Search

EUV emissions from light impurities were observed simultaneously with two spectrometers — one at the limiter, the other 90° away toroidally – on the Alcator A tokamak. A difference in the time history of the emission from the limiter was noted during the low-density phase of the discharge. In addition, differences in the spatial profiles at the limiter are discussed. A simple model is used to explain the difference in the observed time histories. The model indicates that, if either re-cycling or a local source is present at the limiter, the emission will exhibit a toroidal asymmetry in agreement with the experimental data.

High‐speed lithium pellets have been injected into Alcator C tokamak plasmas in order to measure the internal magnetic field, and thus current density profiles. In the pellet ablation cloud, intense visible line radiation from the Li+ ion (λ≊5485 A, 1s2s 3S−1s2p 3P) is polarized due to the Zeeman effect, and measurement of the polarization angle yields the direction of the total local magnetic field. A ‘‘snap shot’’ of the q profile is obtained as the pellet penetrates from the edge into the center of the discharge, in a time of about 300 μs. The spatial resolution of the measurement is about 1 cm. At a toroidal field of BT=10 T, the emission in the unshifted π component of the Zeeman triplet is more than 80% polarized, and q profiles have been obtained. The pellets are perturbative (〈Δne〉/〈ne〉≊1), but the total pellet penetration time is at least a factor of 1000 smaller than the classical skin time. It can thus be anticipated that the current density profile should not be perturbed significantly during ...

Experiments are underway on the Alcator C tokamak with over 1 MW of RF power injected into the plasma at a frequency of 4.6 GHz to study both heating and current drive effects. During these studies, impurity generation from limiter structures has been observed. The RF induced impurity influx is a strongly nonlinear function of net injected power. For PRF P RF = 1.0 MW , n e = 1.3 × 10 14 cm −3 , and the SiC coated limiters, Zeff is seen to increase from 1.5 before the RF pulse to about 4 during the heating. At the same time, central Te increases from 2000 to 3000 eV and central Ti from 1200 to 1800 eV. Similar effects are seen in both H2 and D2 working gas discharges. The contribution to impurity generation of nonthermal electrons, which are produced by the RF, is under investigation. Changes in edge plasma temperature and density, as well as the possibility that the particle transport is affected by the RF, are also being examined. Results of the experiments with the three different limiter materials are compared, and contributions of impurity radiation to the overall power balance are estimated.

X-ray spectra of He-like argon (Ar/sup +16/) have been obtained for the transitions 1snp..-->..1s/sup 2/, with 3< or =n..Ar/sup +16asterisk/(1snp /sup 1/P)+H/sup +/ are presented and their cross sections are estimated.

The Alcator C-Mod tokamak is a new device presently under construction at Massachusetts Institute of Technology (M.I.T.) which is scheduled to begin operation in mid-1990. The projected operating parameters are as follows: Toroidal field of 9 T; I p ≤ 3 MA, R = 66.5 cm, a = 21 cm, κ ≤ 2.0, δ ≤ 0.5, n e ≤ 10 21 m −3 , P ICRF ≤ 6 MW. The divertor configuration includes mechanical baffling as opposed to an ‘open’ geometry. Under strictly ohmic heating conditions, central T i and T e are predicted to be in the range 2.5–3.5 keV over the density range (4–8) × 10 20 m −3 . With the addition of 6 MW of ICRF heating, T i should vary from 4–8 keV over the same density range (assuming either Kaye-Goldston or Neo-Alcator scalings for electron confinement). Based on edge plasma characterizations from Alcator-C and divertor tokamaks, the scrape-off layer (SOL) properties are predicted to be: λ n ≈ 10mm, density at the divertor plate 21 m −3 , H 0 ionization mean free path between 1 and 10 mm. Maximum heat loads on various internal components are predicted to be in the range 5–10 MW/m 2 . The flexibility of the poloidal field system in forming a number of flux surface geometries will provide further comparisons of the relative impurity control capabilities of double-null, single-null and limiter plasmas.

Impurities have been injected into Heliotron E in order to study the transport properties of currentless plasmas. In ECH-produced plasmas, the observed impurity transport can be described by purely anomalous diffusion, with a coefficient of the order of D = 2000–5000 cm2 s−1. During neutral beam operation, the impurity transport can be characterized by considerably weaker diffusion, combined with inward convection. Convection velocities as large as a few hundred centimetres per second are inferred.

Impurity source mechanisms which appear during the heating of the Alcator C tokamak by injection of lower hybrid frequency waves are described. Silicon is a dominant impurity in these experiments where SiC was the limiter surface material. At low values of injected power, the silicon source rate is dominated by physical sputtering. As the amount of injected power is raised, evaporation caused by the tail electron heat flux to the limiter becomes the primary source for impurities entering the plasma. Measurements of silicon line emission from the plasma as well as other central and edge parameters are presented as functions of injected radiofrequency power.

Impurity transport in currentless Heliotron E plasmas is examined. The main aim of the experiment is to clarify the density dependence of impurity transport both in electron cyclotron heated (ECH) and neutral beam injected (NBI) plasmas. In experiments with a stationary electron density, the confinement time of impurities is determined as a function of the line averaged electron density. The inherent difference between ECH and NBI plasmas, observed in an earlier measurement, seems to have been masked by the operating conditions. From a direct comparison between NBI plasmas and simultaneous ECH and NBI heated plasmas of the same density, a pumping-out effect of impurities by the ECH pulse is established. From numerical analyses, the unexpected discrepancy between observed and calculated VUV spectral data can be explained by a charge exchange process. The observed impurity transport is found to be well described by a model which incorporates the effects of radial diffusion and inward convection. The inward flow does not seem to be due to the plasma potential because the flow is also observed in ECH plasmas where the heating power is expected to contribute to a reversed potential. The flow velocity is of the order of the classical friction between background ions and seems to play only a minor role in various aspects of impurity transport in NBI plasmas, such as impurity accumulation. The variety in impurity transport in Heliotron E plasmas is best understood by changes in the diffusion coefficient.

Highly peaked carbon and molybdenum profiles occured after hydrogen pellet injection. 40 ms after injection, carbon was well fit by (T-italic/sub e-italic//T/sub e/(0))/sup =/ for r-italicapprox.

Lines belonging to ionization states ranging from Mo VI to Mo XIV have been observed in the Alcator A tokamak plasma. New lines of Mo XIII, among them a forbidden magnetic quadrupole line, have been identified and classified.

A spatially resolving visible light detector system has been constructed and used to measure continuum radiation in the wavelength region near 5360 A on the Alcator C tokamak. The instrument measures the line integral brightnesses from 20 chords through the plasma. For high‐electron temperature regimes and relatively line‐free wavelength regions, Zeff(r, t) is inferred from Abel inverted brightness profiles. During the steady‐state portion of the discharge, Zeff(r) is usually found to be constant within 10% out to r/a = 0.75.

The wavelengths of lines from Mg- and Na-like Kr ions were precisely determined by puffing Kr gas into Alcator C tokamak plasmas and observing with a time-resolving spectrograph equipped with a multi-element spectral detector.

ICRF ion Bernstein wave heating has been studied on the Alcator C tokamak in both the hydrogen 3/2 (B=7.6T) and deuterium 5/2(B=9.3T, ω=3ωcD at the plasma edge, x/a≂0.87) subharmonic regimes. Rf power of up to 180 kW at 183.6 MHz was coupled into the plasma for up to 150 ms using a toroidal current loop with a loading resistance between 1 and 2Ω. Ion heating effieciencies, neΔTi/Prf, of up to 5.5 eV m−3/kW at B=9.3T and 2.5 eV m−3/kW at B=7.6T at ne(Oh)=0.8×1020 m−3 were observed. A two temperature minority (deuterium) (TD

The report summarizes biological responses in sheep unilaterally exposed at three different dose rates of reactor-produced neutron-gamma radiations. The radiations employed in the studies were from a fission weapon simulator, the Sandia Pulsed Reactor (SPR), yielding a 5:1 neutron-to-gamma ratio.

L'energie stockee dans la queue d'electrons porteuse de courant peut etre une large fraction de l'energie cinetique totale dans un plasma induit par courant RF, alors que l'energie residant dans le plasma est comparable a celle d'une decharge ohmique similaire. Aux densites n e ≤3×10 13 cm −3 , le temps de confinement global du plasma RF est semblable a celui d'un plasma ohmique, mais se degrade par rapport a son contre-partie ohlique aux densites ou puissances plus elevees

We report on initial results from lower hybrid wave heating experiments carried out on the MIT Alcator C and Versator II tokamaks. In the Alcator C experiments a 4 waveguide array, with internally brazed ceramic windows has been used to inject 160 kW of microwave power at 4.6 GHz into the plasma with n O ≤ 1 × 10 15 cm −3 , and B O ≤ 12 T. An RF power density of 8 kW/cm 2 has been transmitted into the plasma without RF breakdown. RF coupling studies show optimal coupling (R ≤ 10%) when the local density at the waveguide mouth is 25–50 times overdense. Initial heating experiments show an ion tail formation in hydrogen discharge peaking at a density of at B = 8.9 T, and bulk ion heating at a density of at B ≃ 11 T. Evidence of RF current enhancement has been observed at a density of n ≃ 3 × 10 13 cm −3 .

The toxicity of Pu was investigated in miniature swine. Results are summarized from studies of blood concentra, tion, organ burden, and excretion patterns following various routes of Pu administration, the toxicity of Pu as compared to some other bone-seeking nuclides, and the penetration and removal of Pu from skin when injected or applied topically. (C.H.)

Velocity fields and density fluctuations of edge turbulence are studied in I-mode [F. Ryter et al., Plasma Phys. Controlled Fusion 40, 725 (1998)] plasmas of the Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] tokamak, which are characterized by a strong thermal transport barrier in the edge while providing little or no barrier to the transport of both bulk and impurity particles. Although previous work showed no clear geodesic-acoustic modes (GAM) on C-Mod, using a newly implemented, gas-puff-imaging based time-delay-estimate velocity inference algorithm, GAM are now shown to be ubiquitous in all I-mode discharges examined to date, with the time histories of the GAM and the I-mode specific [D. Whyte et al., Nucl. Fusion 50, 105005 (2010)] Weakly Coherent Mode (WCM, f = 100–300 kHz, Df/f approx. 0.5; and k_theta approx. 1.3 cm-1) closely following each other through the entire duration of the regime. Thus, the I-mode presents an example of a plasma state in which zero frequency zonal flows and GAM continuously coexist. Using two-field (density-velocity and radial-poloidal velocity) bispectral methods, the GAM are shown to be coupled to the WCM and to be responsible for its broad frequency structure. The effective nonlinear growth rate of the GAM is estimated, and its comparison to the collisional damping rate seems to suggest a new view on I-mode threshold physics.

Central toroidal rotation and impurity transport coefficients have been determined in Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] Ohmic high confinement mode (H-mode) plasmas from observations of x-ray emission following impurity injection. Rotation velocities up to 3×104 m/sec in the co-current direction have been observed in the center of the best Ohmic H-mode plasmas. Purely ohmic H-mode plasmas display many characteristics similar to ion cyclotron range of frequencies (ICRF) heated H-mode plasmas, including the scaling of the rotation velocity with plasma parameters and the formation of edge pedestals in the electron density and temperature profiles. Very long impurity confinement times (∼1 sec) are seen in edge localized mode-free (ELM-free) Ohmic H-modes and the inward impurity convection velocity profile has been determined to be close to the calculated neoclassical profile.

Injecting Lower Hybrid Range of Frequency (LHRF) waves into Alcator C-Mod's high-density H-mode plasmas has led to enhanced global energy confinement by increasing pedestal temperature and pressure gradients, decreasing the separatrix density, modifying the pedestal radial electric field and rotation, and decreasing edge turbulence. These experiments indicate that edge LHRF can be used as an actuator to increase energy confinement via modification of boundary quantities. H-98-factor increases of up to similar to 35% (e.g., H-98 from 0.75 to 1.0) are seen when moderate amounts of LH power (P-LH/P-tot similar to 0.15) are applied to H-modes of densities (n) over tilde (e) similar to 3 x 10(20) m(-3), corresponding to values similar to 0.5 of the Greenwald density. However, the magnitude of the improvement is reduced if the confinement quality of the target H-mode plasma is already good (i.e., H-98(target) similar to 1). Ray-tracing modeling and accessibility calculations for the LH waves indicate that they do not penetrate to the core. The LHRF power appears to be deposited in plasma boundary region, with a large fraction of the injected power increment appearing promptly on the outer divertor target. There is no evidence that the LH waves are driving current in these plasmas. The LHRF-actuated improvements are well correlated with suppressed pedestal density fluctuations in the 100-300 kHz range. There is also a correlation between the improved confinement and a drop in separatrix density, a correlation that is consistent with previous H-mode results with no LHRF. (c) 2015 AIP Publishing LLC.

The observed intensity ratio of the 3s 2 1 S 0 - 3s3p 1 P 1 to the 3s 2 1 S 0 - 3s3p 3 P 1 transitions in MgI-like ion has always presented those who model plasma spectra with a challenge; the obseved intensity of the intercombination line is always several times greater than what simple models predict. We offer a model that takes into account the contribution to the MgI-like emission features from autoionizing levels of the adjacent All-like charge state. Models in the present work give good agreement with the observed resonant/intercombination (R/I) emission ratio only when a departure from ionization equilibrium is assumed. We also identify, for the first time, intercombination lines of the form 3s3p 1 P 1 · 3p 2 3 P 2 in several elements relevant to both astrophysical and magnetically-confined fusion plasmas.

Using three different x‐ray arrays simultaneously, we have made measurements of the transport of aluminum (Z=13) or silicon (Z=14) injected into MIT’s Alcator tokamak plasma. Each array has ∼2 cm spatial and 15 μs temporal resolution. Since Alcator core temperatures are typically between 1 and 2 keV, fully stripped, H‐ and He‐like ions are the only ionization states of the injected impurity with significant populations for r/a≲0.7. The spectral responses of the arrays have been tailored so that each array has a markedly different response to the x radiations from these three ionization states. Through the use of two of these arrays, time‐resolved density profiles of fully stripped and total injected impurities, and studies of their transport, have been previously reported. Current work and description of the analysis method will be presented.

Differently filtered x‐ray arrays have been used to instantaneously measure central carbon and molybdenum profiles, Alcator’s dominant light and heavy impurities, following pellet injection. Approximately 40 ms after the pellet, the width of the carbon profile was close to the neoclassical prediction of a source‐free equilibrium state. (Carbon is a plateau impurity.) The experimental molybdenum profile, which is more uncertain, was between a factor of 1.3 and 3.0 that of the asymptotic prediction of neoclassical theory. Carbon, the dominant nonhydrogenic contributor to Zeff, was found to dramatically affect sawtooth dynamics by altering the central resistivity. Specifically, following injection of the pellet the carbon profile became progressively more peaked, the effect of which was to lengthen the sawtooth period. Approximately 40 ms after the pellet, an internal disruption occurred which reduced on‐axis carbon and molybdenum by a factor of 3. (In comparison, the corresponding temperature drop was only ...

After completing the main construction phase of Wendelstein 7-X (W7-X) and successfully commissioning the device, first plasma operation started at the end of 2015. Integral commissioning of plasma start-up and operation using electron cyclotron resonance heating (ECRH) and an extensive set of plasma diagnostics have been completed, allowing initial physics studies during the first operational campaign. Both in helium and hydrogen, plasma breakdown was easily achieved. Gaining experience with plasma vessel conditioning, discharge lengths could be extended gradually. Eventually, discharges lasted up to 6 s, reaching an injected energy of 4 MJ, which is twice the limit originally agreed for the limiter configuration employed during the first operational campaign. At power levels of 4 MW central electron densities reached 3 × 1019 m−3 , central electron temperatures reached values of 7 keV and ion temperatures reached just above 2 keV. Important physics studies during this first operational phase include a first assessment of power balance and energy confinement, ECRH power deposition experiments, 2nd harmonic O-mode ECRH using multi-pass absorption, and current drive experiments using electron cyclotron current drive. As in many plasma discharges the electron temperature exceeds the ion temperature significantly, these plasmas are governed by core electron root confinement showing a strong positive electric field in the plasma centre. EURATOM 633053

A region of steep plasma gradients, the so-called "narrow-feature," has been found in the near scrape-off layer (SOL) of inner-wall limited (IWL) discharges. Dedicated IWL discharges were carried out in Alcator C-Mod [Marmar et al., Nucl. Fusion 55, 104020 (2015)] to study this phenomenon, allowing detailed observations of the plasma profiles and fluctuations. Langmuir probe (LP) measurements show a clear two-decay length ne and T-e profile structure at the outer midplane. The Gas-Puff Imaging (GPI) diagnostic shows large turbulent fluctuations across the last closed flux-surface, hence supporting the hypothesis that turbulent phenomena play a role in setting the profile steepness. Flux-driven non-linear turbulence simulations of two C-Mod discharges have been carried out, allowing a three-way comparison between LP, GPI, and simulation data. Observations and simulations correlate the steep gradient region characterizing the narrow feature with sheared poloidal flows and a deviation of the plasma potential from its floating value. The E x B shear rate exceeds the linear ballooning growth rate, indicating that the narrow feature could result from the effects of sheared flows, although causality could not be established. The fluctuation level in the narrow feature remains of order unity across the entire SOL, indicating that the transport reduction in the near-SOL cannot result from a simple quench rule. Published by AIP Publishing.

A 1-2 kHz electromagnetic mode is present in a large fraction of W7-X discharges in the standard magnetic field configurations with 5/5 edge island structure. The power in the mode is significantly reduced in other magnetic configurations. The mode can appear spontaneously and persist for the full duration of a discharge. Its full width at half maximum is typically 100-300 Hz. The 1-2 kHz mode was first observed in visible light emissions from the divertor [1] . Matching spectral peaks were found in measurements of both global and edge plasma parameters, including line-integrated density, diamagnetic energy, and edge floating potential and ion saturation current [2] . An infrared camera recording shows a temperature modulation of ∼2.5 K due to this mode on a divertor tile where the average temperature was 600 °C. Separate correlation analyses of the high-speed camera recordings, Langmuir probe signals, and Mirnov coil signals all indicate a poloidal velocity of 0.5-10 km/s and m=2 as a likely poloidal mode number. Langmuir probes mounted on the Multi-Purpose Manipulator show that the fluctuation is present in one of the magnetic islands over a radial distance of at least 5 millimeters. A database of various diagnostic measurements and dB/dt signals from a Mirnov coil was created using a majority of the discharges from the 2018 campaign on W7-X, in order to shed light on the plasma parameters that are correlated with the existence of the 1-2 kHz mode. The power in the mode is positively correlated to the toroidal current, total external heating power, and core electron temperature. It is negatively correlated with plasma density.

We model the plasma edge of Alcator C -Mod with the B2-Eirene SOLPS5.0 code. The drifts, currents and neutral effects contribute to strong plasma rotation in the edge, as measured experimentally [B. LaBombard, S. Gangadhara, B. Lipschultz, C.S. Pitcher, J. Nucl. Mater. 313–316 (2003) 995; B. LaBombard, N. Smick, B. Lipschultz, J.L. Terry, J. Rice, these Proceedings; N. Smick, B. LaBombard, C.S. Pitcher, these Proceedings]. The plasma flow patterns are a complex superposition of several competing effects, including Pfirsch–Schluter flows and currents, toroidal momentum generation and transport (with main chamber neutral recycling and drag playing an important role), as well as the E × B and diamagnetic drift flows. In the standard field direction the E × B flow in the core region adds to the toroidal plasma rotation, while in the reverse direction their respective contributions are roughly of the same order of magnitude but with opposite signs in the plasma core. However, in the edge, all terms can be of the same sign over most if not all of the radial extent of the SOL, leading to complex patterns and very strong flows.