H. P. Summers

Beam emission spectroscopy as a comprehensive plasma diagnostic tool

A beam of injected fast atomic hydrogen presents a superb probe for hot fusion plasmas. The neutral particles experience excitation and ionization by collisions with electrons and ions as they penetrate into a plasma. The emitted characteristic line radiation is Doppler shifted and the spectral lines are split due to motional Stark fields. Measurements of wavelength, intensity and polarization of the Balmer- alpha emission reveal information about the neutral beam, such as beam attenuation, beam-geometry, beam-divergence and species mix. Local pitch angles and toroidal fields can be derived from the simultaneous measurement of the polarization pattern and the wavelength separation of the Stark multiplet. The implementation and application of beam emission spectroscopy as a quantitative diagnostic tool on the Joint European Torus (JET) experiment is reviewed.

Visible charge exchange spectroscopy at JET

Recent developments and results of the JET CXRS diagnostic are reported. The measurements of radial profiles of ion temperatures and densities are based on CXR spectra of fully stripped ions of either carbon or beryllium. Considerable effort has been expended in ensuring consistency between radial profiles of low Z impurity densities and those from other diagnostics. The contributions of the main light impurities are used to reconstruct radial profiles of Zeff which can be compared with Abel‐inverted signals from visible bremsstrahlung or soft x‐ray emission. Active Balmer‐Alpha spectroscopy (ABAS) is being introduced as a diagnostic tool providing data on local magnetic fields, neutral beam densities, and dilution factors. The effects of collision‐energy‐dependent CXR cross sections on observed CXR spectra are calculated. Corrections for the values of deduced ion temperatures, toroidal velocities, and impurity densities are discussed for the case of plasmas with high ion temperatures and high toroidal ro...

Radiative power loss from laboratory and astrophysical plasmas. I. Power loss from plasmas in steady-state ionisation balance

Described a calculation of the power radiated by an optically thin low-density plasma in steady-state ionisation balance taking account of spectral lines, recombination continuum and bremsstrahlung. The calculations have been done in the temperature range 104-1010K for all ions of the elements carbon, oxygen, silicon and argon and for all ions of iron and molybdenum of charge greater than 7 and 23 respectively. Collision strength data of good accuracy have been used where they are available and extrapolation methods developed where they are not. The bremsstrahlung power loss is considered in detail in the appendix. Results are presented in tabular form for each of the ions treated. These are combined with steady-state ionisation balance data in a further table to give the radiated power-loss functions for each element in turn. The role of impurity radiation loss for a deuterium-tritium power-producing plasma is investigated. A composite calculation of the radiated power loss from the solar atmosphere is also presented.

An R-matrix with pseudo-states approach to the electron-impact excitation of H I for diagnostic applications in fusion plasmas

Electron-impact excitation cross sections for transitions between terms up to the n = 5 shell of neutral hydrogen have been calculated using the R -matrix method with pseudostates and compared with previous studies. Maxwell-averaged effective collision strengths have been prepared and used in population calculations to examine effective emission coefficients for diagnostic applications in fusion plasmas. Our results remove an uncertainty in the reaction rates of an important class of atomic processes governing H I emission in plasmas.

Analytical approximation of cross-section effects on charge exchange spectra observed in hot fusion plasmas

An analytical procedure is presented which enables a fast estimate of collision-energy-dependent cross-section effects on thermal charge exchange spectra. The model is based both on experimental evidence and numerical simulations showing that the observed charge exchange (CX) spectra are essentially Gaussian in their shape. The collision-energy-dependent emission rate leads effectively to a lineshift (apparent velocity), usually to a reduction in linewidth (apparent temperature), and to a change in the effective emission rate averaged over the entire thermal velocity distribution function. It is demonstrated that the cross-section effect can be treated analytically introducing an approximated emission rate factor which retains the characteristics of a Maxwellian velocity distribution function using an exponential expression with only linear and quadratic velocity terms in its exponent. An algebraic deconvolution procedure is described, which enables the reconstruction of true temperature, velocity and intensities from measured CX spectra. Examples taken from a recent JET experimental campaign are used to illustrate the cross-section effects on low-Z impurity CX spectra for a comprehensive variety of neutral beams (deuterium, tritium or helium), target densities, temperatures and toroidal rotation speeds. An overview is given of representative correction factors established for high-power, high-temperature plasmas, as well as for plasmas with combined neutral beam and radiofrequency heating, and for the case of locked modes.

Neutral beam stopping and emission in fusion plasmas I : deuterium beams

The charge transfer reaction of neutral deuterium beams with impurities enables one of the principle quantitative diagnostic measurements of the hot core fusion plasma; that is, charge exchange spectroscopy. The complementary measurement of beam emission spectroscopy has been fruitful in motional Stark wavelength shift and fluctuation studies, but less so in using absolute measured intensities. In the last two years we have achieved substantial improvement in the quantitative analysis and agreement between the observed and modelled beam emission at the JET Joint Undertaking. This has depended on improved spectral fitting of the overlayed Dα motional Stark multiplet, self-consistent beam emission and impurity charge exchange modelling and analysis, and revision of the data entering the modelling of the beam emission process. The paper outlines the present JET beam emission diagnostic system and the collisional radiative modelling of deuterium beam stopping and emission. The nature and organization of the effective derived data directly used in experimental interpretation at JET are described and some results of spectral analysis of deuterium beam emission given. The practical implementation of the methods described here is part of the ADAS Project.

Observation of alpha particle slowing-down spectra in JET helium beam fuelling and heating experiments

The first experimental results are reported of anisotropic slowing-down features observed in JET helium beam fuelling experiments. Two independent observation ports, one with a view perpendicular to the magnetic field in the centre of the plasma and a second multichord viewing arrangement, approximately tangential to the toroidal field, provide radially and temporally resolved information on the velocity distribution function comprising the populations of both fast and thermalized alpha particles. The fuelling process is characterized by a change-over from a distinctly non-Maxwellian distribution function to a dominantly Maxwellian distribution and also by a broadening of the deduced fast ion density radial profile. The fast particle component in the observed composite charge exchange spectrum is found to be in excellent agreement with predictions are based on anisotropic velocity distribution functions obtained from the analytical solution of the neutral injection Fokker-Planck equation. Signal-to-noise levels in the measurement of fast alpha particle in the JET helium fuelling campaign are extrapolated to thermonuclear-fusion alpha particle density levels expected for the D-T phase of JET. It is shown that beam penetration and not competing continuum radiation is a major constraint, and that acceptable (hydrogen or helium) neutral beam power and energy requirements promise a feasible CX alpha particle diagnosis in the core of next-step devices such as ITER.

A radiation-damped R -matrix approach to the electron-impact excitation of helium-like ions for diagnostic application to fusion and astrophysical plasmas

Electron-impact excitation collision strengths for transitions between all singly excited levels up to the n = 4 shell of helium-like argon and the n = 4 and 5 shells of helium-like iron have been calculated using a radiation-damped R-matrix approach. The theoretical collision strengths have been examined and associated with their infinite-energy limit values to allow the preparation of Maxwell-averaged effective collision strengths. These are conservatively considered to be accurate to within 20% at all temperatures, 3×105-3×108 K for Ar16+ and 106-109 K for Fe24+. They have been compared with the results of previous studies, where possible, and we find a broad accord. The corresponding rate coefficients are required for use in the calculation of derived, collisional-radiative, effective emission coefficients for helium-like lines for diagnostic application to fusion and astrophysical plasmas. The uncertainties in the fundamental collision data have been used to provide a critical assessment of the expected resultant uncertainties in such derived data, including redistributive and cascade collisional-radiative effects. The consequential uncertainties in the parts of the effective emission coefficients driven by excitation from the ground levels for the key w, x, y and z lines vary between 5% and 10%. Our results remove an uncertainty in the reaction rates of a key class of atomic processes governing the spectral emission of helium-like ions in plasmas.

Observations and modelling of diatomic molecular spectra from JET

Spectra in the visible range may be obtained along multiple lines of sight directed at the axisymmetric poloidal divertor of the JET tokamak. In X-point magnetic field configurations, the scrape-off-layer plasma contacts the divertor target plates. We have conducted observations and analysis of diatomic spectra in the divertor plasma along lines of sight in the strike zone vicinity. We have identified unambiguously CD, and BeD. Synthetic spectra have been calculated. In addition to identification of the molecular features and separation of overlapping atomic lines, the comparison of simulations and observations allow quite sensitive determination of rotational and vibrational temperatures for the above diatomics in a divertor fusion plasma. Systematic studies show that and are comparable for BeD but is significantly less than for CD and . The BeD spectrum time history during a pulse (modulated by sweeping) appears as localized emission from the strike points similar to that of the atomic BeI visible spectrum lines. On the other hand, the CD bands during a swept pulse indicate a more distributed source. Thus both the BeD spectrum and time history suggest physical sputtering of BeD at the strike points. In contrast, the CD spectrum and time history indicate chemical release of higher deuterides followed by catabolism to CD. This work constitutes the first detailed study of diatomic species in the JET tokamak.

INVESTIGATION OF THERMAL AND SLOWING-DOWN ALPHA-PARTICLES ON JET USING CHARGE-EXCHANGE SPECTROSCOPY

Thermal alpha particles are observed in JET during helium discharges using spectral emission in He II (n=4 to 3) near 4685 AA following charge transfer reactions along the path of the neutral deuterium heating beams. New and reappraised He22/H charge transfer cross-sections are presented. The effects of cross-section energy dependence on temperatures, velocities and absolute densities deduced from thermal alpha particle charge exchange spectra are evaluated. The possibility of detecting fusion alpha particles produced at 3.5 MeV and slowing down by collisions with plasma electrons and ions using visible charge exchange spectroscopy is addressed. The spectral signature of slowing-down fusion alpha particles expected during the deuterium-tritium phase of JET is modelled and its identification against thermal alpha particle and background radiation is investigated.