Thermal X-ray spectra and impurities in the ST Tokamak

Abstract

A study is made of the spectral distribution of the soft X-ray emission produced by the thermal part of the electron velocity distribution in the ST Tokamak. The slopes of the spectra are in good agreement with the prediction from laser measurement of electron temperature if radial profile effects are taken into account. The absolute intensity is a factor 5 to 100 larger than is expected for hydrogen bremsstrahlung. This enhancement can be quantitatively accounted for by recombination radiation from oxygen and heavy-metal impurities. The enhancement factor ζ was measured at different radii, in order to study the impurity distribution in the tokamak. These, as well as other experiments, in which high-Z gases (Xe, Kr, A) were pulsed into the discharge, indicated that, within the measuring accuracy, Zeff does not vary substantially with radius.

Low energy x-ray emission from magnetic fusion plasmas

“…The enhancement factor y^ is t/pically <" 10 for clean discharges [8,15] but under some circumstances can be as large as ~ 1000 [15,36] . This factor depends strongly on the ionization stage of an iiapurity ion [6] .…”

“…These instruments permit measurement of a variety of important plasma parameters. These include electron [8,37,45] and ion [22] temperatures, density [31], plasma equilibrium and stability [23,24], plasma position, and plasma rotation [6] . Other parameters inferred from X rays relate to impurities and include radiative energy loss [10,[32][33][34][35], concentration [6,8,10] and motion [36], charge-state distribution [21,371, wavelengths of emission lines [38][39][40][41][42], and atomic physics parameters such as cross sections for excitation, ionization, and recombination [43,44,46,47].…”

Multichannel light detector system for visible continuum measurements on Alcator C

“…Radiative methods, which consist of measuring either line emission or bremsstrahlung emission, are particularly useful in diagnosing equilibrium plasmas.-Spectroscopic theoryl 1 ] allows accurate calculations of Zeff (the effective ion charge of the plasma) from absolute bremsstrahlung emissivity. Relating Zeff to recombination radiation as is done in the x-ray region, [2] proves to be both difficult and imprecise. If, however, the spectrum is relatively free of both recombination and line emission, the analysis becomes much less complicated.…”

Passive Spectroscopic Diagnostics for Magnetically-confined Fusion Plasmas

“…26 As a result of energy conservation, the radiative recombination spectrum has steps at the ionization energies of the various energy levels of the ion; i. e., recombination into the ground level produces only photons with energies greater than the ionization energy. Recombination radiation is therefore negligible in the visible region of the spectrum, so γ(T e ,Z)=1 and the continuum is pure bremmstrahlung as given by Eqs.…”

“…In the soft x-ray region, recombination radiation dominates and γ(T e ,Z i )>1 except for low-Z elements at high T e , for which bremmstrahlung is the main source of continuum radiation. 26 The relative contributions of line radiation, bremsstrahlung and recombination radiation to the iron spectrum in the soft x-ray region are shown in Fig. 5.…”