B. Lipschultz

Chapter 4: Power and particle control

Progress, since the ITER Physics Basis publication (ITER Physics Basis Editors et al 1999 Nucl. Fusion 39 2137–2664), in understanding the processes that will determine the properties of the plasma edge and its interaction with material elements in ITER is described. Experimental areas where significant progress has taken place are energy transport in the scrape-off layer (SOL) in particular of the anomalous transport scaling, particle transport in the SOL that plays a major role in the interaction of diverted plasmas with the main-chamber material elements, edge localized mode (ELM) energy deposition on material elements and the transport mechanism for the ELM energy from the main plasma to the plasma facing components, the physics of plasma detachment and neutral dynamics including the edge density profile structure and the control of plasma particle content and He removal, the erosion of low- and high-Z materials in fusion devices, their transport to the core plasma and their migration at the plasma edge including the formation of mixed materials, the processes determining the size and location of the retention of tritium in fusion devices and methods to remove it and the processes determining the efficiency of the various fuelling methods as well as their development towards the ITER requirements. This experimental progress has been accompanied by the development of modelling tools for the physical processes at the edge plasma and plasma–materials interaction and the further validation of these models by comparing their predictions with the new experimental results. Progress in the modelling development and validation has been mostly concentrated in the following areas: refinement in the predictions for ITER with plasma edge modelling codes by inclusion of detailed geometrical features of the divertor and the introduction of physical effects, which can play a major role in determining the divertor parameters at the divertor for ITER conditions such as hydrogen radiation transport and neutral–neutral collisions, modelling of the ion orbits at the plasma edge, which can play a role in determining power deposition at the divertor target, models for plasma–materials and plasma dynamics interaction during ELMs and disruptions, models for the transport of impurities at the plasma edge to describe the core contamination by impurities and the migration of eroded materials at the edge plasma and its associated tritium retention and models for the turbulent processes that determine the anomalous transport of energy and particles across the SOL. The implications for the expected performance of the reference regimes in ITER, the operation of the ITER device and the lifetime of the plasma facing materials are discussed.

First results from Alcator-C-MOD

Early operation of the Alcator‐C‐MOD tokamak [I.H. Hutchinson, Proceedings of IEEE 13th Symposium on Fusion Engineering, Knoxville, TN, edited by M. Lubell, M. Nestor, and S. Vaughan (Institute of Electrical and Electronic Engineers, New York, 1990), Vol. 1, p. 13] is surveyed. Reliable operation, with plasma current up to 1 MA, has been obtained, despite the massive conducting superstructure and the associated error fields. However, vertical disruptions are not slowed by the long vessel time constant. With pellet fueling, peak densities up to 9×1020 m−3 have been attained and ‘‘snakes’’ are often seen. Initial characterization of divertor and scrape‐off layer is presented and indicates approximately Bohm diffusion. The edge plasma shows a wealth of marfe‐like phenomena, including a transition to detachment from the divertor plates with accompanying radiative divertor regions. Energy confinement generally appears to exceed the expectations of neo‐Alcator scaling. A transition to Ohmic H mode has been observed. Ion cyclotron heating experiments have demonstrated good power coupling, in agreement with theory.

Transport-driven Scrape-Off-Layer flows and the boundary conditions imposed at the magnetic separatrix in a tokamak plasma

Plasma profiles and flows in the low- and high-field side scrape-off-layer (SOL) regions in Alcator C-Mod are found to be remarkably sensitive to magnetic separatrix topologies (upper-, lower- and double-null) and to impose topology-dependent flow boundary conditions on the confined plasma. Near-sonic plasma flows along magnetic field lines are observed in the high-field SOL, with magnitude and direction clearly dependent on X-point location. The principal drive mechanism for the flows is a strong ballooning-like poloidal transport asymmetry: parallel flows arise so as to re-symmetrize the resulting poloidal pressure variation in the SOL. Secondary flows involving a combination of toroidal rotation and Pfirsch–Schluter ion currents are also evident. As a result of the transport-driven parallel flows, the SOL exhibits a net co-current (counter-current) volume-averaged toroidal momentum when B × ∇B is towards (away from) the X-point. Depending on the discharge conditions, flow momentum can couple across the separatrix and affect the toroidal rotation of the confined plasma. This mechanism accounts for a positive (negative) increment in central plasma co-rotation seen in L-mode discharges when B × ∇B is towards (away from) the X-point. Experiments in ion-cyclotron range-of-frequency-heated discharges suggest that topology-dependent flow boundary conditions may also play a role in the sensitivity of the L–H power threshold to X-point location: in a set of otherwise similar discharges, the L–H transition is seen to be coincident with central rotation achieving roughly the same value, independent of magnetic topology. For discharges with B × ∇B pointing away from the X-point (i.e. with the SOL flow boundary condition impeding co-current rotation), the same characteristic rotation can only be achieved with higher input power.

Principal physics developments evaluated in the ITER design review

As part of the ITER Design Review and in response to the issues identified by the Science and Technology Advisory Committee, the ITER physics requirements were reviewed and as appropriate updated. The focus of this paper will be on recent work affecting the ITER design with special emphasis on topics affecting near-term procurement arrangements. This paper will describe results on: design sensitivity studies, poloidal field coil requirements, vertical stability, effect of toroidal field ripple on thermal confinement, material choice and heat load requirements for plasma-facing components, edge localized modes control, resistive wall mode control, disruptions and disruption mitigation.

Review of MARFE phenomena in tokamaks

Abstract This paper presents a review of MARFEs and detached plasmas. A MARFE is a toroidally symmetric tokamak phenomenon occuring at minor radii near the plasma boundary, defined by a limiter or magnetic separatrix. The MARFE poloidal width is approximately 30° in extent at the smaller major radius edge of the plasma. It is characterized by greatly increased radiation, high ion densities and density fluctuations, and relatively low electron temperature. These changes in the edge plasma are consistent with the MARFE being the manifestation of a thermal instability. MARFEs tend to occur in most tokamaks at similar values of ρ = n e κJ , a fraction of the density disruption limit. A related phenomenon, the detached plasma, is different in that the boundary layer radiation emission is poloidally symmetric. The radiated power, in this case, is approximately equal to the input power, thereby reducing power flows to the limiter to negligible values. A review of experimental data and theoretical treatments of MARFEs and detached plasmas are provided herein. The relationship between boundary layer parameter scalings, the MARFE threshold and the density limit is explored.

Particle transport in the scrape-off layer and its relationship to discharge density limit in Alcator C-Mod

Cross-field particle transport in the scrape-off layer (SOL) of Alcator C-Mod [Phys. Plasmas 1, 1511 (1994)] can be characterized by an effective particle diffusivity (Deff) that increases markedly with distance from the separatrix. As a consequence, recycling onto the main-chamber walls is large compared to plasma flows into the divertor volume. The SOL exhibits a two-layer structure: Steep gradients and moderate fluctuation levels are typically found in a ∼5 mm region near the separatrix (near SOL) where parallel electron conduction typically dominates energy losses. Small gradients and larger fluctuation levels with longer correlation times are found outside this region (far SOL). Deff in the near SOL increases strongly with local plasma collisionality normalized to the magnetic connection length. As the discharge density limit is approached, Deff and associated fluctuation levels become large across the entire SOL and cross-field heat convection everywhere exceeds parallel conduction losses, impacting...

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.

I-mode: an H-mode energy confinement regime with L-mode particle transport in Alcator C-Mod

An improved energy confinement regime, I-mode, is studied in Alcator C-Mod, a compact high-field divertor tokamak using ion cyclotron range of frequencies (ICRFs) auxiliary heating. I-mode features an edge energy transport barrier without an accompanying particle barrier, leading to several performance benefits. H-mode energy confinement is obtained without core impurity accumulation, resulting in reduced impurity radiation with a high-Z metal wall and ICRF heating. I-mode has a stationary temperature pedestal with edge localized modes typically absent, while plasma density is controlled using divertor cryopumping. I-mode is a confinement regime that appears distinct from both L-mode and H-mode, combining the most favourable elements of both. The I-mode regime is investigated predominately with ion ∇B drift away from the active X-point. The transition from L-mode to I-mode is primarily identified by the formation of a high temperature edge pedestal, while the edge density profile remains nearly identical to L-mode. Laser blowoff injection shows that I-mode core impurity confinement times are nearly identical with those in L-mode, despite the enhanced energy confinement. In addition, a weakly coherent edge MHD mode is apparent at high frequency ~100–300 kHz which appears to increase particle transport in the edge. The I-mode regime has been obtained over a wide parameter space (BT = 3–6 T, Ip = 0.7–1.3 MA, q95 = 2.5–5). In general, the I-mode exhibits the strongest edge temperature pedestal (Tped) and normalized energy confinement (H98 > 1) at low q95 ( 4 MW). I-mode significantly expands the operational space of edge localized mode (ELM)-free, stationary pedestals in C-Mod to Tped ~ 1 keV and low collisionality , as compared with EDA H-mode with Tped . The I-mode global energy confinement has a relatively weak degradation with heating power; leading to increasing H98 with heating power.

Volume recombination and opacity in Alcator C-Mod divertor plasmas

Volume recombination within the Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] divertor plasma is measured and found to be a significant fraction of the total ion sink under detached divertor conditions. The recombination occurs in regions where Te∼1 eV and ne∼1021 m−3. Measurements of the spatial distribution of the recombination are presented. The determinations of the recombination rates are made by measuring the D0 Balmer spectrum and by using a collisional radiative model describing the level populations, ionization, and recombination of D0. The concept of “recombinations per Balmer series photon” is developed to simplify the determinations. Measurements of the opacity of Lyβ emission are presented. It is observed that up to 50% of the Lyβ emission is trapped, indicating that Lyα is strongly trapped in some cases. The effects of Lyα,β trapping on the “recombinations per photon” curves are calculated and considered in the recombination rate determinations. Observations indicatin...

Edge radial electric field structure and its connections to H-mode confinement in Alcator C-Mod Plasmas

High-resolution charge-exchange recombination spectroscopic measurements of B5+ ions have enabled the first spatially resolved calculations of the radial electric field (Er) in the Alcator C-Mod pedestal region [E. S. Marmar, Fusion Sci. Technol. 51, 261 (2006)]. These observations offer new challenges for theory and simulation and provide for important comparisons with other devices. Qualitatively, the field structure observed on C-Mod is similar to that on other tokamaks. However, the narrow high-confinement mode (H-mode) Er well widths (5 mm) observed on C-Mod suggest a scaling with machine size, while the observed depths (up to 300 kV/m) are unprecedented. Due to the strong ion-electron thermal coupling in the C-Mod pedestal, it is possible to infer information about the main ion population in this region. The results indicate that in H-mode the main ion pressure gradient is the dominant contributor to the Er well and that the main ions have significant edge flow. C-Mod H-mode data show a clear correl...