Princeton Large Torus Research Articles

Theory of Feedthroughs: Wave Transmission and Voltage Characteristics of High-Power Current Feeds for ICRH

Using circuit theory and solutions of a scalar twodimensional Laplace equation for the electrostatic potential, we examine various feedthroughs of interest for voltage holding, voltage standing wave ratio (VSWR) and impedance characteristics. Suitable inhomogeneous Dirichlet, scalar boundary conditions are imposed for the potentials for the calculation of the electrostatic fields. The inhomogeneous vector Newman boundary conditions on the surface of the dielectric are dispatched by use of a conformal mapping. Several feedthroughs are examined: old Princeton Large Torus (PLT), JULICH designs, and several Oak Ridge National Laboratory (ORNL) designs that have been actually fabricated and exhaustively tested. Some of these feedthrough configurations have been optimized to provide constant 50-..cap omega.. impedance or minimum VSWR.

Technological Issues of Ion Cyclotron Heating of Fusion Plasmas

With the recent promising results of plasma heating using electromagnetic waves (EM waves) in the ion cyclotron range of frequency (ICRF) on the Princeton Large Torus (PLT) tokamak the feasibility of employing ICRF heating to a reactor-like magnetic confinement device is increasing. The high power ICRF experiments funded on JET (Joint European Torus in England) and JT-60 (in Japan) will have rf source power in the range of 10-30 MW. The time scale for the duration of the rf pulse will range from seconds up to steady-state. The development of new rf components that can transmit and launch such high power, long pulse length, EM waves in a plasma environment is a major technological task. In general, the technology issues may be divided into two categories. The first category concerns the region where the plasma comes in contact with the wave launchers. The problems here are dominated by plasma-material interaction, heat deposition by the plasma onto the wave launcher, and erosion of the launcher material. It is necessary to minimize the heat deposition from the plasma, the losses of the rf wave energy in the structure, and to prevent sputtering of the antenna components. A solution involves a combined design using special materials and optimal shaping of the Faraday shield (the electrostatic shields which can be used both for an EM wave polarization adjustment and as a particle shield for the launcher). Recent studies by PPPL and McDonnell Douglas Corp. on the Faraday shield designs will be discussed.1 The second important area where technology development will be necessary is the transmission of high power rf waves through a gas/vacuum interface region. In the past, the vacuum feedthrough has been the bottle neck which prevented high power operation of the PLT antennae. However, with major improvements of the feedthrough design, the power level attainable has risen from 200 kW per antenna to over 1.25 MW per antenna. In the present experiment, the power limit of the vacuum/gas interface has not been reached. As the pulse length of the rf is increased, modification (or cooling) of the rf components will be essential. According to the recent studies of vacuum feedthroughs2 by PPPL and the Grumman Corp., it is expected that with proper cooling the present components will be able to withstand steady-state operation.

Conceptual Design of a Fast Wave Current Drive (FWCD) Coupler for the PLT

This paper describes the conceptual design of a Fast Wave Current Drive (FWCD) alumina ceramic loaded coupler for the Princeton Large Torus (PLT). An antenna capable of launching RF at 800 MHz was required. While phased waveguide arrays allow the coupling of correctly phased waves to the plasma, their large size (when used in air or vacuum) would preclude their application for PLT. However, the utilization of a dielectric loaded waveguide results in a reduction in size of waveguide elements by the square root of the dielectric constant1,3. A description of the various approaches considered during the RF design, mechanical design, and component fabrication studies is included.

The effects of ICRF heating on plasma edge conditions in PLT

The interaction of ICRF heating with the plasma edge in Princeton large torus (PLT) is reviewed and possible mechanisms responsible for the observations are discussed. Ion and neutral sputtering are found to be the most significant producers of impurities, which are observed to increase in moderate amounts during ICRF heating. Collisional edge heating by the rf is evaluated as a possible explanation of measured electron density increases and particle fluxes. Antenna conditioning is also discussed, relating to the processing necessary to achieve the highest power handling capability per antenna in PLT.

Absolute calibration of a ten-channel grating polychromator for electron temperature measurements on Princeton large torus

It has been possible to calibrate absolutely the ten-channel grating polychromator installed on PLT (Princeton large torus). Radiation from a blackbody source (eccosorb at liquid-nitrogen temperature) was chopped at 275 Hz and the signal detected using a lock-in amplifier with an integration time constant set at 125 s. A cascade of six grating filters was used to eliminate higher-order radiation from the input to the instrument. Each channel could be calibrated in approximately 2 h; under these conditions the rms calibration signal to noise for a typical channel is about 60.

Relative intensities of 2s22pk-2s2pk+1 transitions in F

Measured relative intensities of a number of allowed 2${s}^{2}$2${p}^{k}$-2s2${p}^{k+1}$ transitions (60--200 A\r{}) in the F i-- to B i--like ions of titanium, chromium, iron, nickel, and germanium are compared with values from level-population calculations. The measurements are from Princeton Large Torus (PLT) tokamak plasmas with electron densities of \ensuremath{\sim}2.5\ifmmode\times\else\texttimes\fi{}${10}^{13}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. For titanium and chromium, data from plasmas with densities of \ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ are also presented; a number of density-dependent line-intensity ratios are found. The spectra were obtained with use of a grazing-incidence time-resolving spectrograph which was radiometrically calibrated with use of synchrotron radiation from the National Bureau of Standards Synchrotron Ultraviolet Radiation Facility (SURF II). The measured relative intensities are therefore reliable. For the majority of the observed lines, agreement between the measured and calculated relative intensities is within 30%, the estimated accuracy of the measurements; significant discrepancies are found in the titanium ions at the low density. The discrepancies, some of which are due to blends, are discussed. Thus, the level-population calculations may be used with some confidence for spectroscopic plasma diagnostics. In the C i--like ions, there is some evidence that calculations which include proton-collisional excitation and deexcitation between the levels of the ground configuration are in better agreement with the measurements than those that do not, indicating that proton collisions should be included in the calculations for these ions.

The effect of inner-shell ionization on intensities of lines from Mg I-like ions in Tokamak plasmas

Tokamak spectra of the Mg I-like ions of high-Z elements ( Z = 21−24), have consistently shown that the 3s 21S 0-3s3p 3P 1 intercombination line is brighter than expected relative to the 3s 21S 0-3s3p 3P 1 resonance line. The 3s3p 3P 2-3s3d 3D 3 transition is also found to be anomalously strong. The results of modeling measured relative intensities of Mg I-like selenium (Se XXIII) lines from Princeton large torus (PLT) tokamak spectra are presented. The observations cannot be explained by a level population model including only electron collisions and radiative decay between the levels of the Mg I-like ion. Agreement with the data is found when direct population of the 3s3p levels by inner-shell ionization of the Al I-like ion is included; indicating that these processes are important for Mg I-like ions at the temperatures and densities of tokamak plasmas.

Coupling to Fast Waves Near the Lower Hybrid Frequency

Calculations show that the fast wave near the lower hybrid frequency may be launched efficiently into large tokamak plasmas. Coupling efficiency is calculated for waves, which may be useful for current drive experiments in the Princeton Large Torus, the Tokamak Fusion Test Reactor, and reactor grade plasmas.

Parametric instabilities in the fast-wave heating of tokamaks

A high-power whistler wave launched into a tokomak is susceptible to resonant decay into ion-cyclotron and lower-hybrid waves when the lower-hybrid resonance layer ωlh=ω0 exists in the outer region of the plasma x2/a2≳0.6, where ω0 is the frequency of the whistler, ωlh is the lower-hybrid frequency, x is the distance away from the center of the plasma, and a is the minor radius. For Princeton Large Torus parameters, this requires a line average density n≳6×1013 cm−3 when the whistler frequency ω0≂800 MHz and the threshold pump power for the onset of the instability is 100 kW. The threshold power is determined by the convection losses and it increases nonlinearly with the temperature and very rapidly with decreasing density. The parametric instability would tend to deposit the momentum and energy of the pump wave in the outer region of the plasma.

Spectrometer-sensitivity calibration in the extreme ultraviolet by means of branching ratios of magnetic-dipole lines

Relative intensity measurements of various line pairs resulting from magnetic-dipole transitions within the configurations s2p2 and s2p4, in conjunction with calculated transition probabilities, have been used to determine the wavelength dependence of the sensitivity of a grazing-incidence spectrometer in the range 400–1000 Å. Emissions from Cr xix, Fe xxi, Ni xxi, Ni xxiii, Cu xxiv, and Zr xxvii ions in Princeton Large Torus tokamak discharges were used for this purpose. Absolute sensitivity of the spectrometer at selected wavelengthshad been determined by the traditional hydrogen, helium, carbon, and oxygen electric-dipole line pairs from the same discharges. Similar attempts to use transitions in the s2p3 configurations in Cr xviii, Zr xxvi, and Mo xxviii ions resulted in significant discrepancies that are ascribed to uncertainties in the corresponding calculated transition probabilities.

Observation of lines above 2000 Å in O VIII and C VI in the Princeton Large Torus due to charge-exchange processes: Diagnostic applications

Hydrogenlike oxygen and carbon lines from transitions (Δn=1) between levels of high principal quantum number n, with wavelengths above 2000 Å have been observed. Observations of such transitions were possible due to charge-exchange processes during neutral beam injection of hydrogen atoms into the Princeton Large Torus tokamak. The lines are O VIII 2976 Å (8–7 transitions), C VI 3434 Å (7–6), and C VI 5291 Å (8–7). Application of these lines for ion temperature measurements and initial observations of neutral beam vertical distributions in the plasma are presented.

Intercombination lines of AlVIII, AlIX, and AlX ions

Several aluminum lines observed in the Princeton Large Torus tokamak discharges have been identified as intersystem transitions, establishing the energies of the Al VIII 2s2p/sup 3/ /sup 5/S, Al IX 2s2p/sup 2/ /sup 4/P, and Al X 2s2p /sup 3/P terms. Some observations of isoelectronic transitions in scandium and titanium ions are also reported.

On the ground configuration of the phosphorus sequence from copper to molybdenum

The energy-level structure of the 3s23p3 configuration of the phosphorus sequence ions Mo xxviii and Zr xxvi has been determined from magnetic-dipole line wavelengths and emissivities measured in the Princeton Large Torus tokamak discharges. The new measurements have caused some changes in the tentative assignments reported earlier for this sequence, especially for the 4S3/2–2D3/2 transitions. Isoelectronic interpolations together with some experimental confirmations in the Se xx and Ge xviii spectra now permit a reasonably accurate and complete reconstruction of this configuration from Z = 42 to previously established Z ≤ 28 elements. The measured wavelengths and relative intensities are in substantial agreement with recent semiempirical evaluations and ab initio calculations.

Formation of a 100-kA Tokamak Discharge in the Princeton Large Torus by Lower Hybrid Waves

The development of noninductive current drive is of great importance in establishing the tokamak as a long-pulse or steady-state fusion reactor. Lower hybrid waves, carrying 200 kW of power at 800 MHz, have been launched into the Princeton Large Torus tokamak to initiate and drive the discharge current to a level in excess of 100 kA.

Dielectronic satellite spectra of hydrogenlike titanium (Ti xxii)

High-resolution spectra of the Ly-${\ensuremath{\alpha}}_{1}$ and Ly-${\ensuremath{\alpha}}_{2}$ lines of hydrogenlike titanium, Ti XXII, and the associated dielectronic satellites which are due to transitions $1snl\ensuremath{-}2pnl$ with $n\ensuremath{\ge}2$, have been observed from tokamak discharges with auxiliary ion-cyclotron heating with central electron temperatures of 2 keV and central electron densities of 8\ifmmode\times\else\texttimes\fi{} ${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ on the Princeton Large Torus (PLT). The data have been used for a detailed comparison with theoretical predictions based on the $Z$-expansion method and on the multiconfiguration Thomas-Fermi---potential wave-function method (SUPERSTRUCTURE code). The predictions from the $Z$-expansion method are in excellent agreement with the observed spectral data except for minor discrepancies between the theoretical and experimental wavelengths of 0.0003 \AA{} for the $n=2$ satellites and of 0.0001 \AA{} for the separation of the Ly-${\ensuremath{\alpha}}_{1}$ and Ly-${\ensuremath{\alpha}}_{2}$ lines. The latter discrepancy is, however, removed if the Lamb shift and relativistic effects of order ${\ensuremath{\alpha}}^{4}$ are taken into account. Very good agreement with the experimental wavelengths is also obtained for the results from SUPERSTRUCTURE though somewhat larger and systematic discrepancies (\ensuremath{\simeq}0.0009 \AA{}) exist. The observed spectra have been used for diagnosis of the central ion and electron temperatures of the PLT discharges and for a measurement of the total dielectronic recombination rate coefficient of Ti XXII. The measured rate coefficient is in good agreement with the predictions from both the detailed calculations and Burgess's general formula.

Grazing incidence time-resolving spectrograph for magnetic fusion plasma diagnostics

A time-resolving grazing incidence spectrograph based on an image intensified photodiode array detector has been constructed and has demonstrated its utility as a fusion plasma diagnostic. The total wavelength coverage is 15–360 Å with 0.7 Å resolution (line profile FWHM); portions of this range may be observed during a single plasma discharge with a bandwidth of ∼40 Å when the detector is centered at 40 Å and ∼80 Å with it centered at 200 Å. Integration times from 5.4 to 13.1 ms are available when the entire photodiode array is read out; integration times as short as 1 ms can be obtained for a few lines of interest through the readout of selected photodiodes. The spectrograph has been radiometrically calibrated over the 60- to 360-Å range at the NBS SURF II electron storage ring and is currently in operation on the Princeton Large Torus (PLT) tokamak.

Interpretation of High Resolution X-Ray Observations of Coronal Plasmas

Soft X-ray spectra with high resolution represent a rich source of information on the nature of solar plasma as well as on the power loss of the fusion plasma due to impurity radiation. These emission spectra correspond to mutliply ionized atoms and for their interpretation in terms of physical parameters it is necessary to understand the atomic processes involved and to compute the corresponding parameters. These X-ray diagnostic techniques are used mainly for active region and flare plasmas and have determined the design of the X-ray Polychromator (XRP) flown on the NASA Solar Maximum Mission. Spectra from calcium and iron have been observed and fitted with the best available theoretical spectra giving the temperature and density, ionization state, abundances, velocity,... A similar analysis of the iron spectrum emitted from the PLT (Princeton Large Torus) tokamak plasma has confirmed the precision of the electron temperature diagnostic. Experience with such high resolution X-ray spectroscopy offers the possibility of extension to the analysis of optically thin astronomical plasmas such as stellar coronal and wind, supernovae remnants,...

Synchrotron emission from runaway electron distributions

Synchrotron emission from a relativistic anti-loss-cone (runaway) distribution is investigated numerically and compared with various analytical approximations. The results are applied to recent measurements of enhanced emission during current-drive experiments on the Princeton Large Torus (PLT) as well as to impulsive solar microwave bursts.

Heating of Plasma Ions in a Tokamak by the Second-Harmonic Ion-Cyclotron Resonance Interaction with Radio-Frequency Waves

The radio-frequency-wave heating of ions in a tokamak plasma employing the second-harmonic ion-cyclotron resonance interaction has been demonstrated in recent high-power (3-MW) experiments on the Princeton Large Torus tokamak.

Measurements of fluctuations in the flux of runaway electrons to the PLT tokamak limiter

Fluctuations in the flux of runaway electrons to the limiter are measured during many PLT (Princeton Large Torus) discharges. Oscillations at 60, 120, and 720 Hz are driven by variations in the vertical magnetic field which moves the plasma major radius. Fluctuations are seen with frequencies in the range of 2–20 kHz because of magnetohydrodynamic (MHD) magnetic islands which extend to the plasma surface. A continuous spectrum of fluctuations is observed up to 200 kHz which correlates with drift-wave turbulence. The magnitude of the driven fluctuations can be used to measure transport properties of the runaway electrons. The amplitude of electron motion due to the MHD and drift-wave oscillations, and hence a measure of the radial size of the instability, can be determined as a function of frequency. The slope of the frequency power spectrum of the drift-wave-induced fluctuations steepens with increasing runaway-electron drift orbit displacement during the current drop at the end of the discharge, and as the power in the MHD oscillations increases. A magnetic probe is used to confirm the presence of oscillating magnetic fields capable of perturbing the electron orbits.