Neutral particle analyzer/isotope separator for measurement of hydrogen isotope composition of JET plasmas

Abstract

This article describes a neutral particle analyzer/isotope separator (ISEP) developed for measurement of the relative hydrogen isotope composition of Joint European Torus (JET) plasmas. The ISEP deployed on the JET can be regarded as a prototype of an instrument proposed for measurement of the spatial profile of the ratio of the density of deuterium and tritium ions in the plasma, $n_{\mathrm{D}}{\mathrm{(r)/n}}_{\mathrm{T}}\mathrm{(r),}$ in the International Thermonuclear Experimental Reactor (ITER). The ISEP makes simultaneous measurements of the energy distribution of efflux of hydrogen isotope atoms (H, D, and T) from the plasma. From such measurements it is possible to deduce the radial profile of the relative hydrogen isotope ion composition of the plasma and radial transport of ions of one isotope across the plasma of another isotope species. The main elements of the ISEP are (a) use of a thin carbon foil for reionization of the incident atoms, thereby eliminating gas stripping cells and gas sources of conventional neutral particle analyzers (NPAs), (b) acceleration of secondary ions in order to access the regime of higher detection efficiency of the NPA and to better separate ion pulses from neutron/γ-ray induced pulses in scintillator detectors, (c) $\mathrm{E\Vert B}$ dispersion of the secondary ions in specially designed nonuniform magnetic and electric fields to provide focusing in the detector plane, increased throughput and greater contrast between neighboring isotopes, and (d) counting of energy and mass analyzed secondary ions using detectors consisting of thin $\text{[1⩽t\hspace{0.17em}(μ}\mathrm{m}\text{)⩽7]}$ CsI(Tl) scintillators deposited directly on miniature thin window photomultiplier tubes mounted in vacuum. The ISEP has high contrast between atoms of neighboring masses ${\text{(⩾10}}^3$ for $\text{E≈5\hspace{0.17em}}\mathrm{keV}$ and much greater at higher energies), and high detection efficiency $\text{(0.06⩽ε⩽0.83}$ for atoms of $\text{5⩽\hspace{0.17em}(}\mathrm{keV}\text{)⩽150.}$ ISEP detectors have very low sensitivity to neutrons and γ rays ${\text{(⩽10}}^{\text{−7}}$ of ion sensitivity), making it feasible to use the ISEP in JET DT experiments without any shielding. Only a modest amount of neutron/γ-ray shielding would be required in the ITER for similar applications of the ISEP. The initial experiments on JET plasmas using the ISEP demonstrate well the capabilities of the instrument for measurement of the hydrogen isotope composition of the plasma and the energy distribution function of isotope ions.