C. Kurz

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.

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.

Dissipative divertor operation in the Alcator C-Mod tokamak

The achievement of large volumetric power losses (dissipation) in the Alcator C-Mod divertor region is demonstrated in two operational modes: radiative divertor and detached divertor. During radiative divertor operation, the fraction of SOL power lost by radiation is P R /P SOL 0.8 with single null plasmas, n e < 2 × 10 20 m −3 and I p < 1 MA. THESE PLASMAS SOMETIMES HAVE VERY HIGH RECYCLING, WITH N e, div ≤ 6 x 10 20 m −3 . As the divertor radiation and density increase, the plasma eventually detaches abruptly from the divertor plates: I SAT drops at the target and the divertor radiation peak moves to the X-point region. Probe measurements at the divertor plate show that the transition occurs when T e 5 eV. The critical n e for detachment depends linearly on the input power. This abrupt divertor detachment is preceded by a comparatively long period (∼ 1-200 ms) where a partial detachment is observed to grow at the outer divertor plate

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.

Density measurements in the edge, divertor and X‐point regions of Alcator C‐Mod from Balmer series emission

The electron density in the edge, divertor and X‐point regions of Alcator C‐Mod [Proceedings of the IEEE 13th Symposium on Fusion Engineering (Institute of Electrical and Electronic Engineers, New York, 1990), Vol. 1, p. 13] has been measured using the Stark broadening of high‐n (n=8 through 11) transitions of the Balmer series of deuterium. These measurements have been made during typical single null, diverted operation and during gas‐puffing through capillaries located within the first wall and divertor. Electron densities up to 1.6×1021 m−3 have been measured in the X‐point region and slightly lower densities (5−7×1020 m−3) have been measured in the divertor and edge regions. These results are factors of 2 to 5 larger than the density measurements from the CO2 laser interferometer system in the main chamber (3×1020 m−3) and comparable with probe measurements in the divertor. The ratios of the widths of adjacent lines within the Balmer series have been compared with ratios calculated for the Stark broad...

Transport experiments in Alcator‐C‐Mod

A series of transport experiments has been carried out in Alcator‐C‐Mod. [Phys Plasmas 1, 1511 (1994)]. Data from both Ohmic and ICRF (ion cyclotron range of frequencies) heated plasmas can be fitted with an L‐mode (low mode) scaling law. The Ohmic τE’s show no scaling with density in any regime and can reach values of 2–3 times neo‐Alcator. Impurity confinement has been studied with the laser blow‐off technique with τI showing nearly linear scaling with plasma current. Ohmic and ICRF H modes are obtained over a wide range of discharge parameters, extending the range in the international database for nB, by almost a factor of 10. The power threshold for ELM‐free (edge localized mode) discharges is in rough agreement with the scaling P/S=0.044nB. Energy diffusivities of Ohmic and ICRF heated plasmas have been measured from local analysis of plasma profiles and power fluxes. The same analysis produces a value for plasma resistivity which lies between the Spitzer and neoclassical calculations. Analysis of pl...

Power balance and scaling of the radiated power in the divertor and main plasma of Alcator C-Mod

Abstract Measurements have been made of the radiation in the main plasma and divertor of Alcator C-Mod using bolometer arrays. It is found that the power radiated from the main plasma is Prad,main/Pohmic ≈ 0.3–0.4 over the range 0.2 × 10 20 ≤ n e ≤ 2.5 × 10 20 m −3 . There is significant radiated power coming from the divertor region; Prad,divertor/Pohmic ≈ 0.4. The radiated power in the main plasma and divertor scales linearly with density. When divertor detachment occurs, the power flow to the divertor plates in the region near the strike points is suddenly reduced, the fraction of the input power radiated from the main plasma and divertor increases, and the radiation in the divertor region is spatially rearranged.

The visible, imaging diode arrays on Alcator C‐Mod

A set of multichord, absolutely calibrated viewing arrays has been used to measure the distributions of H/D and C+2 emissions. A total of 227 chordal views are available, with the chords separated at the plasma by ∼1 cm typically. The chord brightness profiles have been combined and inverted (without recourse to symmetry assumptions) to yield local volume emission rates. Because of the thickness of the superstructure surrounding Alcator C‐Mod, the views are from points re‐entrant to the vacuum vessel. All but one of the arrays employ 64‐channel, linear diode arrays, which are read out serially, thus requiring only one digitizer channel per array. Variable frame rates (∼1 Hz to ∼3.5 kHz) result in an extremely large dynamic range for these detectors. A 35‐channel diode array which is read out in parallel and can track fast events like ELMs or pellets is also in use.

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.

Plasma-surface interactions in the Alcator C-Mod tokamak

Abstract Alcator C-Mod is a high-field tokamak with which experiments were initiated in April 1993. The divertor and the first-wall have plasma-facing surfaces of molybdenum. The divertor has a configuration which is closed in comparison to other, currently operating tokamaks. The combination of divertor geometry and high field (density) make Alcator C-Mod an ideal experiment to investigate dissipative (radiative/CX) divertor scenarios. Single-null divertor operation has become the standard mode of operation. Relatively clean plasmas ( Z eff ≤ 1.3) are obtained with short periods of baking and electron cyclotron discharge cleaning only. Impurity source rates and screening have been investigated with an extensive set of wavelength-filtered diode arrays and spectrographs. Carbon and oxygen and molybdenum dominate the impurity levels. The molybdenum source from the divertor proper is negligible.