The Experts below are selected from a list of 147 Experts worldwide ranked by ideXlab platform
Stephen Wukitch - One of the best experts on this subject based on the ideXlab platform.
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characterization of sol Plasma flows and Potentials in icrf heated Plasmas in alcator c mod
Plasma Physics and Controlled Fusion, 2017Co-Authors: R Hong, J L Terry, Stephen Wukitch, I Cziegler, M L Reinke, G R TynanAbstract:Gas-puff imaging techniques are employed to determine the far SOL region radial electric field and the Plasma potential in ICRF heated discharges in the Alcator C-Mod tokamak. The 2-dimensional velocity fields of the turbulent structures, which are advected by RF-induced $ \mathbf{E\times B} $ flows, are obtained via the time-delay estimation (TDE) techniques. Both the magnitude and radial extension of the radial electric field $ E_r $ are observed to increase with the toroidal magnetic field strength $ B_\varphi $ and the ICRF power. In particular, the RF-induced $ E_r $ extends from the vicinity of the ICRF antenna to the separatrix when $ B_\varphi=7.9\,\mathrm{T} $ and $ P_{\mathrm{ICRF}} \gtrsim 1\,\mathrm{MW} $. In addition, low-Z impurity seeding near the antenna is found to substantially reduce the sheath potential associated with ICRF power. The ICRF-induced Potentials are also estimated in different antenna configurations: (1) conventional toroidally-aligned (TA) antenna versus field-aligned (FA) antenna; (2) FA monopole versus FA dipole. Results show that FA and TA antennas produce similar magnitude of Plasma Potentials, and the FA monopole induced greater potential than the FA dipole phasing.
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icrf enhanced Plasma Potentials in the sol of alcator c mod
RADIOFREQUENCY POWER IN PLASMAS: Proceedings of the 20th Topical Conference, 2014Co-Authors: R Ochoukov, D G Whyte, D Brunner, D A Dippolito, B Labombard, B Lipschultz, J R Myra, J L Terry, Stephen WukitchAbstract: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 th...
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icrf enhanced Plasma Potentials in the sol of alcator c mod
Plasma Physics and Controlled Fusion, 2014Co-Authors: R Ochoukov, D G Whyte, D Brunner, D A Dippolito, B Labombard, B Lipschultz, J R Myra, J L Terry, Stephen WukitchAbstract:An extensive experimental survey of Plasma Potentials induced by ion cyclotron range-of frequency (ICRF) heating was carried out in the scrape-off layer (SOL) Plasmas on the Alcator C-Mod tokamak. Enhanced Plasma Potentials >100?V are observed at locations where local magnetic fields map to active ICRF antennas. In these cases, the enhanced potential appears only when a local Plasma density threshold is surpassed?a threshold that is quantitatively consistent with slow wave (SW) RF rectification theory. However, in many cases large potential enhancements are found in locations that do not map along magnetic field lines to active antennas without obstruction, i.e. locations that are inaccessible to SWs launched by the active antennas. Enhanced Potentials in these ?unmapped? locations are correlated with local Plasma parameters, ICRF electromagnetic fields associated with the fast wave (FW) and SW, launched wave spectra, and the boundary surface geometry. It is found that enhanced Plasma Potentials in unmapped locations correlate with the FW field strength. These observations are qualitatively consistent with a model that accounts for the conversion of FWs to SWs at conducting surfaces oriented at an oblique angle with respect to the magnetic field, with the SW leading to sheath rectification. In addition, enhanced Plasma Potentials are found far into the shadow of passive limiter structures. These are correlated with the magnitude of the local FW field strength, yet the effect does not follow any present model. Overall, ICRF-induced Plasma Potentials may appear in regions far removed from the active antennas, yet due to the complex response of the SOL Potentials at a variety of boundary surfaces, it remains unclear what part of the Plasma-facing wall should be targeted to mitigate ICRF-induced impurities. The results also suggest that operating active ICRF antennas in a high single pass absorption regime is crucial in minimizing the effects of the FW fields on Plasma?material interactions.
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investigation of rf enhanced Plasma Potentials on alcator c mod
Journal of Nuclear Materials, 2013Co-Authors: R Ochoukov, D G Whyte, D Brunner, B Labombard, B Lipschultz, J R Myra, J L Terry, I Cziegler, Stephen WukitchAbstract: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 ( Φ P ) in RF-heated discharges on C-Mod reveals that significant Φ P 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 Φ P 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 Φ P 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.
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Interpretation and implementation of an ion sensitive probe as a Plasma potential diagnostic.
Review of Scientific Instruments, 2010Co-Authors: R Ochoukov, Dennis Whyte, Bruce Lipschultz, Brian Labombard, Stephen WukitchAbstract:An ion sensitive probe (ISP) is developed as a robust diagnostic for measuring Plasma Potentials (ΦP) in magnetized Plasmas. The ISP relies on the large difference between the ion and electron gyroradii (ρi/ρe∼60) to reduce the electron collection at a collector recessed behind a separately biased wall distance ∼ρi. We develop a new ISP method to measure the Plasma potential that is independent of the precise position and shape of the collector. ΦP 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 ΦP measured with an emissive probe over a wide range of Plasma conditions in a small magnetized Plasma.
R Ochoukov - One of the best experts on this subject based on the ideXlab platform.
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icrf enhanced Plasma Potentials in the sol of alcator c mod
RADIOFREQUENCY POWER IN PLASMAS: Proceedings of the 20th Topical Conference, 2014Co-Authors: R Ochoukov, D G Whyte, D Brunner, D A Dippolito, B Labombard, B Lipschultz, J R Myra, J L Terry, Stephen WukitchAbstract: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 th...
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icrf enhanced Plasma Potentials in the sol of alcator c mod
Plasma Physics and Controlled Fusion, 2014Co-Authors: R Ochoukov, D G Whyte, D Brunner, D A Dippolito, B Labombard, B Lipschultz, J R Myra, J L Terry, Stephen WukitchAbstract:An extensive experimental survey of Plasma Potentials induced by ion cyclotron range-of frequency (ICRF) heating was carried out in the scrape-off layer (SOL) Plasmas on the Alcator C-Mod tokamak. Enhanced Plasma Potentials >100?V are observed at locations where local magnetic fields map to active ICRF antennas. In these cases, the enhanced potential appears only when a local Plasma density threshold is surpassed?a threshold that is quantitatively consistent with slow wave (SW) RF rectification theory. However, in many cases large potential enhancements are found in locations that do not map along magnetic field lines to active antennas without obstruction, i.e. locations that are inaccessible to SWs launched by the active antennas. Enhanced Potentials in these ?unmapped? locations are correlated with local Plasma parameters, ICRF electromagnetic fields associated with the fast wave (FW) and SW, launched wave spectra, and the boundary surface geometry. It is found that enhanced Plasma Potentials in unmapped locations correlate with the FW field strength. These observations are qualitatively consistent with a model that accounts for the conversion of FWs to SWs at conducting surfaces oriented at an oblique angle with respect to the magnetic field, with the SW leading to sheath rectification. In addition, enhanced Plasma Potentials are found far into the shadow of passive limiter structures. These are correlated with the magnitude of the local FW field strength, yet the effect does not follow any present model. Overall, ICRF-induced Plasma Potentials may appear in regions far removed from the active antennas, yet due to the complex response of the SOL Potentials at a variety of boundary surfaces, it remains unclear what part of the Plasma-facing wall should be targeted to mitigate ICRF-induced impurities. The results also suggest that operating active ICRF antennas in a high single pass absorption regime is crucial in minimizing the effects of the FW fields on Plasma?material interactions.
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investigation of rf enhanced Plasma Potentials on alcator c mod
Journal of Nuclear Materials, 2013Co-Authors: R Ochoukov, D G Whyte, D Brunner, B Labombard, B Lipschultz, J R Myra, J L Terry, I Cziegler, Stephen WukitchAbstract: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 ( Φ P ) in RF-heated discharges on C-Mod reveals that significant Φ P 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 Φ P 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 Φ P 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.
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Interpretation and implementation of an ion sensitive probe as a Plasma potential diagnostic.
Review of Scientific Instruments, 2010Co-Authors: R Ochoukov, Dennis Whyte, Bruce Lipschultz, Brian Labombard, Stephen WukitchAbstract:An ion sensitive probe (ISP) is developed as a robust diagnostic for measuring Plasma Potentials (ΦP) in magnetized Plasmas. The ISP relies on the large difference between the ion and electron gyroradii (ρi/ρe∼60) to reduce the electron collection at a collector recessed behind a separately biased wall distance ∼ρi. We develop a new ISP method to measure the Plasma potential that is independent of the precise position and shape of the collector. ΦP 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 ΦP measured with an emissive probe over a wide range of Plasma conditions in a small magnetized Plasma.
J L Terry - One of the best experts on this subject based on the ideXlab platform.
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characterization of sol Plasma flows and Potentials in icrf heated Plasmas in alcator c mod
Plasma Physics and Controlled Fusion, 2017Co-Authors: R Hong, J L Terry, Stephen Wukitch, I Cziegler, M L Reinke, G R TynanAbstract:Gas-puff imaging techniques are employed to determine the far SOL region radial electric field and the Plasma potential in ICRF heated discharges in the Alcator C-Mod tokamak. The 2-dimensional velocity fields of the turbulent structures, which are advected by RF-induced $ \mathbf{E\times B} $ flows, are obtained via the time-delay estimation (TDE) techniques. Both the magnitude and radial extension of the radial electric field $ E_r $ are observed to increase with the toroidal magnetic field strength $ B_\varphi $ and the ICRF power. In particular, the RF-induced $ E_r $ extends from the vicinity of the ICRF antenna to the separatrix when $ B_\varphi=7.9\,\mathrm{T} $ and $ P_{\mathrm{ICRF}} \gtrsim 1\,\mathrm{MW} $. In addition, low-Z impurity seeding near the antenna is found to substantially reduce the sheath potential associated with ICRF power. The ICRF-induced Potentials are also estimated in different antenna configurations: (1) conventional toroidally-aligned (TA) antenna versus field-aligned (FA) antenna; (2) FA monopole versus FA dipole. Results show that FA and TA antennas produce similar magnitude of Plasma Potentials, and the FA monopole induced greater potential than the FA dipole phasing.
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icrf enhanced Plasma Potentials in the sol of alcator c mod
RADIOFREQUENCY POWER IN PLASMAS: Proceedings of the 20th Topical Conference, 2014Co-Authors: R Ochoukov, D G Whyte, D Brunner, D A Dippolito, B Labombard, B Lipschultz, J R Myra, J L Terry, Stephen WukitchAbstract: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 th...
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icrf enhanced Plasma Potentials in the sol of alcator c mod
Plasma Physics and Controlled Fusion, 2014Co-Authors: R Ochoukov, D G Whyte, D Brunner, D A Dippolito, B Labombard, B Lipschultz, J R Myra, J L Terry, Stephen WukitchAbstract:An extensive experimental survey of Plasma Potentials induced by ion cyclotron range-of frequency (ICRF) heating was carried out in the scrape-off layer (SOL) Plasmas on the Alcator C-Mod tokamak. Enhanced Plasma Potentials >100?V are observed at locations where local magnetic fields map to active ICRF antennas. In these cases, the enhanced potential appears only when a local Plasma density threshold is surpassed?a threshold that is quantitatively consistent with slow wave (SW) RF rectification theory. However, in many cases large potential enhancements are found in locations that do not map along magnetic field lines to active antennas without obstruction, i.e. locations that are inaccessible to SWs launched by the active antennas. Enhanced Potentials in these ?unmapped? locations are correlated with local Plasma parameters, ICRF electromagnetic fields associated with the fast wave (FW) and SW, launched wave spectra, and the boundary surface geometry. It is found that enhanced Plasma Potentials in unmapped locations correlate with the FW field strength. These observations are qualitatively consistent with a model that accounts for the conversion of FWs to SWs at conducting surfaces oriented at an oblique angle with respect to the magnetic field, with the SW leading to sheath rectification. In addition, enhanced Plasma Potentials are found far into the shadow of passive limiter structures. These are correlated with the magnitude of the local FW field strength, yet the effect does not follow any present model. Overall, ICRF-induced Plasma Potentials may appear in regions far removed from the active antennas, yet due to the complex response of the SOL Potentials at a variety of boundary surfaces, it remains unclear what part of the Plasma-facing wall should be targeted to mitigate ICRF-induced impurities. The results also suggest that operating active ICRF antennas in a high single pass absorption regime is crucial in minimizing the effects of the FW fields on Plasma?material interactions.
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investigation of rf enhanced Plasma Potentials on alcator c mod
Journal of Nuclear Materials, 2013Co-Authors: R Ochoukov, D G Whyte, D Brunner, B Labombard, B Lipschultz, J R Myra, J L Terry, I Cziegler, Stephen WukitchAbstract: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 ( Φ P ) in RF-heated discharges on C-Mod reveals that significant Φ P 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 Φ P 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 Φ P 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.
Noah Hershkowitz - One of the best experts on this subject based on the ideXlab platform.
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experimental studies of the difference between Plasma Potentials measured by langmuir probes and emissive probes in presheaths
Plasma Sources Science and Technology, 2020Co-Authors: Peixuan Li, Noah Hershkowitz, Eugene Wackerbarth, Greg SevernAbstract:The Plasma potential measured by cylindrical and planar Langmuir probes has been shown to differ from the Plasma potential measured by emissive probes in the neighborhood of the presheath near a negatively biased electrode immersed in a weakly collisional low temperature argon Plasma. There are two principal results demonstrated in this paper. First, while it is well known that Langmuir probes cannot reliably measure Plasma Potentials inside of sheaths, results presented here demonstrate that the problem persists in presheaths, the quasineutral Plasma bordering sheaths. It is known that emissive probes analyzed in the limit of zero emission accurately measure the Plasma potential in the sheath. It is now clear that they are the only known electrostatic probe technique able to measure the Plasma potential accurately throughout the presheath. Second, it is shown that the difference between potential measurements made by Langmuir probes and emissive probes in the body of the Plasma, farther than a presheath distance from the boundary, is not proportional to Te as has been previously claimed.
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Negative Plasma potential in a chamber with a dielectric coated Plasma boundary
2012 Abstracts IEEE International Conference on Plasma Science, 2012Co-Authors: J. P. Sheehan, Noah HershkowitzAbstract:Summary form only given. Negative Plasma Potentials with respect to a grounded wall that was coated with a dielectric have been achieved in an electropositive Plasma confined by a multidipole device. A Langmuir probe was used to measure the density and temperature of the electrons and an emissive probe was used to measure the Plasma potential profile near the Plasma boundary. For many discharge parameters, the potential profile was that of a typical electropositive sheath, but it was shifted negative due to negative charge accumulated on the Plasma-surface boundary. A virtual cathode was observed near the boundary when the neutral pressure, primary electron energy, and/or discharge current were low (for example, 2×10−4 Torr, 60 eV, and 80 mA, respectively). The behavior of the sheath potential was shown to be consistent with that predicted by electron creation and loss balance and a qualitative mechanism for wall charging is presented.
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Negative Plasma potential in a multidipole chamber with a dielectric coated Plasma boundary
Journal of Vacuum Science and Technology, 2012Co-Authors: J. P. Sheehan, Noah HershkowitzAbstract:Negative Plasma Potentials with respect to a grounded wall that was coated with a dielectric have been achieved in an electropositive Plasma confined by a multidipole device. A Langmuir probe was used to measure the density and temperatures of the bi-Maxwellian distribution electrons and an emissive probe was used to measure the Plasma potential profile near the Plasma boundary. For many discharge parameters, the potential profile was that of a typical electropositive sheath, but it was shifted negative due to negative charge accumulated on the Plasma-surface boundary. A virtual cathode was observed near the boundary when the neutral pressure, primary electron energy, and/or discharge current were low (∼2 × 10−4 Torr, ∼60 eV, and 80 mA, respectively). The behavior of the sheath potential was shown to be consistent with that predicted by particle balance and a qualitative mechanism for wall charging is presented.
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Negative Plasma potential in unmagnetized DC electropositive Plasma with conducting walls
Physics Letters A, 2011Co-Authors: Lutfi Oksuz, Noah HershkowitzAbstract:Abstract Negative Plasma Potentials were obtained in DC hot filament unmagnetized electropositive argon Plasma for sufficiently low neutral pressure. Double layers provide ion and electron confinement near the walls. The potential profiles from the center of the Plasma to the potential minima are quite similar in shape to those observed when the Plasma has positive Plasma Potentials. The primary electrons emitted from the filaments are important for charge conservation and for modification of the Bohm criteria but are not important for current balance.
D Brunner - One of the best experts on this subject based on the ideXlab platform.
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icrf enhanced Plasma Potentials in the sol of alcator c mod
RADIOFREQUENCY POWER IN PLASMAS: Proceedings of the 20th Topical Conference, 2014Co-Authors: R Ochoukov, D G Whyte, D Brunner, D A Dippolito, B Labombard, B Lipschultz, J R Myra, J L Terry, Stephen WukitchAbstract: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 th...
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icrf enhanced Plasma Potentials in the sol of alcator c mod
Plasma Physics and Controlled Fusion, 2014Co-Authors: R Ochoukov, D G Whyte, D Brunner, D A Dippolito, B Labombard, B Lipschultz, J R Myra, J L Terry, Stephen WukitchAbstract:An extensive experimental survey of Plasma Potentials induced by ion cyclotron range-of frequency (ICRF) heating was carried out in the scrape-off layer (SOL) Plasmas on the Alcator C-Mod tokamak. Enhanced Plasma Potentials >100?V are observed at locations where local magnetic fields map to active ICRF antennas. In these cases, the enhanced potential appears only when a local Plasma density threshold is surpassed?a threshold that is quantitatively consistent with slow wave (SW) RF rectification theory. However, in many cases large potential enhancements are found in locations that do not map along magnetic field lines to active antennas without obstruction, i.e. locations that are inaccessible to SWs launched by the active antennas. Enhanced Potentials in these ?unmapped? locations are correlated with local Plasma parameters, ICRF electromagnetic fields associated with the fast wave (FW) and SW, launched wave spectra, and the boundary surface geometry. It is found that enhanced Plasma Potentials in unmapped locations correlate with the FW field strength. These observations are qualitatively consistent with a model that accounts for the conversion of FWs to SWs at conducting surfaces oriented at an oblique angle with respect to the magnetic field, with the SW leading to sheath rectification. In addition, enhanced Plasma Potentials are found far into the shadow of passive limiter structures. These are correlated with the magnitude of the local FW field strength, yet the effect does not follow any present model. Overall, ICRF-induced Plasma Potentials may appear in regions far removed from the active antennas, yet due to the complex response of the SOL Potentials at a variety of boundary surfaces, it remains unclear what part of the Plasma-facing wall should be targeted to mitigate ICRF-induced impurities. The results also suggest that operating active ICRF antennas in a high single pass absorption regime is crucial in minimizing the effects of the FW fields on Plasma?material interactions.
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investigation of rf enhanced Plasma Potentials on alcator c mod
Journal of Nuclear Materials, 2013Co-Authors: R Ochoukov, D G Whyte, D Brunner, B Labombard, B Lipschultz, J R Myra, J L Terry, I Cziegler, Stephen WukitchAbstract: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 ( Φ P ) in RF-heated discharges on C-Mod reveals that significant Φ P 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 Φ P 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 Φ P 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.