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.

Sphalerite q-hexagonal pyrrhotite ñ pyrite were recrystalHzed hydrothermally between 322 ø and 584 ø C by transport across a temperature gradient of about 12 ø C. Sphalerite compositions in equilibrium with pyrrhotite are close to those found by Barton and Toulmin (1966) by extrapolation from above 580 ø C; the composition of sphalerite is given by Mole % FeS --72.26695 --15900.5/T q-0.01448 log [s2 --0.38918 (10S/T 2) --(7205.5/T) log rs2 --0.34486 (log/s•) • and is consistent with our data and those of Barton and Toulmin. Addition of pyrite to the system restricts sphalerite composition to a constant value of 20.7 ñ 0.6 mole % FeS below 550 ø C, in agreement with Boorman's (1967) results; therefore, the slope of the sphalerite q-pyrite q-pyrrhotite phase boundary cannot be used as a geothermometer. However, the pressure effect of the FeS-content of sphalerite is large, making sphalerite a useful geobarometer when in equilibrium with pyrite and hexagonal pyrrhotite. Available analyses of sphalerite give pressures of 900 ñ 300 bars for central and southern Bolivia, •2.5 kb for-the Nairne Pyritic Formation, South Australia, 4.5-4-1 'kb for Balmat, New York, 5 ñ0.5 kb for the Highland-Surprise Mine, Coeur D'Alene, Idaho and 5.5 ñ 0.5 kb for the Quemont Mine, Noranda, Quebec. Improved values of fsa for the pyrite q-hexagonal pyrrhotite solvus were obtained by substituting into the above equation measured compositions of sphalerite coexisting with these minerals. From these, AGf ø of pyrite is calculated, --57.013 _4-0.800 kcal/mole and zxHf ø, --71.430 kcal/mole, both at 25 ø C.Sphalerite crystals grown with pyrite and pyrrhotite contain iron-rich patches, detected by microprobe, which are in roetastable equilibrium with the homogeneous matrix of the crystals. The compositional difference between the patches and the matrix is nearly independent of pressure but is strongly temperature dependent from about A = 4.4 mole % FeS at 350 ø C to 0.0 at 525 ø C.

Staphylococcus aureus causes pathologies ranging from minor skin infections to life-threatening diseases. Pathogenic effects are largely due to production of bacterial toxin, which is regulated by an RNA molecule, RNAIII. The S. aureus protein called RAP (RNAIII activating protein) activates RNAIII, and a peptide called RIP (RNAIII inhibiting peptide), produced by a nonpathogenic bacteria, inhibits RNAIII. Mice vaccinated with RAP or treated with purified or synthetic RIP were protected from S. aureus pathology. Thus, these two molecules may provide useful approaches for the prevention and treatment of diseases caused by S. aureus.

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After many years of fusion research, the conditions needed for a D–T fusion reactor have been approached on the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)]. For the first time the unique phenomena present in a D–T plasma are now being studied in a laboratory plasma. The first magnetic fusion experiments to study plasmas using nearly equal concentrations of deuterium and tritium have been carried out on TFTR. At present the maximum fusion power of 10.7 MW, using 39.5 MW of neutral-beam heating, in a supershot discharge and 6.7 MW in a high-βp discharge following a current rampdown. The fusion power density in a core of the plasma is ≊2.8 MW m−3, exceeding that expected in the International Thermonuclear Experimental Reactor (ITER) [Plasma Physics and Controlled Nuclear Fusion Research (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 239] at 1500 MW total fusion power. The energy confinement time, τE, is observed to increase in D–T, relative to D plasmas, by 20% and the ni(0) Ti(0) τE product by 55%. The improvement in thermal confinement is caused primarily by a decrease in ion heat conductivity in both supershot and limiter-H-mode discharges. Extensive lithium pellet injection increased the confinement time to 0.27 s and enabled higher current operation in both supershot and high-βp discharges. Ion cyclotron range of frequencies (ICRF) heating of a D–T plasma, using the second harmonic of tritium, has been demonstrated. First measurements of the confined alpha particles have been performed and found to be in good agreement with TRANSP [Nucl. Fusion 34, 1247 (1994)] simulations. Initial measurements of the alpha ash profile have been compared with simulations using particle transport coefficients from He gas puffing experiments. The loss of alpha particles to a detector at the bottom of the vessel is well described by the first-orbit loss mechanism. No loss due to alpha-particle-driven instabilities has yet been observed. D–T experiments on TFTR will continue to explore the assumptions of the ITER design and to examine some of the physics issues associated with an advanced tokamak reactor.

Long-wavelength (k ±pi < 1) density turbulence has been measured with good spatial localization in the core region of a high temperature tokamak plasma with auxiliary heating. Density fluctuations of h/n > 0.5% exist for /cj. < 2 cm "' with radial and poloidal correlation lengths typically 1-2.5 cm in the confinement region, corresponding to /:j.p/=^ 0.1-0.3. An anisotropic wave-number spectrum is observed, and estimates of the turbulence-driven transport are comparable to the anomalous transport observed in tokamaks.