E. S. Marmar

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

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Publications (385)561.49 Total impact

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    ABSTRACT: We present inboard (HFS) and outboard (LFS) radial electric field (Er) and impurity temperature (Tz) measurements in the I-mode and H-mode pedestal of Alcator C-Mod. These measurements reveal strong Er wells at the HFS and the LFS midplane in both regimes and clear pedestals in Tz, which are of similar shape and height for the HFS and LFS. While the H-mode Er well has a radially symmetric structure, the Er well in I-mode is asymmetric, with a stronger ExB shear layer at the outer edge of the Er well, near the separatrix. Comparison of HFS and LFS profiles indicates that impurity temperature and plasma potential are not simultaneously flux functions. Uncertainties in radial alignment after mapping HFS measurements along flux surfaces to the LFS do not, however, allow direct determination as to which quantity varies poloidally and to what extent. Radially aligning HFS and LFS measurements based on the Tz profiles would result in substantial inboard-outboard variations of plasma potential and electron density. Aligning HFS and LFS Er wells instead also approximately aligns the impurity poloidal flow profiles, while resulting in a LFS impurity temperature exceeding the HFS values in the region of steepest gradients by up to 70%. Considerations based on a simplified form of total parallel momentum balance and estimates of parallel and perpendicular heat transport time scales seem to favor an approximate alignment of the Er wells and a substantial poloidal asymmetry in impurity temperature.
    Nuclear Fusion. 06/2014; 54:083017.
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    ABSTRACT: Microwave backscattering experiments have been performed on the Alcator C-Mod tokamak in order to investigate the propagation of lower hybrid (LH) waves in reactor-relevant, high-density plasmas. When the line-averaged density is raised above 1 × 1020 m-3, lower hybrid current drive efficiency is found to be lower than expected [Wallace et al., Phys. Plasmas 19, 062505 (2012)] and LH power is thought to be dissipated at the plasma edge. Using a single channel (60 GHz) ordinary-mode (O-mode) reflectometer system, we demonstrate radially localized LH wave measurements in the scrape-off layer of high density plasmas (n ≳ 0.9×10 m-3). Measured backscattered O-mode power varies depending on the magnetic field line mapping, suggesting the resonance cone propagation of LH waves. Backscattered power is also sensitive to variations in plasma density and the launched parallel refractive index of the LH waves. LH ray-tracing simulations have been carried out to interpret the observed variations. To understand the measured LH waves in regions not magnetically connected to the launcher, two hypotheses are examined. One is the weak single pass absorption and the other is scattering of LH waves by non-linear effects.
    12/2013; 21(1).
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    ABSTRACT: X-ray imaging crystal spectrometers with high spectral and spatial resolution are currently being used on magnetically confined fusion devices to infer the time history profiles of ion and electron temperatures as well as plasma flow velocities. The absolute measurement of flow velocities is important for optimizing various discharge scenarios and evaluating the radial electric field in tokamak and stellarator plasmas. Recent studies indicate that the crystal temperature must be kept constant to within a fraction of a degree to avoid changes of the interplanar 2d-spacing by thermal expansion that cause changes in the Bragg angle, which could be misinterpreted as Doppler shifts. For the instrumental parameters of the x-ray crystal spectrometer on Alcator C-Mod, where those thermal effects were investigated, a change of the crystal temperature by 1 °C causes a change of the lattice spacing of the order of Δd = 1 × 10-5 Å introducing a fictitious velocity drift of the order of ˜3 km s-1. This effect must be considered for x-ray imaging crystals spectrometers installed on LHD, KSTAR, EAST, J-TEXT, NSTX and, in the future, W7-X and ITER.
    Plasma Physics and Controlled Fusion 12/2013; 55(12):5011-. · 2.37 Impact Factor
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    ABSTRACT: Application of lower hybrid (LH) current drive in tokamak plasmas can induce both co- and countercurrent directed changes in toroidal rotation, depending on the core q profile. For discharges with q_{0}<1, rotation increments in the countercurrent direction are observed. If the LH-driven current is sufficient to suppress sawteeth and increase q_{0} above unity, the core toroidal rotation change is in the cocurrent direction. This change in sign of the rotation increment is consistent with a change in sign of the residual stress (the divergence of which constitutes an intrinsic torque that drives the flow) through its dependence on magnetic shear.
    Physical Review Letters 09/2013; 111(12):125003. · 7.73 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. · 2.73 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. · 2.73 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. · 7.73 Impact Factor
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    ABSTRACT: Several seemingly unrelated effects in Alcator C-Mod ohmic L-mode plasmas are shown to be closely connected: non-local heat transport, core toroidal rotation reversals, energy confinement saturation and up/down impurity density asymmetries. These phenomena all abruptly transform at a critical value of the collisionality. At low densities in the linear ohmic confinement regime, with collisionality ν* ≤ 0.35 (evaluated inside of the q = 3/2 surface), heat transport exhibits non-local behaviour, core toroidal rotation is directed co-current, edge impurity density profiles are up/down symmetric and a turbulent feature in core density fluctuations with kθ up to 15 cm−1 (kθρs ~ 1) is present. At high density/collisionality with saturated ohmic confinement, electron thermal transport is diffusive, core rotation is in the counter-current direction, edge impurity density profiles are up/down asymmetric and the high kθ turbulent feature is absent. The rotation reversal stagnation point (just inside of the q = 3/2 surface) coincides with the non-local electron temperature profile inversion radius. All of these observations suggest a possible unification in a model with trapped electron mode prevalence at low collisionality and ion temperature gradient mode domination at high collisionality.
    Nuclear Fusion 02/2013; 53(3):033004. · 2.73 Impact Factor
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    ABSTRACT: New experimental data from the Alcator C-Mod tokamak are used to benchmark predictive modelling of the edge pedestal in various high-confinement regimes, contributing to greater confidence in projection of pedestal height and width in ITER and reactors. ELMy H-modes operate near stability limits for ideal peeling–ballooning modes, as shown by calculations with the ELITE code. Experimental pedestal width in ELMy H-mode scales as the square root of βpol at the pedestal top, i.e. the dependence expected from theory if kinetic ballooning modes (KBMs) were responsible for limiting the pedestal width. A search for KBMs in experiment has revealed a short-wavelength electromagnetic fluctuation in the pedestal that is a candidate driver for inter-edge localized mode (ELM) pedestal regulation. A predictive pedestal model (EPED) has been tested on an extended set of ELMy H-modes from C-Mod, reproducing pedestal height and width reasonably well across the data set, and extending the tested range of EPED to the highest absolute pressures available on any existing tokamak and to within a factor of three of the pedestal pressure targeted for ITER. In addition, C-Mod offers access to two regimes, enhanced D-alpha (EDA) H-mode and I-mode, that have high pedestals, but in which large ELM activity is naturally suppressed and, instead, particle and impurity transport are regulated continuously. Pedestals of EDA H-mode and I-mode discharges are found to be ideal magnetohydrodynamic (MHD) stable with ELITE, consistent with the general absence of ELM activity. Invocation of alternative physics mechanisms may be required to make EPED-like predictions of pedestals in these kinds of intrinsically ELM-suppressed regimes, which would be very beneficial to operation in burning plasma devices.
    Nuclear Fusion 01/2013; 53(4):043016. · 2.73 Impact Factor
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    ABSTRACT: Transport in ohmically heated plasmas in Alcator C-Mod was studied in both the linear (LOC) and saturated (SOC) ohmic L-mode confinement regimes and the importance of turbulent transport in the region r/a = 0.5–0.8 was established. After an extensive analysis with TGLF and GYRO, it is found that using an effective impurity ion species with Zi = 8, and moderately high Zeff (2.0–5.6), in the LOC regime electron transport becomes dominant due to TEM turbulence. The key ingredient in the present results is the observation that dilution of the main ion species (deuterium) by impurity species of moderate charge state reduces dominant ITG turbulence, in contrast to the SOC regime with little, if any dilution. The turbulent spectrum measured with the phase contrast imaging (PCI) diagnostic is in qualitative agreement with predictions of a synthetic PCI diagnostic adopted to Global GYRO. The toroidal rotation in the low-density LOC regime is in the co-current direction but as the density is raised in the SOC regime the rotation reverses to the counter current drive direction. The impurity content of the plasma was measured recently and an effective Zi of 9 was deduced.
    Plasma Physics and Controlled Fusion 11/2012; 54(12):124029. · 2.37 Impact Factor
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    ABSTRACT: Transitions from the L-mode regime to the I-mode regime, with an energy transport barrier, and to the H-mode regime with both an energy and particle transport barrier are studied on the Alcator C-Mod tokamak. Steady I-mode plasmas have been produced over a wide range of plasma field (3–6 T), current (0.8–1.35 MA), density and shaping in the unfavourable ion B × ∇B configuration. The power threshold for the L–I transition is higher than scalings for the L–H transition with favourable drift, and increases with plasma current as well as density. Threshold conditions for the I–H transition are more variable. In some conditions I-mode is maintained up to the maximum available ICRF power of 5 MW, nearly a factor of two above the L–I threshold, giving a robust operating window. Edge Te at the L–I transition is in the range 250–450 eV, over a range of current and density, about a factor of two higher than with favourable drift, while at the I–H transition it can be much higher (up to 1.1 keV) but is again widely variable. Heat pulses due to sawteeth may play a role in transitions. Controlled I–L back transitions indicate little power hysteresis.
    Nuclear Fusion 10/2012; 52(11):114009. · 2.73 Impact Factor
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    ABSTRACT: Backscattering experiments to detect lower-hybrid (LH) waves have been performed in Alcator C-Mod, using the two modified channels (60 GHz and 75 GHz) of an ordinary-mode reflectometry system with newly developed spectral recorders that can continuously monitor spectral power at a target frequency. The change in the baseline of the spectral recorder during the LH wave injection is highly correlated to the strength of the X-mode non-thermal electron cyclotron emission. In high density plasmas where an anomalous drop in the lower hybrid current drive efficiency is observed, the observed backscattered signals are expected to be generated near the last closed flux surface, demonstrating the presence of LH waves within the plasma. This experimental technique can be useful in identifying spatially localized LH electric fields in the periphery of high-density plasmas.
    The Review of scientific instruments 10/2012; 83(10):10E325. · 1.52 Impact Factor
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    ABSTRACT: Previous studies of turbulence using the reduced gyro-landau fluid code TGLF, and the gyrokinetic code GYRO, have predicted that in C-Mod ohmic plasmas a dilution of the main ions by a significant amount causes a reduction of turbulent transport in the ion channel [1]. This could be a factor in the LOC-SOC transition. To test this effect, experiments were performed where nitrogen was puffed into ohmic target plasmas with the density kept constant. This seeding reduced the turbulence in the ion diamagnetic direction as measured by phase contrast imaging. To determine impurity concentrations in the plasma, line brightnesses for the relevant impurity species (N, O, Ar, and Mo) were compared to Zeff from neoclassical conductivity and from continuum measurements. The turbulence and transport simulated with GYRO and TGLF were compared to that measured experimentally. Work supported by US DOE awards DE-FG02-94-ER54235 and DE-FC02-99-ER54512.[4pt] [1] M. Porkolab, et al, Bull. Am Phys. Soc. 56, no 12 139 (2011).
    10/2012;
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    ABSTRACT: The I-mode regime of operation has been extended in recent Alcator C-Mod campaigns in duration and robustness, over a wide range of parameters. It features an edge thermal barrier, with L-mode like density profiles and particle transport [1]. I-modes are now routinely maintained in stationary conditions for over 10 τE. They are usually ELM free, a key advantage given the concern over divertor heat pulses on ITER. Instead, a continuous pedestal fluctuation, the ``weakly coherent mode,'' appears to enhance selectively particle over thermal transport. High performance I-modes are usually obtained with unfavorable ion drift direction, and extend to low q95=2.5and νped^*=1.3. τE is in the range of H-mode (H98,y2 up to 1.2), and exhibits less power degradation (W˜P^0.7). Power thresholds for I-mode are higher than typical L-H scalings, and increase with Ip as well as density. The widest power range for I-mode, nearly a factor of two above the L-I threshold, has been obtained in reversed field, lower null discharges at moderate ne. The focus of 2012 experiments is on assessing the density range and dependences, important for extrapolating to ITER, and measuring the pedestal profiles and fluctuations in greater detail.[4pt] [1] Whyte D G et al 2010 Nucl. Fusion 50 105005
    10/2012;
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    ABSTRACT: A high-resolution charge exchange recombination spectroscopy (CXRS) diagnostic is installed on Alcator C-Mod to measure edge profiles of B^5+ density, temperature, and flows and to deduce radial electric field profiles. As donor particles for CX, both hydrogen from a modulated diagnostic neutral beam and deuterium from a local gas puff are employed. While the former technique often suffers from low signal intensities near the LCFS, the latter can be complicated by contamination of the signal by molecular lines and cross-section effects. After discussing how these challenges are overcome in data analysis, we present recent measurements in regimes which have not extensively been investigated previously with edge CXRS on C-Mod. The first one is the I-mode regime, which is characterized by H-mode like energy confinement and L-mode like particle transport. Focusing on I-modes with unfavorable ion B x∇B drift, we study profile scalings with plasma parameters such as density, plasma current and heating power. The second regime is ELMy H-mode, where we explore the capabilities of the CXRS diagnostic to measure profile evolutions during and in between ELMs.
    10/2012;
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    ABSTRACT: A three-chord, poloidally-viewing, polarimetry diagnostic measuring the Faraday effect is now operational on Alcator C-Mod. Faraday effect phase shifts observed during Ohmic discharges for all three chords agree well with calculations using EFIT and Thomson scattering density profiles. During lower hybrid current drive experiments, the Faraday rotation measurements, which are sensitive to the current change, indicate that the current density profile flattens and then relaxes back to an Ohmic current profile within ˜150 ms following the RF pulse. A 30% discrepancy between polarimetry measurements and EFIT calculation (without internal constraints) during current drive has been found Using polarimetry data in addition to MSE as constraints on EFIT reconstructions will be discussed. Sources of error, and progress made toward improving the signal-to-noise level of the diagnostic for the lower hybrid current drive experiments will also be described
    10/2012;
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    ABSTRACT: Comprehensive studies on H-mode threshold physics have been conducted on Alcator C-Mod tokamak, covering many ITER-relevant conditions, e.g. similar magnetic field and density range, metallic wall, and divertor configuration. C-Mod experiments confirm that the density dependence of H-mode threshold power (Pth) is U-shaped without clear dependence on plasma current, and the local minimum of Pth in density (nmin) decreases as BT is reduced [1]. An effect of divertor geometry on Pth was identified, with a dramatic (˜50%) reduction in Pth seen in ``slot'' divertor operation accompanied by longer SOL connection length [2]. Experimental results were also compared with a new physics-based model for Pth [3], showing reasonable agreement of density, BT, and divertor geometry dependences with model predictions. A significant implication of this model is that nmin occurs as the SOL transitions from sheath-limited to conduction-limited regime, which also seems to agree with experiments. Supported by USDoE award DE-FC02-99ER54512. [4pt] [1] Y.Ma, et al Nucl. Fusion 52 (2012) 023010.[0pt] [2] Y.Ma, et al PPCF 54 (2012) 082002.[0pt] [3] W.Fundamenski, et al Nucl. Fusion 52 (2012) 062003.
    10/2012;
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    ABSTRACT: A new ``Shoelace'' antenna has been installed on Alcator C-Mod. Its goal is to interact with edge fluctuations, and particularly the quasi-coherent mode (QCM) associated with the steady-state EDA H-mode. With k=1.5 cm-1 and frequency range, 40<f<300 kHz, the antenna is matched to the QCM and the Weakly Coherent Mode (WCM) of the I-mode regime. Here, we present initial results from the first operation of the antenna into ohmic, ohmic EDA H-mode, and I-mode plasmas. The antenna response observed on fluctuations diagnostics, including Mirnov coils and a Gas Puff Imaging (GPI) system, is discussed and compared with BOUT++ simulations. The coherent response on Mirnov coils reveals an antenna-driven field perturbation that competes with background turbulence. The edge plasma stability to antenna perturbations is investigated by searching for poles in the coil response. A reciprocating probe scans through the scrape-off layer. Mounting a magnetic coil head on the probe gives the perturbation's radial decay rate, kR 0.3-1.0 cm-1; a Langmuir probe head examines the antenna's effect on radial transport.
    10/2012;
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    ABSTRACT: The I-mode regime is characterized by the formation of ion and electron temperature pedestals, while maintaining L-mode-like radial density profiles [1]. Changes in edge density, temperature and magnetic field fluctuations accompany the L-mode to I-mode transition with reduction of fluctuations in the 50-150kHz range as well as the appearance of a Weakly Coherent Mode (WCM) in the 150-300kHz range. Previous work[2] has established a connection between the midrange fluctuation reduction and a decrease in the effective thermal diffusivity in the pedestal region. The mechanism for maintaining sufficient particle transport to avoid impurity accumulation has been unclear. In this work, quantitative comparisons are made between particle transport measurements across the LCFS and the intensity of the WCM in the I-mode. The particle transport has been estimated using Dα profiles measured near the outboard midplane and the intensity of the WCM is obtained by spectral analysis of density fluctuations from reflectometry. The measurements show correlations between the particle transport and the WCM intensity, supporting evidence that the WCM causes enhanced transport.[4pt] [1] D.G. Whyte, et al., Nucl. Fusion 50 (2010)105005.[0pt] [2] A.E. Hubbard, et al., Phys. Plasmas 18(2011)056115
    10/2012;
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    ABSTRACT: A laser-based (2.55 THz) mulitchord polarimeter is now operational on Alcator C-Mod and is used to make measurements of the internal magnetic field structure as well as plasma fluctuations. The polarimeter is designed to measure the Faraday effect for high-field (up to 8.3 T) and high-density (up to 5 × 10(20) m(-3)) ITER relevant plasma conditions. Initial 3 chord tests are consistent with magnetic equilibrium reconstructions and indicate no measurable contamination from the toroidal magnetic field due to the Cotton-Mouton effect or misalignment. Time response of <1 μs enables the measurement of fast equilibrium temporal dynamics as well as high-frequency fluctuations.
    The Review of scientific instruments 10/2012; 83(10):10E316. · 1.52 Impact Factor

Publication Stats

3k Citations
561.49 Total Impact Points

Institutions

  • 1978–2013
    • Massachusetts Institute of Technology
      • Plasma Science and Fusion Center (PSFC)
      Cambridge, Massachusetts, United States
  • 2012
    • University of California, Los Angeles
      • Department of Electrical Engineering
      Los Angeles, California, United States
  • 2007
    • Sandia National Laboratories
      Albuquerque, New Mexico, United States
  • 2002–2005
    • Columbia University
      • Department of Physics
      New York City, New York, United States
  • 2004
    • University of Maryland, College Park
      Maryland, United States
  • 1997–2004
    • University of Texas at Austin
      • Fusion Research Center
      Texas City, TX, United States
    • General Atomics
      San Diego, California, United States
  • 1978–2002
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, MD, United States
  • 1975–2002
    • Princeton University
      • Princeton Plasma Physics Laboratory
      Princeton, NJ, United States
  • 1981
    • Oak Ridge National Laboratory
      Oak Ridge, Florida, United States