M. Diesso

Tokamak Fusion Test Reactor horizontal high‐resolution Bragg x‐ray spectrometer

A bent quartz‐crystal spectrometer of the Johann type with a spectral resolution of λ/Δλ= 10000–25000 is used on the Tokamak Fusion Test Reactor (TFTR) to determine central plasma parameters from the spectra of helium‐like and lithium‐like metal impurity ions (Ti, Cr, Fe, and Ni). The spectra are observed along a central radial chord and are recorded by a position‐sensitive multiwire proportional counter with a spatial resolution of 250 μ. Standard delay‐line time‐difference readout is employed. The data are histogrammed and stored in 64 K of memory providing 128 time groups of 512‐channel spectra. The central ion temperature and the toroidal plasma rotation are inferred from the Doppler broadening and Doppler shift of the Kα lines. The central electron temperature, the distribution of ionization states, and dielectronic recombination rates are obtained from satellite‐to‐resonance line ratios. The performance of the spectrometer is demonstrated by measurements of the Tiu2009xxi Kα radiation.

Abel inversions: Error propagation and inversion reliability

Inversion of chordally integrated data to infer the profile of a plasma parameter (plasma density, radiated power, or, in our case, the visible continuum from bremsstrahlung) propagates errors from outer shells inward as the inversion progresses. If the inversion is done by a matrix technique, error propagation can likewise be determined by a matrix technique. Where other considerations make the matrix method undesirable, there is no clearly defined analytical method to test the reliability of the inversion. We have solved this problem by taking real data from the Tokamak Fusion Test Reactor (TFTR) visible bremsstrahlung (VB) diagnostic, applying normally distributed random noise of a known mean value, and inverting the signal. When we have done this hundreds of times at all points in the profile, we can clearly track the error propagation. At the plasma center, for example, the error is several times the average error in the relative (chord to chord) calibration of the system. This error multiplier is a ...

Tokamak Fusion Test Reactor prototype x‐ray pulse‐height analyzer diagnostic

The x‐ray pulse‐height analysis (PHA) diagnostic uses a liquid‐nitrogen cooled array of 5 Si(Li) and one HpGe detectors to do time‐resolved (5–100 ms) x‐ray spectroscopy of the central horizontal chord of the Tokamak Fusion Test Reactor (TFTR) plasmas in the 1–50‐keV range. Central electron temperature Te and concentration of medium and low‐Z impurities are derived from the spectra. Remotely selectable absorber‐foil arrays provide selection of the energy range. Fixed and movable aperture arrays allow approximate equalization of count rates in different energy bands and extend dynamic range. Amplifier pulse shapes are approximately triangular. Main amplifier peaking time is 4 μs yielding 230‐eV FWHM at 5.9 keV. Pileup inspection times are selectable at 0.13, 0.4, or 0.9 μs. Throughput is up to 42 kHz. The PHA has been used to study temperature and impurities over a wide range of TFTR operational parameters. Dramatic variations in metal impurities with density, plasma current, and major radius have been obs...

Observations of neutral beam and ICRF tail ion losses due to Alfven modes in TFTR

Experimental observations from TFTR of fast ion losses resulting from the toroidicity induced Alfven eigenmode (TAE) and the Alfven frequency mode (AFM) are presented. The AFM was driven by neutral beam ions, at low BT, and the TAE was excited by hydrogen minority ion cyclotron range of frequencies (ICRF) tail ions at higher BT. The measurements indicate that the loss rate varies linearly with the mode amplitude for both modes, and that the fast ion losses during the mode activity can be significant, with tens of per cent of the input power lost in the worst cases

STUDIES OF IMPURITY BEHAVIOUR IN TFTR

Central medium and low Z impurity concentrations and Zeff have been measured by X-ray spectrometry in Tokamak Fusion Test Reactor discharges during three periods of operation. These were (1) startup period, (2) Ohmic heating, and (3) Ohmic heating portion of the two neutral beam periods, distinguished mainly by different vacuum vessel internal hardware and increasing plasma current and toroidal field capability. Plasma parameters spanned minor radius a = 0.41−0.83 m, major radius R = 2.1−3.1 m, current Ip = 0.25−2.0 MA, line averaged electron density e = (0.9–4.0) × 1019 m−3, and toroidal magnetic field BT = 1.8−4.0 T. The metal impurities came mostly from the limiter. At low densities titanium or nickel approached 1% of ne during operation on a TiC-coated graphite or Inconel limiter, respectively. Lower levels of Cr, Fe, and Ni (0.1%) were observed with a graphite limiter at similarly low densities; these elements were removed mainly from stainless steel or Inconel hardware within the vacuum vessel during pulse discharge cleaning or plasma operation on an Inconel limiter and then deposited on the graphite limiter. Hardware closest to the plasma contributed most to the deposits. Subsequent discharges slowly eroded the deposits from the limiter and reduced the metal impurity levels in the plasma. Both total low Z and metal impurity concentrations, relative to ne, decreased approximately exponentially with ne and increased with Ip. The metals showed much larger variations than did low Z elements, dropping to 10−5 ne at high density. Low Z impurities, mainly carbon and oxygen, ranged from 10% of ne at low ne to 3% at high ne and usually dominated Zeff and power loss. In low density plasmas with a TiC-coated graphite or Inconel limiter, the respective metals contributed 30–50% of Zeff and 10–20% of the central input power. The metal contributions to Zeff and radiated power were much lower with the graphite limiter.

Fast detection of 14 MeV neutrons on the TFTR neutron collimator

Current mode operation of the NE451 ZnS scintillation detectors of the TFTR neutron collimator has enabled us to record the development of radial neutron emission profiles with much faster speed and higher accuracy than in the pulse counting mode. During high power deuterium–tritium (DT) operation, the intrinsic shot noise on the detector traces was so low that we could observe sawtooth instabilities and disruptions with good precision and, in addition, were able to identify precursor magnetohydrodynamic (MHD) activity and fishbone instabilities. These results demonstrate that in future tritium burning machines like ITER or TPX, the neutron collimator should be designed not only as a monitor of radial fusion power profiles but also as a wave detector for MHD activity.

Measurements of radial profiles of the ion temperature and the plasma rotation velocity with the TFTR vertical x‐ray crystal spectrometer

The TFTR vertical x‐ray crystal spectrometer has now been operating with three crystals and position‐sensitive detectors according to the original design specifications. The observed spectra of heliumlike Ti xxi, Cr xxiii, Fe xxv, and Ni xxvii have permitted a detailed comparison with the predictions from atomic theories, and they have provided data on the radial profiles of the ion temperature and toroidal rotation velocity, as well as the radial ion charge‐state distribution in TFTR discharges. Central ion temperatures of 20 keV and central plasma rotation velocities of 5×105 m/s have been recorded from plasmas with auxiliary neutral beam heating. These experimental results are presented. Also discussed are further instrumental improvements, such as the installation of two additional crystals and detectors and the installation of γ and neutron shielding, which will make it possible to measure under full DD and DT operation with 27 MW of neutral beam injection where neutron production rates of 1019 neutr...

Profile correction to electron temperature and enhancement factor in soft x-ray pulse-height-analysis measurements in tokamaks

Because soft‐x‐ray pulse‐height‐analysis spectra contain chordal information, the electron temperature and the radiation intensity (enhancement factor) measurements do not represent the local values. The correction factors for the electron temperature and the enhancement factor as a function of the temperature and density profile parameters and the energy are obtained. The spectrum distortion due to pulse pileup effects is also evaluated. A set of curves is given from which the distortion of the spectrum can be obtained if the electron temperature, the Be filter thickness, and the electronic parameters of the acquisition system are known.

Tokamak Fusion Test Reactor horizontal high‐resolution Bragg x‐ray spectrometer (abstract)

A bent quartz‐crystal spectrometer of the Johann type with a spectral resolution of λ/Δλ= 10000–25000 is used on the Tokamak Fusion Test Reactor (TFTR) to determine central plasma parameters from the spectra of helium‐like and lithium‐like metal impurity ions (Ti, Cr, Fe, and Ni). The spectra are observed along a central radial chord and are recorded by a position‐sensitive multiwire proportional counter with a spatial resolution of 250 μ. Standard delay‐line time‐difference readout is employed. The data are histogrammed and stored in 64 K of memory providing 128 time groups of 512‐channel spectra. The central ion temperature and the toroidal plasma rotation are inferred from the Doppler broadening and Doppler shift of the Kα lines. The central electron temperature, the distribution of ionization states, and dielectronic recombination rates are obtained from satellite‐to‐resonance line ratios. The performance of the spectrometer is demonstrated by measurements of the Tiu2009xxi Kα radiation.

Upgrade to the multichannel neutron collimator

Spatially resolved fluctuation measurements of the 14 MeV neutron emission due to MHD instabilities have recently been obtained during DT operation, using ZnS based NE451 scintillation detectors operated in the current mode on the TFTR multichannel neutron collimator. The observed fluctuations have a frequency range up to 10 kHz. In order to obtain better spatial resolution of the emission, two additional channels viewing the intermediate regions of the plasma core have been added to the collimator array. Initially, these new channels will serve as testing stations to evaluate several prototype scintillator concepts having improved detection efficiency. Both ZnS and plastic based scintillators are presently being studied. The plastic detectors have the advantage of being transparent to scintillation photons and allow an increase in the bulk scintillation material and hence an increase in the number of neutron interactions, while the ZnS based detectors have a greater immunity to gamma-rays. The increased detector efficiency has reduced neutron shot noise and extends the capability of the detector system to higher frequencies. Pulse height distributions and efficiency measurements obtained for each candidate detector using a 14-MeV generator as a neutron source will be presented. Absolute calibration of the new channels using a jog shot technique will be discussed.