neBT, but H‐modes have been obtained at power levels as much as a factor of two below this scaling. H‐factors of up to 1.5 have been observed in ELM‐free H‐mode plasmas. The highest fusion reactivity of 9×1013 sec−1 was obtained in a PEP H‐mode plasma with lithium pellet injection and on‐axis ICRF heating. These discharges are characterized by highly peaked density and ion temperature profiles, with fusion reactivity enhanced by typically an order of magnitude above similar L‐mode discharges. Effective localized direct electron heating by mode converted IBW was observed in H‐3He plasmas (analogous to D‐T plasmas) at 6.5 T.