S. J. Wukitch

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

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Publications (319)301.93 Total impact

  • Physics of Plasmas 05/2015; 22(5):050701. DOI:10.1063/1.4919964 · 2.25 Impact Factor
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    ABSTRACT: Core impurity transport has been investigated for a variety of confinement regimes in Alcator C-Mod plasmas from x-ray emission following injection of medium and high Z materials. In ohmic L-mode discharges, impurity transport is anomalous (D eff ≫ D nc) and changes very little across the LOC/SOC boundary. In ion cyclotron range of frequencies (ICRF) heated L-mode plasmas, the core impurity confinement time decreases with increasing ICRF input power (and subsequent increasing electron temperature) and increases with plasma current. Nearly identical impurity confinement characteristics are observed in I-mode plasmas. In enhanced D α H-mode discharges the core impurity confinement times are much longer. There is a strong connection between core impurity confinement time and the edge density gradient across all confinement regimes studied here. Deduced central impurity density profiles in stationary plasmas are generally flat, in spite of large amplitude sawtooth oscillations, and there is little evidence of impurity convection inside of r/a = 0.3 when averaged over sawteeth.
    Nuclear Fusion 03/2015; 55(3). DOI:10.1088/0029-5515/55/3/033014 · 3.24 Impact Factor
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    The European Physical Journal Conferences 01/2015; 87:02013. DOI:10.1051/epjconf/20158702013
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    ABSTRACT: Radio frequency (RF) power in the ion cyclotron range of frequencies (ICRF) is one of the primary auxiliary heating techniques for Experimental Advanced Superconducting Tokamak (EAST). The ICRF system for EAST has been developed to support long-pulse high-β advanced tokamak fusion physics experiments. The ICRF system is capable of delivering 12 MW 1000-s RF power to the plasma through two antennas. The phasing between current straps of the antennas can be adjusted to optimize the RF power spectrum. The main technical features of the ICRF system are described. Each of the 8 ICRF transmitters has been successfully tested to 1.5 MW for a wide range of frequency (25–70 MHz) on a dummy load. Part of the ICRF system was in operation during the EAST 2012 spring experimental campaign and a maximum power of 800 kW (at 27 MHz) lasting for 30 s has been coupled for long pulse H mode operation.
    Fusion Engineering and Design 07/2014; 89(11). DOI:10.1016/j.fusengdes.2014.06.017 · 1.15 Impact Factor
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    ABSTRACT: An ion cyclotron range of frequencies (ICRF) system with power up to 6.0 MW and a lower hybrid current drive (LHCD) system up to 4 MW have been applied for heating and current drive experiments in Experimental Advanced Superconducting Tokomak (EAST). Significant progress has been made with ICRF heating and LHCD for realizing the H-mode plasma operation in EAST. During 2010 and 2012 experimental campaigns, ICRF heating experiments were carried out at the fixed frequency of 27MHz, achieving effective ions and electrons heating with the H minority heating (H-MH) mode. The H-MH mode produced good plasma performance, and realized H-mode using ICRF power alone in 2012. In 2010, H-modes were generated and sustained by LHCD alone, where lithium coating and gas puffing near the mouth of the LH launcher were applied to improve the LHCD power coupling and penetration into the core plasmas of H-modes. In 2012, the combination of LHCD and ICRH power extended the H-mode duration up to over 30 s. H-modes with various types of edge localized modes (ELMs) have been achieved with HIPB98(y, 2) ranging from 0.7 to over unity. A brief overview of LHCD and ICRF Heating experiment and their application in achieving H-mode operation during these two campaigns will be presented.
    Physics of Plasmas 06/2014; 21(6):061501. DOI:10.1063/1.4884356 · 2.25 Impact Factor
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    ABSTRACT: Active stub tuning with a fast ferrite tuner (FFT) allows for the system to respond dynamically to changes in the plasma impedance such as during the L-H transition or edge localized modes (ELMs), and has greatly increased the effectiveness of fusion ion cyclotron range of frequency systems. A high power waveguide double-stub tuner is under development for use with the Alcator C-Mod lower hybrid current drive (LHCD) system. Exact impedance matching with a double-stub is possible for a single radiating element under most load conditions, with the reflection coefficient reduced from Γ to Γ2 in the "forbidden region." The relative phase shift between adjacent columns of a LHCD antenna is critical for control of the launched n∥ spectrum. Adding a double-stub tuning network will perturb the phase of the forward wave particularly if the unmatched reflection coefficient is high. This effect can be compensated by adjusting the phase of the low power microwave drive for each klystron amplifier. Cross-coupling of the reflected power between columns of the launcher must also be considered. The problem is simulated by cascading a scattering matrix for the plasma provided by a linear coupling model with the measured launcher scattering matrix and that of the FFTs. The solution is advanced in an iterative manner similar to the time-dependent behavior of the real system. System performance is presented under a range of edge density conditions from under-dense to over-dense and a range of launched n∥.
    Fusion Engineering and Design 01/2014; 1580(1). DOI:10.1063/1.4864589 · 1.15 Impact Factor
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    ABSTRACT: Studies of scrape-off-layer (SOL) density profiles by a reflectometer at three poloidal locations adjacent to the lower hybrid (LH) launcher show that for a wide range of plasma parameters, the application of either LH power only or combined with ion cyclotron range of frequencies (ICRF) power induces large modifications on SOL profiles and also creates strong poloidal density striations. With density striations, calculating LH reflection coefficient assuming a poloidally uniform density profile becomes inadequate to represent experimental conditions. The experimentally measured LH reflection coefficient appears to be close to that calculated using the measured density profile in front of the waveguide mouth, where strong density depletion is observed. Using profiles from other locations can result in a factor of 2 too low in reflection coefficient calculation. The poloidal density striations with LH power can be reproduced by a 2-D diffusive-convective model that includes the dynamics of the LH induced E×B convective eddies. Adding ICRF power, the density modifications and striations become more complicated and often lead to worse LH coupling, especially in cases that the ICRF antenna is magnetically connected to the LH launcher.
    01/2014; 1580(1). DOI:10.1063/1.4864575
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    ABSTRACT: 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 potentials and poor heating efficiency despite calculations indicating low integrated E∥. In mode conversion heating scenario, no core waves were detected in the plasma core indicating poor wave penetration. For monopole phasing, simulations suggest the antenna spectrum is peaked at very short wavelength and full wave simulations show the short wavelength has poor wave penetration to the plasma core.
    01/2014; 1580(1). DOI:10.1063/1.4864504
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    ABSTRACT: We performed an extensive survey of the plasma potential in the scrape-off layer (SOL) of Ion Cyclotron Range-of Frequencies (ICRF)-heated discharges on Alcator C-Mod. Our results show that plasma potentials are enhanced in the presence of ICRF power and plasma potential values of >100 V are often observed. Such potentials are high enough to induce sputtering of high-Z molybdenum (Mo) plasma facing components by deuterium ions on C-Mod. For comparison, the plasma potential in Ohmic discharges is typically less than 10 V, well below the threshold needed to induce Mo sputtering by deuterium ions. ICRF-enhanced plasma potentials are observed in the SOL regions that both magnetically map and do not map to active ICRF antennas. Regions that magnetically map to active ICRF antennas are accessible to slow waves directly launched by the antennas and these regions experience plasma potential enhancement that is partially consistent with the slow wave rectification mechanism. One of the most defining features of the slow wave rectification is a threshold appearance of significant plasma potentials (>100 V) when the dimensionless rectification parameter A, is above unity and this trend is observed experimentally. We also observe ICRF-enhanced plasma potentials >100 V in regions that do not magnetically map to the active antennas and, hence, are not accessible for slow waves launched directly by the active antennas. However, unabsorbed fast waves can reach these regions. The general trend that we observe in these "un-mapped" regions is that the plasma potential scales with the strength of the local RF wave fields with the fast wave polarization and the highest plasma potentials are observed in discharges with the highest levels of unabsorbed ICRF power. Similarly, we find that core Mo levels scale with the level of unabsorbed ICRF power suggesting a link between plasma potentials in the SOL and the strength of the impurity source.
    Plasma Physics and Controlled Fusion 01/2014; 56(1):5004-. DOI:10.1088/0741-3335/56/1/015004 · 2.39 Impact Factor
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    ABSTRACT: We describe results from imaging observations of atomic line and continuum emission in the 550.6 nm region on Alcator C-Mod. Both the 550.6 nm neutral molybdenum emission and the adjacent 549 nm continuum emission are imaged separately to isolate line emission. A few complications of using imaging to infer erosion in this wavelength region are discussed including subtraction of continuum emission and determination of an appropriate S/XB coefficient. Diagnostics of surface erosion and thermography using these emissions are briefly reviewed, and used to study phenomenology during ohmic operation, ion cyclotron range of frequencies heating (ICRH), and lower hybrid current drive (LHCD). In addition to broadening of Mo I emission regions in the outer divertor and main limiter during ICRH compared to ohmic operation, mid-plane localized heating of the main limiter associated with fast-ion impact is observed which exceeds the divertor heat flux. During LHCD operation, several localized regions of increased brightness associated with hot spots are interpreted as heating due to localized density peaking, which re-iterates the importance of imaging continuum emission for subtraction. These sources of surface heating exacerbate plasma-material interactions at the device wall and may require additional mitigation if they cannot be avoided in future machines.
    Plasma Physics and Controlled Fusion 12/2013; 55(12):5010-. DOI:10.1088/0741-3335/55/12/125010 · 2.39 Impact Factor
  • Nuclear Fusion 09/2013; 53(9):093021. DOI:10.1088/0029-5515/53/9/093021 · 3.24 Impact Factor
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    ABSTRACT: Application of lower hybrid range of frequencies (LHRF) waves can induce both co- and counter-current directed changes in toroidal rotation in Alcator C-Mod plasmas, depending on the target plasma current, electron density, confinement regime and magnetic shear. For ohmic L-mode discharges with good core LH wave absorption, and significant current drive at a fixed LH power near 0.8 MW, the interior (r/a < 0.5) rotation increments (on a time scale of order the current relaxation time) in the counter-current direction if ne(1020 m−3) > q95/11.5, and in the co-current direction if ne(1020 m−3) < q95/11.5. All discharges with co-current rotation changes have q0 > 1, indicating a good correlation with driven current fraction, unifying the results observed on various tokamaks. For high density (ne ≥ 1.2 × 1020 m−3) L-mode target discharges, where core LH wave absorption is low, the rotation change is in the co-current direction, but evolves on a shorter momentum transport time scale, and is seen across the entire spatial profile. For H-mode target plasmas, both co- and counter-current direction increments have been observed with LHRF. The H-mode co-rotation is correlated with the pedestal temperature gradient, which itself is enhanced by the LH waves absorbed in the plasma periphery. The H-mode counter-rotation increment, a flattening of the peaked velocity profile in the core, is consistent with a reduction in the momentum pinch correlated with a steepening of the core density profile. Most of these rotation changes must be due to indirect transport effects of LH waves on various parameters, which modify the momentum flux.
    Nuclear Fusion 08/2013; 53(9):093015. DOI:10.1088/0029-5515/53/9/093015 · 3.24 Impact Factor
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    ABSTRACT: A dedicated experiment during simultaneous lower hybrid (LH) and ion cyclotron range-of-frequencies (ICRF) operations is carried out to evaluate and understand the effects of ICRF power on the scrape-off-layer (SOL) density profiles and on the resultant LH coupling for a wide range of plasma parameters on Alcator C-Mod. Operation of the LH launcher with the adjacent ICRF antenna significantly degrades LH coupling while operation with the ICRF antenna that is not magnetically connected to the LH launcher minimally affects LH coupling. An X-mode reflectometer system at three poloidal locations adjacent to the LH launcher and a visible video camera imaging the LH launcher are used to measure local SOL density profile and emissivity modifications with the application of LH and LH + ICRF power. These measurements confirm that the density in front of the LH launcher depends strongly on the magnetic field line mapping of the active ICRF antenna. Reflectometer measurements also observe both ICRF-driven and LH-driven poloidal density profile asymmetries, especially a strong density depletion at certain poloidal locations in front of the LH launcher during operation with a magnetically connected ICRF antenna. The results indicate that understanding both LH-driven flows and ICRF sheath driven flows may be necessary to understand the observed density profile modifications and LH coupling results during simultaneous LH + ICRF operation.
    Plasma Physics and Controlled Fusion 07/2013; 55(9):095003. DOI:10.1088/0741-3335/55/9/095003 · 2.39 Impact Factor
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    ABSTRACT: Radio frequency (RF) sheath rectification is a leading mechanism suspected of causing anomalously high erosion of plasma facing materials in RF-heated plasmas on Alcator C-Mod. An extensive experimental survey of the plasma potential (ΦP) in RF-heated discharges on C-Mod reveals that significant ΦP enhancement (>100 V) is found on outboard limiter surfaces, both mapped and not mapped to active RF antennas. Surfaces that magnetically map to active RF antennas show ΦP enhancement that is, in part, consistent with the recently proposed slow wave rectification mechanism. Surfaces that do not map to active RF antennas also experience significant ΦP enhancement, which strongly correlates with the local fast wave intensity. In this case, fast wave rectification is a leading candidate mechanism responsible for the observed enhancement.
    Journal of Nuclear Materials 07/2013; DOI:10.1016/j.jnucmat.2013.01.189 · 2.02 Impact Factor
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    ABSTRACT: Lower hybrid current drive (LHCD) is an attractive option for non-inductive tokamak operation due to its high current drive efficiency and ability to drive current off axis. The parameters of the Alcator C-Mod LHCD system (f0 = 4.6 GHz, B 5.5 T, ) are similar to the proposed LHCD system on ITER. This paper will describe improvements in LHCD technology on C-Mod designed to increase single-pass absorption at high , extend pulse length (to >3 s), and increase power delivered to the plasma (to ~2 MW). Modelling of lower hybrid (LH) wave propagation indicates that the observed loss of LHCD efficiency at higher can be mitigated by enhancing the single pass power absorption through use of an off mid-plane launcher. The four rows of the launcher are located above the mid-plane (with Ip and B both clockwise viewing from the top down) in order to exploit the poloidal upshift of n|| as rays propagate from the antenna into the plasma. The transmitter protection system (TPS) was redesigned to model the coolant temperature in real time and shut off the klystron beam voltage if the coolant is close to boiling. The TPS upgrade has been installed and operated on C-Mod for pulses up to 4.5 s into dummy loads and 1.0 s into the plasma. A new movable local LH launcher protection limiter was designed to reduce reflection coefficients across a wide range of launcher positions. Finally, a high power waveguide double-stub tuner is under development to provide feedback controlled load matching to reduce power reflected from the antenna under poor coupling conditions.
    Nuclear Fusion 06/2013; 53(7):073012. DOI:10.1088/0029-5515/53/7/073012 · 3.24 Impact Factor
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    ABSTRACT: Long-lived (1, 1) 'snake' modes were discovered nearly three decades ago, but basic questions regarding their formation, stability, and superb particle confinement—shown by surviving tens to hundreds of sawtooth cycles—have remained unanswered. High-resolution spectroscopic imaging diagnostics permit studies of heavy-impurity-ion snakes with unprecedented temporal and spatial resolution, making it possible to positively identify the SXR signals with specific ion charge states and to infer, for the first time, the perturbed impurity density, Zeff, and resistivity at the centre of these long-lived helical modes. The results show a new scenario for the formation of heavy-impurity-ion snakes, which can begin as a broad 1/1 kink asymmetry of the central impurity-ion density, that grows and undergoes a seamless transition to a large crescent-shaped helical island-like structure inside q < 1, with a regularly sawtoothing core. This type of formation departs strongly from the nonlinear island model based on a modified Rutherford equation proposed originally to describe the pellet-induced snakes and expanded further to account for the impurity effects (e.g. and ). These new high-resolution observations show details of their evolution and the accompanying sawtooth oscillations that suggest important differences between the density and temperature dynamics, ruling out a purely pressure-driven process. Instead, many features arise naturally from nonlinear interactions in a 3D MHD model that separately evolves the plasma density and temperature.
    Nuclear Fusion 04/2013; 53(4):043019. DOI:10.1088/0029-5515/53/4/043019 · 3.24 Impact Factor
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    ABSTRACT: 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.
    Physical Review Letters 02/2013; 110(6):065006. DOI:10.1103/PhysRevLett.110.065006 · 7.73 Impact Factor
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    ABSTRACT: Deuterium high-confinement (H-mode) plasmas, lasting up to 3.45 s, have been generated in the EAST by ion cyclotron range of frequency (ICRF) heating. H-mode access was achieved by coating the molybdenum-tiled first wall with lithium to reduce the hydrogen recycling from the wall. H-mode plasmas with plasma currents between 0.4 and 0.6 MA and axial toroidal magnetic fields between 1.85 and 1.95 T were generated by 27 MHz ICRF heating of deuterium plasma with hydrogen minority. The ICRF input power required to access the H-mode was 1.6–1.8 MW. The line-averaged density was in the range (1.83–2.3) × 1019 m−3. 200–500 Hz type-III edge localized mode activity was observed during the H-mode phase. The H-mode confinement factor, H98IPB(y, 2), was ~0.7.
    Nuclear Fusion 02/2013; 53(2-2). DOI:10.1088/0029-5515/53/2/023004 · 3.24 Impact Factor
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    ABSTRACT: A swept-frequency X-mode reflectometer has been used to measure the scrape-off-layer (SOL) density profiles with and without lower hybrid (LH) power at three poloidal locations adjacent to the LH launcher for various plasma parameters in order to understand the coupling of LH waves on Alcator C-Mod. LH power has been observed to create significant poloidal SOL density profile asymmetries that are correlated with visible video camera images of emissivity patterns in front of the LH launcher. The observed density profile asymmetries depend on LH power, , magnetic geometry and magnetic field direction. A 2D diffusive–convective model has been used to show that these density profile modifications are consistent with a LH vortex, where LH power drives E × B drifts that then modify the SOL density profile. In particular, the simulations show that the density profile can possibly create a net poloidally averaged density depletion in front of the waveguide rows. A LH slab coupling model is then used to show that the simulated reflection coefficients strongly depend on the poloidal density profile asymmetries. The simulated LH power reflection coefficients agree with the experimental reflection coefficients only after the observed density depletion is included in the model.
    Plasma Physics and Controlled Fusion 01/2013; 55(2):025008. DOI:10.1088/0741-3335/55/2/025008 · 2.39 Impact Factor
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    ABSTRACT: This paper describes the operation of a double stub fast ferrite tuner (FFT) that we have designed for the Alcator C-Mod 4.6GHz Lower Hybrid Current Drive (LHCD) system. This FFT is unique because it uses a single electromagnet coil and permanent magnet on each tuning stub. The ferrite is located on the center of the broad face of the waveguide. The FFT is required to withstand over 200kW of power (20kW/cm2) at high VSWR (>5) for 1-3 second pulses spaced 10 minutes apart. Breakdown measurements and fabrication considerations will be discussed. Also, simulation of thermal conditions will be shown. The FFT will be computer controlled and must react to matching a load in a few hundred microseconds. This puts a severe requirement on power supply response time and its variation. In addition, the calculation time of the controlling software algorithms must be considered as well as the diffusion time of the controlling magnetic field through the waveguide wall. We will discuss these requirements and what we have done to meet them.
    Fusion Engineering (SOFE), 2013 IEEE 25th Symposium on; 01/2013

Publication Stats

2k Citations
301.93 Total Impact Points


  • 1998–2015
    • Massachusetts Institute of Technology
      • Plasma Science and Fusion Center (PSFC)
      Cambridge, Massachusetts, United States
  • 2002–2013
    • Princeton University
      • Princeton Plasma Physics Laboratory
      Princeton, New Jersey, United States
  • 1995–1996
    • University of Wisconsin, Madison
      • Department of Nuclear Engineering
      Mississippi, United States