Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session JI1: Waves and Energetic Particles |
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Chair: James Van Dam, University of Texas Room: Rosen Centre Hotel Junior Ballroom |
Tuesday, November 13, 2007 2:00PM - 2:30PM |
JI1.00001: Excitation of Alfv\'{e}n Eigenmodes by Low Velocity Beam Ions in the JET and DIII-D Tokamaks Invited Speaker: New data on the DIII-D and JET tokamaks reveal a rich variety of Alfv\'{e}nic activity excited by neutral beam ions traveling at only a small fraction of the local Alfv\'{e}n velocity. These observations challenge our detailed understanding of the excitation of Alfv\'{e}nic phenomena and provide a validation platform for testing fundamental theoretical predictions. In addition, precise internal measurements of density and temperature fluctuations reveal new information on the kinetic properties of Alfv\'{e}n eigenmodes that challenge ideal MHD descriptions of these instabilities. Recent experiments on the JET facility with 3.5 T magnetic field and low plasma density demonstrate that Cascade modes are excited by 50 keV beam ions corresponding to only v$_{A}$/6, where v$_{A}$ is the local Alfv\'{e}n velocity. Toroidal Alfv\'{e}n eigenmodes are excited by ions traveling at only v$_{A}$/4, well below the v$_{A}$/3 sideband condition for the primary resonance. Detailed stability analysis reveals a key role played by finite orbit effects and in particular the beam ion anisotropy for these low energy excitations. Similarly, studies on DIII-D with 2.0 T magnetic fields reveal that the direction of injection of neutral beam injection is a critical factor in the excitation of Alfv\'{e}n eigenmodes. As in JET, a key to directional sensitivity is the finite orbit width of the fast ions. New observations are also obtained on the excitation of $n$=0 modes in both JET and DIII-D driven by low energy (50-80 keV) beam ions. Internal measurements reveal much smaller temperature-to-density fluctuation levels for these modes, suggesting that the fluctuations cannot be interpreted as due to the radial displacement of magnetic field lines. [Preview Abstract] |
Tuesday, November 13, 2007 2:30PM - 3:00PM |
JI1.00002: Alfv\'{e}n Cascade modes at high $\beta_{e}$ in the National Spherical Torus Experiment--structure and suppression Invited Speaker: Beam ions and/or fusion alphas are expected to excite Alfv\'{e}n Cascade (AC) modes (i.e. reversed-shear Alfv\'{e}n eigenmodes) in ITER reversed-shear advanced scenarios. The National Spherical Torus eXperiment (NSTX), where fast-ions with comparable v/v$_{Alfv\mbox{\'{e}}n}$ ($\sim $ 2 -- 4) excite ACs, is an ideal device in which to observe ACs and their impact. Its wide range of \textit{$\beta $}$_{e }$(ratio of electron to magnetic pressure) enables tests of AC theory up to, and beyond, a critical \textit{$\beta $}$_{e}$ where suppression is predicted. A value for critical \textit{$\beta $}$_{e}$, $\sim $ [4$q_{min}^{2}{\rm o}$1+(7/4)(T$_{i}$/T$_{e}))$]$^{-1}$, may be derived from the theory of Breizman, et al. [\textit{Phys. Plasmas }\textbf{\textit{12}}\textit{ (2005) 112506}]. Observations of suppression and frequency evolution in NSTX, including onset and saturation, agree well with this theory and calculations by the NOVA-K linear stability code. The dependence of AC frequency on minimum safety factor ($q_{min})$ enables a sensitive determination of $q_{min}$ from the AC spectrum that agrees well with the minimum of the $q$ profile measured using the motional Stark effect. AC structure measurements near critical \textit{$\beta $}$_{e}$ from three fixed frequency (i.e. spatially localized) reflectometers and three tangential interferometers show a structure consistent with predicted localization near the $q_{min}$ radius. Magnetic measurements indicate shear-wave polarization at $q_{min}$. Fast-ion response is monitored with neutral particle analyzers, a fast lost ion probe and neutron detectors. Profile measurements of $q$, density, electron and ion temperature, and rotation are used by NOVA-K to predict mode structure and frequency, or suppression, for direct comparison with the mode measurements. These novel observations of ACs near critical \textit{$\beta $}$_{e}$ are well explained by theory, allowing us to extrapolate our understanding of this physics with confidence. [Preview Abstract] |
Tuesday, November 13, 2007 3:00PM - 3:30PM |
JI1.00003: Lower Hybrid Current Drive Experiments on Alcator C-Mod: Comparison with Theory and Simulation Invited Speaker: Recently, lower hybrid current drive (LHCD) experiments have been carried out on Alcator C-Mod using an RF system consisting of 12 klystrons at 4.6 GHz, feeding a 4 $\times $ 22 waveguide array. Up to 900 kW of LH power has been coupled in the range1.6 $\le $ n$_{//} \quad \le $ 4), where n$_{// }$is the parallel refractive index. Driven LH currents have been inferred from magnetic measurements by extrapolating to zero loop voltage, yielding an efficiency of n$_{20}$I$_{LH}$R/P$_{LH} \quad \approx $ 0.3 [1]. We have simulated the LH current drive in these discharges using the combined ray tracing / 3D (r, v$_{\bot }$, v$_{//})$ Fokker Planck code GENRAY -- CQL3D [2] and found similar current drive efficiencies. Measurements of nonthermal x-ray emission and electron cyclotron emission (ECE) confirm the presence of a significant fast electron population that varies with waveguide phasing and plasma density. Studies are currently underway to investigate the role of fast electron diffusion and full-wave effects such as diffractional broadening in determining the spatial and velocity space structure of the nonthermal electrons. The 3D (r, v$_{\bot }$, v$_{//})$ electron distribution function from CQL3D has been used in synthetic diagnostic codes to simulate the measured hard x-ray and ECE emissions. Fast electron diffusion times have been inferred from x-ray data by employing a radial diffusion operator in CQL3D and determining the fast electron diffusivities that are required to reproduce the experimentally observed profiles of hard x-ray emission. Finally, we have been performing full-wave LH field simulations using the massively parallel TORIC --LH solver [3] in order to assess spatial and spectral broadening of the incident wave front that can result from diffraction and wave focusing effects. \newline \newline [1] R. Parker, Bull. Am. Phys. Soc. \textbf{51}, 20 (2006). \newline [2] R.W. Harvey and M. McCoy, ``The CQL3D Fokker Planck Code,'' \textit{Proc. IAEA Tech. Comm. Meeting on Simulation and Modeling of Thermonuclear Plasmas}, Montreal, Canada, 1992. \newline [3] J. C. Wright \textit{et al.}, Nucl. Fusion \textbf{45}, 1411 (2005). [Preview Abstract] |
Tuesday, November 13, 2007 3:30PM - 4:00PM |
JI1.00004: ICRF performance with Metallic Plasma Facing Components: Revenge of the Sheath Invited Speaker: Ion cyclotron range of frequency (ICRF) heating is expected to provide auxiliary heating for ITER and future fusion reactors where high Z metallic plasma facing components (PFCs) are envisioned. The advantages of ICRF heating is the availability of relatively inexpensive high power sources and it can directly heat ions. For coupling, the antenna needs to be close to the plasma and antenna operation can be limited by compatibility (impurity generation, density production and erosion). Utilizing high Z PFCs, control of ICRF generated impurities becomes more important because the acceptable fractional high Z material concentration in the plasma is of order 1000 times less than low Z materials. In addition, low Z coatings applied in-situ, ie boronization, is often utilized to mitigate the high Z impurities in the plasma. However, erosion of these typically thin, low Z coatings will limit their effective lifetime. In Alcator C-Mod, we have investigated the compatibility of high power ICRF heating with high performance plasmas and high-Z PFCs with and without boronization. With boronization, record C-Mod stored energy and world record plasma pressures were achieved with 5.25 MW of injected ICRF power. However, impurity control through boronization is temporary and boronization appears to erode 3-5 times faster with ICRF compared with Ohmic H-modes. Experimental evidence suggests that RF-enhanced sheaths on open field lines are responsible for enhanced erosion and impurity influx. Utilizing localized boronization, we have determined that the primary impurity source is outside the divertor and we demonstrated that the erosion location is linked to the active antenna. Furthermore, we observed that erosion rate associated with ICRF heating was unaffected by the heating scenario's single pass absorption. Using a 3-D antenna code coupled to a full wave solver we will present the influence antenna geometry has upon sheaths and possible mitigation strategies. [Preview Abstract] |
Tuesday, November 13, 2007 4:00PM - 4:30PM |
JI1.00005: HHFW Heating Efficiency and Current Drive Enhancement at Longer Wavelengths on NSTX Invited Speaker: High harmonic fast wave (HHFW) heating and current drive (CD) are being developed on NSTX for supporting startup and sustainment of the ST plasma. Considerable enhancement of the core heating efficiency ($\eta$) under CD conditions has been demonstrated, correlating strongly with locating the onset density for fast wave perpendicular propagation -- n$_{onset }\propto {\rm B}_{\phi }\times $ k$_{\vert \vert }^{2}$/$\omega $ -- away from the antenna/wall. FW fields propagating close to the wall with decreasing B$_{\phi }$ and k$_{\vert \vert }$ could enhance both parametric decay instability (PDI) losses and losses in sheaths and structures around the machine. HHFW RF power delivered to the core plasma of NSTX is strongly reduced as k$_{\vert \vert }$ is reduced -- for ${\rm B}_{\Phi }$= 4.5kG, heating is $\sim $ 1/2 as effective at k$_{\phi }$= - 7m$^{-1}$ as at 14 m$^{-1}$ and $\sim $ 1/10 as effective at - 3m$^{-1}$. A dramatic increase in $\eta $ is observed for k$_{\phi }$ - 7m$^{-1}$ when B$_{\phi }$ is increased to 5.5 kG (central T$_{e}$ near 4 keV at P$_{RF}$ = 2 MW), when the density 2 cm in front of the antenna is at or below n$_{onset}$. However, $\eta $ is not improved when the density immediately in front of the antenna is elevated relative to the onset value. Furthermore, $\eta $ at even lower k$_{\phi }$ still falls off rapidly with the k$_{\phi }$value. Measured edge ion heating, attributable to PDI, does not change significantly with B$_{\phi }$ and thus the improvement in $\eta $is attributed to a reduction of surface FW losses. This work is important for understanding the role of perpendicular propagation of fast waves near the antenna/wall on surface power losses generally, and has important implications for FW heating efficiency in the standard minority regimes as well. For example, the antenna bombardment observed for k$_{\vert \vert }$ near zero excitation on TFTR can be attributed to n$_{onset}$ being exceeded at the antenna face. Improved detection of CD effects under conditions of higher coupling efficiency on NSTX will be presented. [Preview Abstract] |
Tuesday, November 13, 2007 4:30PM - 5:00PM |
JI1.00006: EBW harmonic generation in heating and current drive Invited Speaker: Experiments on MST and NSTX are using or proposing to use electron Bernstein waves (EBWs) for heating and current drive, as these modes can couple into the overdense, high-beta plasmas via X-B or O-X-B mode conversions. This work shows that for modest power - typical of those planned for experiments - significant energy can be transferred from the fundamental mode to its harmonic. This arises because of the existence of a resonant point, where both the frequency and wave number match for the fundamental and its harmonic. This resonant point can occur where the harmonic has zero group velocity, hence it is weakly stabilized by propagation, and large power transfer occurs. The required pump power is much lower than the electron thermal energy. It is found that the amplitude of the second harmonic EBW excited can exceed that of the fundamental wave with the plasma and wave parameters used in the experiments. The system is investigated both analytically and through use of the VORPAL computing framework, with the latter via both via both the delta-f and full particle-in-cell (PIC) simulations, which confirm the analytical predictions. This second harmonic EBW generation can cause the wave power absorbed near the resonance layer at the half-harmonic frequencies and thus affect EBW power deposition significantly. [Preview Abstract] |
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