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. Additionally, the decrease in cross-sectional area of a magnetic flux tube connecting from low to high-field regions appears to act as a 'nozzle', increasing flow velocities in the high-field SOL. Secondary flows involving a combination of toroidal rotation and Pfirsch-Schlüter 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 toward (away from) the x-point. Depending on 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 toward (away from) the xpoint. Experiments suggest that topology-dependent flow boundary conditions may also play a role in the sensitivity of 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.

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 (ICRF) 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 (ELMs) 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 favorable elements of both. The I-mode regime is obtained 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 Lmode. 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 (B=3-6 T, I p =0.7-1.3 MA, q 95 =2.5-5). In general the I-mode exhibits the strongest edge T pedestal and normalized energy confinement (H 98 >1) at low q 95 (<3.5) and high heating power (P heat > 4 MW). I-mode significantly expands the operational space of ELM-free, stationary pedestals in C-Mod to T ped~1 keV and low collisionality ν* ped~0 .1, as compared to EDA H-mode with T ped < 0.6 keV, ν* ped >1. The I-mode global energy confinement has a relatively weak degradation with heating power; W th ~ I p P heat 0.7 leading to increasing H 98 with heating power.2

The SPARC tokamak is a critical next step towards commercial fusion energy. SPARC is designed as a high-field ( $B_0 = 12.2$ T), compact ( $R_0 = 1.85$ m, $a = 0.57$ m), superconducting, D-T tokamak with the goal of producing fusion gain $Q>2$ from a magnetically confined fusion plasma for the first time. Currently under design, SPARC will continue the high-field path of the Alcator series of tokamaks, utilizing new magnets based on rare earth barium copper oxide high-temperature superconductors to achieve high performance in a compact device. The goal of $Q>2$ is achievable with conservative physics assumptions ( $H_{98,y2} = 0.7$ ) and, with the nominal assumption of $H_{98,y2} = 1$ , SPARC is projected to attain $Q \approx 11$ and $P_{\textrm {fusion}} \approx 140$ MW. SPARC will therefore constitute a unique platform for burning plasma physics research with high density ( $\langle n_{e} \rangle \approx 3 \times 10^{20}\ \textrm {m}^{-3}$ ), high temperature ( $\langle T_e \rangle \approx 7$ keV) and high power density ( $P_{\textrm {fusion}}/V_{\textrm {plasma}} \approx 7\ \textrm {MW}\,\textrm {m}^{-3}$ ) relevant to fusion power plants. SPARC's place in the path to commercial fusion energy, its parameters and the current status of SPARC design work are presented. This work also describes the basis for global performance projections and summarizes some of the physics analysis that is presented in greater detail in the companion articles of this collection.

Anomalous momentum transport has been observed in Alcator C-Mod tokamak plasmas. The time evolution of core impurity toroidal rotation velocity profiles has been measured with a tangentially viewing crystal x-ray spectrometer array. Following the L-mode to EDA (enhanced D α ) H-mode transition in both Ohmic and ICRF heated discharges, the ensuing co-current toroidal rotation velocity, which is generated in the absence of any external momentum source, is observed to propagate in from the edge plasma to the core with a time scale of order of the observed energy confinement time, but much less than the neo-classical momentum confinement time. The ensuing steady state toroidal rotation velocity profiles in EDA H-mode plasmas are relatively flat, with V φ ∼ 50 km/s, and the momentum transport can be simulated with a simple diffusion model. Assuming the L-H transition produces an instantaneous edge source of toroidal torque (which disappears at the H-to L-mode transition), the momentum transport may be characterized by a diffusivity, with values of ∼ 0.07 m 2 /s during EDA H-mode and ∼ 0.2 m 2 /s in L-mode. These values are large compared to the calculated neoclassical momentum diffusivities, which are of order 0.003 m 2 /s. Velocity profiles of ELM-free H-mode plasmas are centrally peaked (with V φ (0) exceeding 100 km/s in some cases), which suggests the workings of an inward momentum pinch; the observed profiles are consistent with simulations including an edge inward convection velocity of ∼ 10 m/s. In EDA H-mode discharges which develop internal transport barriers, the velocity profiles become hollow in the center, indicating the presence of a negative radial electric field well in the vicinity of the barrier foot. Upper single null diverted and inner wall limited L-mode discharges exhibit strong counter-current rotation (with V φ (0) ∼ −60 km/s in some cases), which may be related to the observed higher Hmode power threshold in these configurations. For plasmas with locked modes, the toroidal rotation is observed to stop (V φ ≤5 km/s). 1