Radiative divertor and scrape-off layer experiments in open and baffled divertors on DIII-D (original) (raw)

Divertor plasma studies on DIII-D: experiment and modelling

Plasma Physics and Controlled Fusion, 1997

In a magnetically diverted tokamak, the scrape-off layer (SOL) and divertor plasma provides separation between the first wall and the core plasma, intercepting impurities generated at the wall before they reach the core plasma. The divertor plasma can also serve to spread the heat and particle flux over a large area of divertor structure wall using impurity radiation and neutral charge exchange, thus reducing peak heat and particle fluxes at the divertor strike plate. Such a reduction will be required in the next generation of tokamaks, for without it, the divertor engineering requirements are very demanding. To successfully demonstrate a radiative divertor, a highly radiative condition with significant volume recombination must be achieved in the divertor, while maintaining a low impurity content in the core plasma.

Influence of Plasma Composition on Divertor Detachment

2000

Abstract The phenomena that lead to the observed power and particle flux detachment have been studied with the code B2-Eirene for DIII-D Helium and Deuterium experiments. Contrary to the usual Deuterium experiments, a significant reduction of the power load to the divertor is observed in Helium discharges while the ion flux remains high. Modelling indicates that this is due to the longer ionisation mean free path of Helium, which can penetrate from the divertor into the bulk plasma with the consequent power loss.

Compatibility of the radiating divertor with high performance plasmas in DIII-D

Journal of Nuclear Materials, 2007

A radiating divertor approach was successfully applied to high performance 'hybrid' plasmas [M.R. Wade et al., in: Proceedings of the 20th IAEA Fusion Energy Conference, Vilamoura, Portugal, 2004]. Our techniques included: (1) injecting argon near the outer divertor target, (2) enhancing the plasma flow into the inner and outer divertors by a combination of particle pumping and deuterium gas puffing upstream of the divertor targets, and (3) isolating the inner divertor from the outer by a structure in the private flux region. Good hybrid conditions were maintained, as the peak heat flux at the outer divertor target was reduced by a factor of 2.5; the peak heat flux at the inner target decreased by 20%. This difference was caused by a higher concentration of argon at the outer target than at the inner target. Argon accumulation in the main plasma was modest (n Ar /n e 6 0.004 on axis), although the argon profile was more peaked than the electron profile.

Comprehensive 2D measurements of radiative divertor plasmas in DIII-D

Journal of Nuclear Materials, 1997

This paper presents a comparison of the total radiated power profile and impurity line emission distributions in the SOL and divertor of DIII-D. This is done for ELMing H-mode plasmas with heavy deuterium injection (Partially Detached Divertor operation, PDD) and those without deuterium puffing. Results are described from a series of dedicated experiments performed on DIII-D to systematically measure the 2-D (R,Z) structure of the divertor plasma. The discharges were designed to optimize measurements with new divertor diagnostics including a divertor Thomson scattering system. Discharge sequences were designed to produce optimized data sets against which SOL and divertor theories and simulation codes could be benchmarked. During PDD operation the regions of significant radiated power shift from the inner divertor leg and SOL to the outer leg and X-point regions. Da emission shifts from the inner strikepoint to the outer strikepoint. Carbon emissions (visible CII and CIII) shift from the inner SOL near the X-point to a distributed region from the X-point to partially down the outer leg during moderate D2 puffing. In heavy puffing discharges the carbon emission coalesces on the outer separatrix near the X-point and for very heavy puffing it appears inside the last closed flux surface above the X-point. Calibrated spectroscopic measurements indicate that hydrogenic and carbon radiation can account for all of the radiated power. La and CIV radiation are comparable and when combined account for as much as 90% of the total radiated power along chords viewing the significant radiating regions of the outer leg.

On Modeling of Plasma Edge Conditions at Divertors with Solid Metal Neutralizers

Fusion Technology, 1992

It is widely recognized that results from divertor models can be quite sensitive to the boundary conditions that are assumed at the divertor neutralizer plate. However, some past models assumed electron and ion heat transmission coefficients with little justification. In fact, energy and momentum fluxes from backscattered neutral deuterium and tritium atoms can significantly contribute to the energy and momentum balance of the divertor plasma and consequently affect the estimates of steady-state plasma conditions. In illustration of this point, a two-point model similar to that of Galambos and Peng is rederived, including momentum and energy sources from chargeexchange and a self-consistent fluid treatment of the sheath heat transmission coefficients. Divertor conditions associated with the International Tokamak Reactor (INTOR) and International Thermonuclear Experimental Reactor (ITER)-like fusion reactors are estimated, and the effects of including the backscattered fluxes are discussed.

Radiative Divertor Plasma Behavior in L- and H-Mode Discharges with Argon Injection in EAST

Plasma Science and Technology, 2013

Introducing strong radiative impurities into divertor plasmas has been considered as an important way to mitigate the peak heat load at the divertor target plate for ITER, and will be employed in EAST for high power long pulse operations. To this end, radiative divertor experiments were explored under both low (L) and high (H)-mode confinement regimes, for the first time in EAST, with the injection of argon and its mixture (25% Ar in D2). The Ar injection greatly reduced particle and heat fluxes to the divertor in L-mode discharges, achieving nearly complete detached divertor plasma regimes for both single null (SN) and double null (DN) configurations, without increasing the core impurity content. In particular, the peak heat flux was reduced by a factor of ∼6, significantly reducing the intrinsic in-out divertor asymmetry for DN, as seen by both the new infra-red camera and the Langmuir probes at the divertor target. Promising results have also been obtained in the H-modes with argon seeding, demonstrating a significant increase in the frequency and decrease in the amplitude of the edge localized modes (ELMs), thus reducing both particle and heat loads caused by the ELMs. This will be further explored in the next experimental campaign with increasing heating power for long pulse operations.

A Comprehensive 2-D Divertor Data Set from DIII-D for Edge Theory Validation

Contributions to Plasma Physics, 1996

Portions of this document may be illegible in electronic imRge products. b a g = are produced from the bgst available originat domnnenL Abstract A comprehensive set of experiments has been carried out on the DIII-D tokamak to measure the 2-D (R,Z) structure of the divertor plasma in a systematic way using new diagnostics. Measurements cover the divertor radially from inside the X-point to the outer target plate and vertically from the target plate to above the X-point. Identical, repeatable shots were made, each having radial sweeps of the X-point and divertor strike points, to allow complete plasma and radiation profile measurements. Data have been obtained in ohmic, L-mode, ELMing H-mode, and reversed BToperation (VB drift away from the X-point).

Initial performance results of the DIII-D Divertor 2000

Journal of Nuclear Materials, 2001

A major upgrade of the DIII-D divertor, with the goal of enhancing impurity and density control and increasing the thermal pulse length limit of Advanced Tokamak (AT) plasmas has been successfully completed and commissioned. The integrated system that includes independent cryopumps at both the inner and the outer leg of the divertor, private flux region and outboard baffles, and improved graphite divertor armor, has been successfully applied to a variety of plasma conditions. Comparison of similar discharges before and after the upgrades show that with the new divertor the core plasma neutral source and carbon content are lower by as much as 50%. Calculations supported by preliminary IR camera measurements show that the new graphite armor design increases the limit on the discharge duration, due to temperature of the tile edges reaching sublimitation point, by an order of magnitude. With the new system we have been able to control density of high confinement H-mode plasmas to less than 1/3 of the Greenwald limit. It is observed that with divertor pumping during the current ramp phase the wall particle inventory and consequently the density rise after the H-mode transition can be significantly reduced. M.A. MAHDAVI, et al.