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© 2020 The Author(s). The SPARC tokamak is a critical next step towards commercial fusion energy. SPARC is designed as a high-field (T), compact (m, m), superconducting, D-T tokamak with the goal of producing fusion gain 2$]]> from a magnetically confined fusion plasma for the first time. Currently under design, SPARC will continue the high-field path of the Alcator series of tokamaks, utilizing new magnets based on rare earth barium copper oxide high-temperature superconductors to achieve high performance in a compact device. The goal of 2$]]> is achievable with conservative physics assumptions () and, with the nominal assumption of, SPARC is projected to attain and MW. SPARC will therefore constitute a unique platform for burning plasma physics research with high density (), high temperature (keV) and high power density () 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.

Real-time fast ferrite tuning (FFT) has been successfully implemented on the ICRF antennas on Alcator C-Mod. The former prototypical FFT system on the E-port 2-strap antenna has been upgraded using new ferrite tuners. A new FFT system with two ferrite tuners and one fixed-length stub has been installed on the transmission line of the D-port 2-strap antenna. These two systems are able to achieve and maintain the reflected power to the transmitters to less than 1% in real time under almost all plasma conditions and help ensure reliable high power operation of the antennas. The loading insensitivity feature vs. plasma conditions of the innovative field-aligned (FA) 4-strap antenna on the J-port allows us to significantly improve the matching by installing a carefully designed stub on each of the two transmission lines. The reduction of the RF voltages in the transmission lines has enabled the J-port FA antenna to deliver 3.7 MW RF power to plasmas out of 4 MW source power. The matching on the J-port antenna can be further improved by adding a single ferrite tuner under real-time control on each transmission line and this scheme will be implemented in the near future., United States. Department of Energy (Agreement DE-FC02-99ER54512)

We describe a new technique for the efficient generation of high-energy ions with electromagnetic ion cyclotron waves in multi-ion plasmas. The discussed 'three-ion' scenarios are especially suited for strong wave absorption by a very low number of resonant ions. To observe this effect, the plasma composition has to be properly adjusted, as prescribed by theory. We demonstrate the potential of the method on the world-largest plasma magnetic confinement device, JET (Joint European Torus, Culham, UK), and the high-magnetic-field tokamak Alcator C-Mod (Cambridge, USA). The obtained results demonstrate efficient acceleration of 3 He ions to high energies in dedicated hydrogen-deuterium mixtures. Simultaneously, effective plasma heating is observed, as a result of the slowing-down of the fast 3 He ions. The developed technique is not only limited to laboratory plasmas, but can also be applied to explain observations of energetic ions in space-plasma environments, in particular, 3 He-rich solar flares.

In recent three-ion species (majority D and H plus a trace level of 3 He) ICRF heating experiments on Alcator C-Mod, double mode conversion on both sides of the 3 He cyclotron resonance has been observed using the phase contrast imaging (PCI) system. The MC locations are used to estimate the species concentrations in the plasma. Simulation using TORIC shows that with the 3 He level < 1%, most RF power is absorbed by the 3 He ions and the process can generate energetic 3 He ions. In mode conversion (MC) flow drive experiment in D( 3 He) plasma at 8 T, MC waves were also monitored by PCI. The MC ion cyclotron wave (ICW) amplitude and wavenumber k R have been found to correlate with the flow drive force. The MC efficiency, wave-number k of the MC ICW and their dependence on plasma parameters like T e0 have been studied. Based on the experimental observation and numerical study of the dispersion solutions, a hypothesis of the flow drive mechanism has been proposed., United States. Department of Energy. Office of Fusion Energy Sciences (Award DE -FC02 -99ER54512)

New experiments on Alcator C-Mod reveal that the favorable impurity screening characteristics of the high-field side (HFS) scrape-off layer (SOL), previously reported for single null geometries, is retained in double null configurations, despite the formation of an extremely thin SOL. In balanced double-null, nitrogen injected locally into the HFS SOL is better screened by a factor of 2.5 compared to the same injection into the low field side (LFS) SOL. This result is insensitive to plasma current and Greenwald fraction. Nitrogen injected into the HFS SOL is not as well screened (only a factor of 1.5 improvement over LFS) in unbalanced double-null discharges, when the primary divertor is in the direction of B×∇B. In this configuration, impurity ‘plume’ emission patterns indicate that an opposing E × B drift competes with the parallel impurity flow to the divertor. In balanced double-null plasmas, the dispersal pattern exhibits a dominant E × B motion. Unbalanced discharges with the primary divertor opposite the direction of B×∇B exhibit excellent HFS screening characteristics – a factor of 5 enhancement compared to LFS. These data support the idea that future tokamaks should locate all RF actuators and close-fitting wall structures on the HFS and employ near-double-null magnetic topologies, both to precisely control plasma conditions at the antenna/plasma interface and to maximally mitigate the impact of local impurity sources arising from plasma-material interactions. Keywords: Alcator C-Mod; Impurity screening; Double null; High field side scrape-off layer, United States. Department of Energy (Contract DE-FC02-99ER54512)

Injecting Lower Hybrid Range of Frequency (LHRF) waves into Alcator C-Mod's high-density H-mode plasmas has led to enhanced global energy confinement by increasing pedestal temperature and pressure gradients, decreasing the separatrix density, modifying the pedestal radial electric field and rotation, and decreasing edge turbulence. These experiments indicate that edge LHRF can be used as an actuator to increase energy confinement via modification of boundary quantities. H98-factor increases of up to ∼35% (e.g., H₉₈ from 0.75 to 1.0) are seen when moderate amounts of LH power (P[subscript LH]/P[subscript tot] ∼ 0.15) are applied to H-modes of densities [n with line above it][subscript e] ∼ 3 × 10²⁰ m⁻³, corresponding to values ∼0.5 of the Greenwald density. However, the magnitude of the improvement is reduced if the confinement quality of the target H-mode plasma is already good (i.e., H₉₈ [superscript target] ∼ 1). Ray-tracing modeling and accessibility calculations for the LH waves indicate that they do not penetrate to the core. The LHRF power appears to be deposited in plasma boundary region, with a large fraction of the injected power increment appearing promptly on the outer divertor target. There is no evidence that the LH waves are driving current in these plasmas. The LHRF-actuated improvements are well correlated with suppressed pedestal density fluctuations in the 100–300 kHz range. There is also a correlation between the improved confinement and a drop in separatrix density, a correlation that is consistent with previous H-mode results with no LHRF., United States. Department of Energy. Office of Fusion Energy Sciences (Award DE-FC02-99ER54512), United States. Department of Energy. Office of Fusion Energy Sciences (Award DE-AC02-09CH11466)

In lower hybrid current drive (LHCD) experiments on tokamaks, the parallel wavenumber of lower hybrid waves is an important physics parameter that governs the wave propagation and absorption physics. However, this parameter has not been experimentally well-characterized in the present-day high density tokamaks, despite the advances in the wave physics modeling. In this paper, we present the first measurement of the dominant parallel wavenumber of lower hybrid waves in the scrape-off layer (SOL) of the Alcator C-Mod tokamak with an array of magnetic loop probes. The electric field strength measured with the probe in typical C-Mod plasmas is about one-fifth of that of the electric field at the mouth of the grill antenna. The amplitude and phase responses of the measured signals on the applied power spectrum are consistent with the expected wave energy propagation. At higher density, the observed k|| increases for the fixed launched k||, and the wave amplitude decreases rapidly. This decrease is correlated with the loss of LHCD efficiency at high density, suggesting the presence of loss mechanisms. Evidence of the spectral broadening mechanisms is observed in the frequency spectra. However, no clear modifications in the dominant k|| are observed in the spectrally broadened wave components, as compared to the measured k|| at the applied frequency. It could be due to (1) the probe being in the SOL and (2) the limited k|| resolution of the diagnostic. Future experiments are planned to investigate the roles of the observed spectral broadening mechanisms on the LH density limit problem in the strong single pass damping regime.

Mode conversion of fast waves in the ion cyclotron range of frequencies (ICRF) is known to result in current drive and flow drive under optimised conditions, which may be utilized to control plasma profiles and improve fusion plasma performance. To describe these processes accurately in a realistic toroidal geometry, numerical simulations are essential. Quantitative comparison of these simulations and the actual experimental measurements is important to validate their predictions and to evaluate their limitations. The phase contrast imaging (PCI) diagnostic has been used to directly detect the ICRF waves in the Alcator C-Mod tokamak. The measurements have been compared with full-wave simulations through a synthetic diagnostic technique. Recently, the frequency response of the PCI detector array on Alcator C-Mod was recalibrated, which greatly improved the comparison between the measurements and the simulations. In this study, mode converted waves for D-{superscript 3]He and D-H plasmas with various ion species compositions were re-analyzed with the new calibration. For the minority heating cases, self-consistent electric fields and a minority ion distribution function were simulated by iterating a full-wave code and a Fokker-Planck code. The simulated mode converted wave intensity was in quite reasonable agreement with the measurements close to the antenna, but discrepancies remain for comparison at larger distances., United States. Department of Energy (Grant DE-FG02- 94ER54235)

This paper presents an overview of recent highlights from research on Alcator C-Mod. Significant progress has been made across all research areas over the last two years, with particular emphasis on divertor physics and power handling, plasma–material interaction studies, edge localized mode-suppressed pedestal dynamics, core transport and turbulence, and RF heating and current drive utilizing ion cyclotron and lower hybrid tools. Specific results of particular relevance to ITER include: inner wall SOL transport studies that have led, together with results from other experiments, to the change of the detailed shape of the inner wall in ITER; runaway electron studies showing that the critical electric field required for runaway generation is much higher than predicted from collisional theory; core tungsten impurity transport studies reveal that tungsten accumulation is naturally avoided in typical C-Mod conditions., United States. Department of Energy (DE-FC02-99ER54512-CMOD), United States. Department of Energy (DE-AC02-09CH11466), United States. Department of Energy (DE-FG02-96ER-54373), United States. Department of Energy (DE-FG02-94ER54235)

Ion cyclotron resonance heating is the primary auxiliary heating on the Alcator C-Mod tokamak and is commonly used on other devices, and is planned for use on ITER. The RF-power density on C-Mod is above 5 MW m−3 providing for a unique opportunity to study wave–particle effects in the high RF power per particle regime. Minority heating produces a highly energetic tail in the minority distribution function which is measured using a compact neutral particle analyser. In this paper, we present the measurements of the fast-ion spectrum between 200 and 2 MeV, compiled over an entire experimental campaign. We also estimate the effective tail temperatures for the fast-ion distribution. We find that the fast-ion distribution is less energetic and less dense with increasing electron density; is more energetic with increasing plasma current; and is more dense but has no measurable change in energy with increasing RF power. Some possible explanations for these findings are discussed., United States. Dept. of Energy (Award DE-FC02-99ER54512)

In the Alcator C-Mod tokamak, strong, steady-state variations of molybdenum density within a flux surface are routinely observed in plasmas using hydrogen minority ion cyclotron resonant heating. In/out asymmetries, up to a factor of 2, occur with either inboard or outboard accumulation depending on the major radius of the minority resonance layer. These poloidal variations can be attributed to the impurity's high charge and large mass in the neoclassical parallel force balance. The large mass enhances the centrifugal force, causing outboard accumulation while the high charge enhances ion-impurity friction and makes impurities sensitive to small poloidal variations in the plasma potential. Quantitative comparisons between existing parallel high-Z impurity transport theories and experimental results for r/a < 0.7 show good agreement when the resonance layer is on the high-field side of the tokamak but disagree substantially for low-field side heating. Ion-impurity friction is insufficient to explain the experimental results, and the accumulation of impurity density on the inboard side of flux surface is shown to be driven by a poloidal potential variation due to magnetic trapping of non-thermal, cyclotron heated minority ions. Parallel impurity transport theory is extended to account for cyclotron effects and shown to agree with experimentally measured impurity density asymmetries., United States. Dept. of Energy (Agreement DE-FC02-99ER54512), United States. Dept. of Energy. Office of Fusion Energy Sciences (Postdoctoral Research Program)

Impurity contamination and localized heat loads associated with ion cyclotron range of frequency (ICRF) antenna operation are among the most challenging issues for ICRF utilization.. Another challenge is maintaining maximum coupled power through plasma variations including edge localized modes (ELMs) and confinement transitions. Here, we report on an experimental assessment of a field aligned (FA) antenna with respect to impurity contamination, impurity sources, RF enhanced heat flux and load tolerance. In addition, we compare the modification of the scrape of layer (SOL) plasma potential of the FA antenna to a conventional, toroidally aligned (TA) antenna, in order to explore the underlying physics governing impurity contamination linked to ICRF heating. The FA antenna is a 4-strap ICRF antenna where the current straps and antenna enclosure sides are perpendicular to and the Faraday screen rods are parallel to the total magnetic field. In principle, alignment with respect to the total magnetic field minimizes integrated E∥ (electric field along a magnetic field line) via symmetry. Consistent with expectations, we observed that the impurity contamination and impurity source at the FA antenna are reduced compared to the TA antenna. In both L and H-mode discharges, the radiated power is 20–30% lower for a FA-antenna heated discharge than a discharge heated with the TA-antennas. Further we observe that the fraction of RF energy deposited upon the antenna is less than 0.4 % of the total injected RF energy in dipole phasing. The total deposited energy increases significantly when the FA antenna is operated in monopole phasing. The FA antenna also exhibits an unexpected load tolerance for ELMs and confinement transitions compared to the TA antennas. However, inconsistent with expectations, we observe RF induced plasma potentials to be nearly identical for FA and TA antennas when operated in dipole phasing. In monopole phasing, the FA antenna has the highest plasma potenti, United States. Dept. of Energy (DOE award DE-FC02-99ER54512), United States. Dept. of Energy (Fusion Energy Postdoctoral Research Program administered by ORISE)

Radio frequency waves in the ion cyclotron range of frequencies (ICRF) are widely used to heat tokamak plasmas. In ICRF heating schemes involving multiple ion species, the launched fast waves convert to ion cyclotron waves or ion Bernstein waves at the two-ion hybrid resonances. Mode converted waves are of interest as actuators to optimise plasma performance through current drive and flow drive. In order to describe these processes accurately in a realistic tokamak geometry, numerical simulations are essential, and it is important that these codes be validated against experiment. In this study, the mode converted waves were measured using a phase contrast imaging technique in D-H and D-[superscript 3]He plasmas. The measured mode converted wave intensity in the D-[superscript 3]He mode conversion regime was found to be a factor of ∼ 50 weaker than the full-wave predictions. The discrepancy was reduced in the hydrogen minority heating regime, where mode conversion is weaker., United States. Dept. of Energy (DE-FG02-94-ER54235), United States. Dept. of Energy (DE-FC02-99-ER54512), United States. Dept. of Energy (DE-FC02-01ER54648)

This paper reports on new experiments at Alcator C-Mod that were performed in order to investigate the long-standing, unresolved discrepancy between Thomson scattering (TS) and electron cyclotron emission (ECE) measurements of electron temperature in high temperature tokamak plasmas. Ion cyclotron range of frequency (ICRF) heating is used to produce high temperature conditions where the type of TS-ECE discrepancy observed in the past at JET and TFTR should become observable. At Alcator C-Mod, plasmas with Te(0) up to 8 keV are obtained using ion cyclotron resonance heating (ICRH), ICRF mode conversion heating and a combination of the two heating methods in order to explore the hypothesis that the presence of ICRH-generated fast ions may be related to the discrepancy. In all high temperature cases, the TS and ECE measurements of electron temperature agree to within experimental uncertainties. We find no evidence for the type of discrepancy reported at JET and TFTR. These results show that the TS-ECE discrepancy does not depend on high temperatures alone and also that the presence of ICRH-generated fast ions is insufficient to cause the TS-ECE discrepancy., United States. Dept. of Energy (DE-FC02-99ER54512)

New observations of the formation and dynamics of long-lived impurity-induced helical “snake” modes in tokamak plasmas have recently been carried out on Alcator C-Mod. The snakes form as an asymmetry in the impurity ion density that undergoes a seamless transition from a small helically displaced density to a large crescent-shaped helical structure inside q<1, with a regularly sawtoothing core. The observations show that the conditions for the formation and persistence of a snake cannot be explained by plasma pressure alone. Instead, many features arise naturally from nonlinear interactions in a 3D MHD model that separately evolves the plasma density and temperature., United States. Dept. of Energy (Contract DE-FC02-99ER54512)

An ion sensitive probe (ISP) is developed as a robust diagnostic for measuring plasma potentials (Φ[subscript P]) in magnetized plasmas. The ISP relies on the large difference between the ion and electron gyroradii (ρ[subscript i]/ρ[subscript e] ∼ 60) to reduce the electron collection at a collector recessed behind a separately biased wall distance ∼ ρ[subscript i]. We develop a new ISP method to measure the plasma potential that is independent of the precise position and shape of the collector. Φ[subscript P] is found as the wall potential when charged current to the probe collector vanishes during the voltage sweep. The plasma potentials obtained from the ISP match Φ[subscript P] measured with an emissive probe over a wide range of plasma conditions in a small magnetized plasma., United States. Dept. of Energy (Grant No. DE-FC02-99ER54512)

Parametric dependences of the heat flux footprint on the outer divertor target plate are explored in EDA H-mode and ohmic L-mode plasmas over a wide range of parameters with attached plasma conditions. Heat flux profile shapes are found to be independent of toroidal field strength, independent of power flow along magnetic field lines and insensitive to x-point topology (single-null versus double-null). The magnitudes and widths closely follow that of the “upstream” pressure profile, which are correlated to plasma thermal energy content and plasma current. Heat flux decay lengths near the strike-point in H- and L-mode plasmas scale approximately with the inverse of plasma current, with a diminished dependence at high collisionality in L-mode. Consistent with previous studies, pressure gradients in the boundary scale with plasma current squared, holding the magnetohydrodynamic ballooning parameter approximately invariant at fixed collisionality—strong evidence that critical-gradient transport physics plays a key role in setting the power exhaust channel., United States. Dept. of Energy (Award DE-AC52-06NA25396), United States. Dept. of Energy (Coop. Agreement DE-FC02-99ER54512)

An ion sensitive probe (ISP) is developed as a robust diagnostic for measuring plasma potentials (Φ[subscript P]) in magnetized plasmas. The ISP relies on the large difference between the ion and electron gyroradii (ρ[subscript i]/ρ[subscript e] ∼ 60) to reduce the electron collection at a collector recessed behind a separately biased wall distance ∼ ρ[subscript i]. We develop a new ISP method to measure the plasma potential that is independent of the precise position and shape of the collector. Φ[subscript P] is found as the wall potential when charged current to the probe collector vanishes during the voltage sweep. The plasma potentials obtained from the ISP match Φ[subscript P] measured with an emissive probe over a wide range of plasma conditions in a small magnetized plasma., United States. Dept. of Energy (Grant No. DE-FC02-99ER54512)

The Lower Hybrid Current Drive (LHCD) system on Alcator C-Mod is designed to drive off-axis current in the core plasma, e.g. r/a<=0.8, however several interesting observations indicate that the LH waves are depositing significant power in the Scrape Off Layer (SOL) in high density L-Mode discharges. These phenomena are observed in accessible plasmas below the so-called “density limit” attributed to Parametric Decay Instabilities as omega-->2omegalh. Parallel current densities of ~500 kA/m [superscript 2] are measured on divertor Langmuir probes located just outside the separatrix during high power LH operation above n-bare~1×10[superscript 20] m[superscript −3]. These SOL currents rise at the same density at which the line integrated hard X-ray Bremsstrahlung emission drops sharply. Ray tracing/Fokker-Planck simulations over-estimate the line integrated X-ray emissivity by a factor of ~500 at n-bare = 1.4 10[superscript 20] m[superscript −3] as compared to a factor of 3–4 at n-bare = 0.6×10[superscript 20] m. Ray tracing simulations including a realistic SOL show similar trends in X-ray emission compared with the experimental observations. ©2009 American Institute of Physics, United States. Dept. of Energy (contract DE-AC02-76CH03073), United States. Dept. of Energy (contract DE-FC02-99ER54512)

The goal of the Lower Hybrid Current Drive (LHCD) system on the Alcator C-Mod tokamak [ Hutchinson et al., Phys. Plasmas 1, 1511 (1994) ] is to investigate current profile control under plasma conditions relevant to future tokamak experiments. Experimental observations of a LHCD “density limit” for C-Mod are presented in this paper. Bremsstrahlung emission from relativistic fast electrons in the core plasma drops suddenly above line averaged densities of 10[superscript 20] m[superscript −3] (ω/ω[subscript LH] ~ 3–4), well below the density limit previously observed on other experiments (ω/ωLH ∼ 2). Electric currents flowing through the scrape off layer (SOL) between the inner and outer divertors increase dramatically across the same density range that the core bremsstrahlung emission drops precipitously. These experimental x-ray data are compared to both conventional modeling, which gives poor agreement with experiment above the density limit and a model including collisional absorption in the SOL, which dramatically improves agreement with experiment above the observed density limit. These results show that strong absorption of LH waves in the SOL is possible on a high density tokamak and the SOL must be included in simulations of LHCD at high density., United States. Dept. of Energy (contract DE-FC02-99ER54512), United States. Dept. of Energy (contract DE-AC02-76CH03073)

Mode converted ion cyclotron wave (ICW) has been observed with phase contrast imaging (PCI) in D-[superscript 3]He plasmas in Alcator C-Mod [1, 2]. The measurements were carried out with the optical heterodyne technique using acousto-optic modulators which modulate the CO2 laser beam intensity near the ion cyclotron frequency [3]. With recently improved calibration of the PCI system using a calibrated sound wave source, the measurements have been compared with the full-wave code TORIC, as interpreted by a synthetic diagnostic. Because of the line-integrated nature of the PCI signal, the predictions are sensitive to the exact wave field pattern. The simulations are found to be in qualitative agreement with the measurements., United States. Dept. of Energy (DE-FG02-94-ER54235), United States. Dept. of Energy (DE-FC02-99-ER54512)

Plasma flow drive via ICRF mode conversion (MC) has been demonstrated on Alcator C-Mod. The toroidal rotation in these D([superscript 3]He) MC plasmas is typically more than twice above the empirically determined intrinsic rotation scaling in ICRF minority heated plasmas. In L-mode plasmas at 3 MW ICRF power input, up to 90 km/s toroidal rotation and 2 km/s localized ( r/a~0.4) poloidal rotation has been observed. The MC ion cyclotron wave (ICW) was detected by a phase contrast imaging system in heterodyne setup. Through TORIC 2-D full wave simulation, and comparison with other experimental evidence, we hypothesize that the interaction between the MC ICW and the [superscript 3]He ions may be the mechanism for the observed MC flow drive. TORIC simulation suggests that similar flow drive scenario may be realized on JET D(3He) plasmas. The promising scenarios on ITER are the inverted minority scenario (T)D and high field launch for T-D-([superscript 3]He) plasma. In non-radioactive phase, these correspond to ([superscript 3]He)-H and [superscript 4]He([superscript 3]He) plasmas respectively., United States. Dept. of Energy (grant DE-FC02-99-ER54512)

Mode converted ion cyclotron wave (ICW) has been observed with phase contrast imaging (PCI) in D-[superscript 3]He plasmas in Alcator C-Mod [1, 2]. The measurements were carried out with the optical heterodyne technique using acousto-optic modulators which modulate the CO2 laser beam intensity near the ion cyclotron frequency [3]. With recently improved calibration of the PCI system using a calibrated sound wave source, the measurements have been compared with the full-wave code TORIC, as interpreted by a synthetic diagnostic. Because of the line-integrated nature of the PCI signal, the predictions are sensitive to the exact wave field pattern. The simulations are found to be in qualitative agreement with the measurements., United States. Dept. of Energy (DE-FG02-94-ER54235), United States. Dept. of Energy (DE-FC02-99-ER54512)

Parametric dependences of the heat flux footprint on the outer divertor target plate are explored in EDA H-mode and ohmic L-mode plasmas over a wide range of parameters with attached plasma conditions. Heat flux profile shapes are found to be independent of toroidal field strength, independent of power flow along magnetic field lines and insensitive to x-point topology (single-null versus double-null). The magnitudes and widths closely follow that of the “upstream” pressure profile, which are correlated to plasma thermal energy content and plasma current. Heat flux decay lengths near the strike-point in H- and L-mode plasmas scale approximately with the inverse of plasma current, with a diminished dependence at high collisionality in L-mode. Consistent with previous studies, pressure gradients in the boundary scale with plasma current squared, holding the magnetohydrodynamic ballooning parameter approximately invariant at fixed collisionality—strong evidence that critical-gradient transport physics plays a key role in setting the power exhaust channel., United States. Dept. of Energy (Award DE-AC52-06NA25396), United States. Dept. of Energy (Coop. Agreement DE-FC02-99ER54512)

The Lower Hybrid Current Drive (LHCD) system on Alcator C-Mod is designed to drive off-axis current in the core plasma, e.g. r/a<=0.8, however several interesting observations indicate that the LH waves are depositing significant power in the Scrape Off Layer (SOL) in high density L-Mode discharges. These phenomena are observed in accessible plasmas below the so-called “density limit” attributed to Parametric Decay Instabilities as omega-->2omegalh. Parallel current densities of ~500 kA/m [superscript 2] are measured on divertor Langmuir probes located just outside the separatrix during high power LH operation above n-bare~1×10[superscript 20] m[superscript −3]. These SOL currents rise at the same density at which the line integrated hard X-ray Bremsstrahlung emission drops sharply. Ray tracing/Fokker-Planck simulations over-estimate the line integrated X-ray emissivity by a factor of ~500 at n-bare = 1.4 10[superscript 20] m[superscript −3] as compared to a factor of 3–4 at n-bare = 0.6×10[superscript 20] m. Ray tracing simulations including a realistic SOL show similar trends in X-ray emission compared with the experimental observations. ©2009 American Institute of Physics, United States. Dept. of Energy (contract DE-AC02-76CH03073), United States. Dept. of Energy (contract DE-FC02-99ER54512)

ICRF operations with Alcator C-Mod consists of 4 transmitters, of which two are tunable within the 40 MHz to 80 MHz band. These transmitters were comprised of 3 stages all of which employed vacuum tube amplifiers. As a part of the general plan to upgrade the two tunable transmitters, a project was implemented to replace the first stage, the intermediate power amplifier, with a 10 kilowatt solid state broadband amplifier. There were two primary benefits from the upgrade. The tuning of the transmitters was considerably simplified and better performance was obtained under a variety of operational conditions by having a conservative power margin. The improvements obtained are an important first step toward advancing the overall radio frequency performance of the transmitters and minimizing the time intensive task of changing the frequency of operation., United States. Dept. of Energy (Cooperative Grant No. DE-FC02-99ER54512-C-Mod)

The Ion Cyclotron RF Transmitter System (ICRF) at Alcator C-Mod comprises four separate transmitters each capable of driving 2 MW of power into plasma loads. Four separate transmission lines guide RF power into three antennas, each mounted in a separate horizontal port, in the C-Mod Tokamak. Protection for the antennas, matching elements and transmission line is accomplished by two unique but interdependent subsystems encompassed by the ICRF Fault System. The Antenna Protection System evaluates antenna phasing and voltage, sets fault thresholds, generates fault signals, and passes fault information to the Master Fault Processor. During operation, the Master Fault Processor is responsible for detecting hazards along the transmission line, generating faults, processing faults from the Antenna Protection System, terminating RF drive and extinguishing faults within 10 mus. In addition, the system controls various delays and sets the boundaries for RF retries. The ICRF Control System provides amplitude regulation for all antennas and phase control for a four-strap antenna. We are modifying some of the fault processing components and control elements of these systems in an effort to improve reliability and serviceability, and increase flexibility. This upgrade will reduce wired interconnections, add remote features to improve access to key operating parameters, improve RF isolation with new switching components, simplify phase control, and expand the RF regulation system to an active control regime whereby plasma parameters may become direct feedback elements for RF regulation. Details of the proposed upgrade to the system will be presented, and implementation of any new technological tools will be discussed., United States. Dept of Energy (Cooperative Grant No. DE-FC02-99ER54512-C-Mod)

Groups of frequency chirping modes observed between sawtooth crashes in the Alcator C-Mod tokamak are interpreted as reversed shear Alfve´n eigenmodes near the q ¼ 1 surface. These modes indicate that a reversed shear q profile is generated during the relaxation phase of the sawtooth cycle. Two important parameters, qmin and its radial position, are deduced from comparisons of measured density fluctuations with calculations from the ideal MHD, Department of Energy

We have developed a design for a new rotated 4-strap ICRF antenna. The design is based on a modification of the existing C-Mod antennas with the antenna rotated 10deg such that the entire structure is perpendicular to total magnetic field. This rotation is implemented in an attempt to reduce ICRF impurity production. The rotation results in an antenna with less surface area than the previous antennas and therefore a higher power density will be obtained for a given input power of 3 MW. The power density would reach 15 MW/m2, near the world record power density obtained in Tore Supra. We will describe the RF of this new design. The RF analysis was accomplished using the CST computer code. The fields were studied to minimize E field breakdown in the feed system and to accomplish the symmetry required for reduced impurity production in the four straps., United States. Dept. of Energy (cooperative grant no. DE-FC02-99ER54512-CMOD)

We have developed a design for a new rotated 4-strap ICRF antenna. The design is based on a modification of the existing C-Mod antennas with the antenna rotated 10deg such that the entire structure is perpendicular to total magnetic field. This rotation is implemented in an attempt to reduce ICRF impurity production. The rotation results in an antenna with less surface area than the previous antennas and therefore a higher power density will be obtained for a given input power of 3 MW. The power density would reach 15 MW/m2, near the world record power density obtained in Tore Supra. We will describe the RF of this new design. The RF analysis was accomplished using the CST computer code. The fields were studied to minimize E field breakdown in the feed system and to accomplish the symmetry required for reduced impurity production in the four straps., United States. Dept. of Energy (Cooperative Grant No DE-FC02-99ER54512-CMOD)

ICRF operations with Alcator C-Mod consists of 4 transmitters, of which two are tunable within the 40 MHz to 80 MHz band. These transmitters were comprised of 3 stages all of which employed vacuum tube amplifiers. As a part of the general plan to upgrade the two tunable transmitters, a project was implemented to replace the first stage, the intermediate power amplifier, with a 10 kilowatt solid state broadband amplifier. There were two primary benefits from the upgrade. The tuning of the transmitters was considerably simplified and better performance was obtained under a variety of operational conditions by having a conservative power margin. The improvements obtained are an important first step toward advancing the overall radio frequency performance of the transmitters and minimizing the time intensive task of changing the frequency of operation., United States. Dept. of Energy (Cooperative Grant No. DE-FC02-99ER54512-C-Mod)

Recent advances in gyrokinetic simulation of core turbulence and associated transport requires an intensified experimental effort to validate these codes using state of the art synthetic diagnostics to compare simulations with experimental data. A phase contrast imaging (PCI) diagnostic [M. Porkolab, J. C. Rost, N. Basse et al., IEEE Trans. Plasma Sci. 34, 229 (2006)] is used to study H-mode plasmas in Alcator C-Mod [M. Greenwald, D. Andelin, N. Basse et al., Nucl. Fusion 45, S109 (2005)]. The PCI system is capable of measuring density fluctuations with high temporal (2 kHz–5 MHz) and wavenumber (0.5–55 cm[superscript −1]) resolution. Recent upgrades have enabled PCI to localize the short wavelength turbulence in the electron temperature gradient range and resolve the direction of propagation (i.e., electron versus ion diamagnetic direction) of the longer wavelength turbulence in the ion temperature gradient (ITG) and trapped electron mode range. The studies focus on plasmas before and during internal transport barrier formation in an enhanced Dalpha H-mode plasma assisted with ion cyclotron resonance frequency heating. Nonlinear GYRO simulations have also been performed [J. Candy and R. E. Waltz, Phys. Rev. Lett. 91, 045001 (2003)] and the predicted fluctuation is compared against experimental measurements through a synthetic PCI diagnostic method. The simulated fluctuations from GYRO agree with experimental measurements in the ITG regime. GYRO also shows good agreement in transport predictions with experimental measurements after reducing the ion temperature gradient (~15%) and adding E×B shear suppression, all within the experimental uncertainty., U.S. Department of Energy

At modest [superscript 3]He levels (n[subscript 3He]/n[subscript e]~8%–12%), in relatively low density D([superscript 3]He) plasmas, [overline n]e<=1.3×10[superscript 20] m[superscript −3], heated with 50 MHz rf power at B[subscript t0]~5.1 T, strong (up to 90 km/s) toroidal rotation (Vphi) in the cocurrent direction has been observed by high-resolution x-ray spectroscopy on Alcator C-Mod. The change in central Vphi scales with the applied rf power (<=30 km s[superscript −1] MW[superscript −1]), and is generally at least a factor of 2 higher than the empirically determined intrinsic plasma rotation scaling. The rotation in the inner plasma (r/a<=0.3) responds to the rf power more quickly than that of the outer region (r/a>=0.7), and the rotation profile is broadly peaked for r/a<=0.5. Localized poloidal rotation (0.3<=r/a<=0.6) in the ion diamagnetic drift direction (~2 km/s at 3 MW) is also observed, and similarly increases with rf power. Changing the toroidal phase of the antenna does not affect the rotation direction, and it only weakly affects the rotation magnitude. The mode converted ion cyclotron wave (MC ICW) has been detected by a phase contrast imaging system and the MC process is confirmed by two-dimensional full wave TORIC simulations. The simulations also show that the MC ICW is strongly damped on 3He ions in the vicinity of the MC layer, approximately on the same flux surfaces where the rf driven flow is observed. The flow shear in our experiment is marginally sufficient for plasma confinement enhancement based on the comparison of the E×B shearing rate and gyrokinetic linear stability analysis., U.S. Department of Energy

Reversed shear Alfvén eigenmodes (RSAEs) have been observed with the phase contrast imaging diagnostic and Mirnov coils during the sawtooth cycle in Alcator C-mod [M. Greenwald et al., Nucl. Fusion 45, S109 (2005)] plasmas with minority ion-cyclotron resonance heating. Both down-chirping RSAEs and up-chirping RSAEs have been observed during the sawtooth cycle. Experimental measurements of the spatial structure of the RSAEs are compared to theoretical models based on the code NOVA [C. Z. Cheng and M. S. Chance, J. Comput. Phys. 71, 124 (1987)] and used to derive constraints on the q profile. It is shown that the observed RSAEs can be understood by assuming a reversed shear q profile (up chirping) or a q profile with a local maximum (down chirping) with q[approximate]1., U.S. Department of Energy