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, nD(r)/nT(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) E‖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 [1⩽t (μm)⩽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 (⩾103 for E≈5 keV and much greater at higher energies), and high detection efficiency (0.06⩽ε⩽0.83 for atoms of 5⩽ (keV)⩽150. ISEP detectors have very low sensitivity to neutrons and γ rays (⩽10−7 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.

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Articles you may be interested inCompact, accurate description of diagnostic neutral beam propagation and attenuation in a high temperature plasma for charge exchange recombination spectroscopy analysisa) Rev. Sci. Instrum. 79, 10F315 (2008); Neutral beam diagnostics for the HT-7 tokamak Rev. Sci. Instrum. 75, 3496 (2004); 10.1063/1.1787952Plasma potential profile determination by a new diagnostic based on neutral particle beam injection and laser induced fluorescence Rev. Sci. Instrum. 70, 908 (1999); Experimental apparatus and data analysis techniques used in neutral particle analyzer ͑NPA͒ diagnostics on the Tokamak à Configuration Variable ͑TCV͒ are described. Two NPAs are used on TCV to measure the energy spectrum of neutral particle fluxes from the plasma. The "five-channel energy analyzer of atomic particles" used in double electrical analysis mode with fast voltage sweeping detect particles without atomic mass discrimination in the energy range of 0.6-8.0 keV with a time resolution of 0.5-2.0 ms and an energy resolution of 7%-20%. The 28-channel "compact neutral particle analyzer" ͑CNPA͒ is an EIIB spectrometer with mass and energy separations designed for medium sized fusion machines featuring a carbon neutral stripping foil, a permanent magnet for dispersion, and channel-electron multiplier detectors. The CNPA simultaneously detects two mass species ͓hydrogen ͑H͒ and deuterium ͑D͒ or D and helium ͑He͔͒ in the 0.5-50 keV energy range with a resolution of 60%-10% and a time resolution of 0.5-4.0 ms. The CNPA views the plasma across the path of the diagnostic neutral beam and can perform active charge-exchange NPA measurement. Data analysis procedures and numerical algorithms developed for NPA measurement are routinely used on TCV to obtain information on the plasma ion temperature, ion energy distribution function, plasma isotope ratios, and other plasma characteristics.

In this paper we present the fusion related activities of the Plasma Physics Division at the Ioffe Institute. The first experiments on lower hybrid current drive (LHCD) in a spherical tokamak performed at the Globus-M tokamak (R = 0.36 m, a = 0.24 m, B t = 0.4 T, I p = 200 kA) with a novel poloidally oriented grill resulted in an RF driven current of up to 30 kA at (100 kW, 2.5 GHz), exceeding the modelling predictions. At the FT-2 tokamak (R = 0.56 m, a = 0.08 m, B t = 3 T, I p = 30 kA) experiments with a traditional toroidally oriented grill revealed no strong dependence of the LHCD density limit on the H/D ratio in spite of LH resonance densities differing by a factor of 3. Microwave Doppler reflectometry (DR) at the Globus-M, and DR and heavy ion beam probe measurements at the tokamak TUMAN-3M (R = 0.53 m, a = 0.24 m, B t = 1.0 T, I p = 190 kA) demonstrated geodesic acoustic mode (GAM) suppression at the L to H transition. Observations at FT-2 using Doppler Enhanced Scattering showed that the GAM amplitude is anti-correlated both spatially and temporally to the drift turbulence level and electron thermal diffusivity. For the first time turbulence amplitude modulation at the GAM frequency was found both experimentally and in global gyrokinetic modelling. A model of the L-H transition is proposed based on this effect. The loss mechanisms of energetic ions' (EI) were investigated in the neutral beam injection (NBI) experiments on Globus-M and TUMAN-3M. Empirical scaling of the 2.45 MeV DD neutron rate for the two devices shows a strong dependence on toroidal field B 1.29 t and plasma current I 1.34 p justifying the B t and I p increase by a factor of 2.5 for the proposed upgrade of Globus-M. Bursts of ∼1 MHz Alfvenic type oscillations correlating with sawtooth crashes were observed in ohmic TUMAN-3M discharges. The possibility of low threshold parametric excitation of Bernstein and upper hybrid waves trapped in drift-wave eddies resulting in anomalous absorption in electron cyclotron resonance heating (ECRH) experiments in toroidal plasmas was identified theoretically. A novel method of radial correlation Doppler reflectometry is shown to be capable of measuring the turbulence wave-number spectrum in realistic 2D geometry. On the progress in design and fabrication of three diagnostics for ITER developed in the Ioffe institute is reported: neutral particle analysis, divertor Thomson scattering and gamma spectroscopy.

A new Compact Neutral Particle Analyzer (CNPA) [1] has been developed at A.F. Ioffe Physical-Technical Institute. The CNPA is an energy and mass spectrometer of reduced size (169´302´326 mm). It is designed for the simultaneous analysis of H 0 (0.8 -80 keV) and D 0 (0.66 -36 keV) fluxes emitted by plasma. Significant reduction of spectrometer size and weight (42.5 kg) is achieved by two innovations: 1) employing of stripping in a thin (100 Å) diamond-like carbon foil instead of the traditional stripping in gas; 2) using a high-fieldstrength (1 T) NdFeB permanent magnets instead of the traditional electromagnets for generation of the analysing magnetic field. An acceleration of particles scattered in the stripping foil is employed in the CNPA, which together with a magnetic field configuration providing two-coordinate focusing is used to achieve a better detection efficiency. The CNPA has the following significant advantages in comparison with conventional NPAs:1) The compact spectrometer can be installed practically at any position around plasma machine. It can be easily moved or replaced in the case of need.2) The CNPA shielding against n-gamma radiation and against stray magnetic field can be made more compact.3) The arrays of such instruments for a purpose of multichord diagnostic can be easily formed.4) The CNPA has a high detection sensitivity (10-100 times higher than for conventional NPAs) due to wide solid angle of observation and high detection efficiency.5) The CNPA does not require its own high-vacuum pumping system (because of absence of gas inlet) and does not require magnet power supply (because of permanent magnets usage).The CNPA has been tested on the Wendelstein 7-AS stellarator at IPP,

A neutral particle analyzer is used to measure the time-resolved energy spectrum of neutral hydrogen leaving a spheromak plasma. A gas cell filled with 10-50 mTorr of helium is used to strip electrons from incoming neutral hydrogen, lowering the minimum detectable energy well below that obtained with thin foils. Effective neutral particle temperature is calculated by fitting a Maxwellian energy distribution to the measured energy spectrum above and below approximately 300 eV. A computational model with approximated profiles of plasma density and neutral density is used with the measured neutral hydrogen flux to estimate the ion temperature. Measurement of the power flux due to neutral hydrogen emitted at the measurement location is extended to the whole plasma surface to estimate the total charge exchange power loss from the plasma. The initial results indicate that the charge exchange power loss represents only 2% of the total input gun power during the sustainment phase of the discharge.

Recent research at three small tokamaks with different parameters located at the Ioffe Institute-the spherical tokamak Globus-M, the large aspect ratio tokamak FT-2 and the compact tokamak TUMAN-3M-are reviewed. This overview covers energy confinement (Globus-M and FT-2), L-H transition (TUMAN-3M and FT-2), Alfvén waves (Globus-M and TUMAN-3M), ion cyclotron emission (TUMAN-3M), major plasma discharge disruption (Globus-M) and scrape-off layer (Globus-M) studies. A full-f global gyrokinetic modeling benchmark using synthetic diagnostics in FT-2 is described. Anomalous absorption and emission in electron cyclotron resonance heating experiments due to the parametric excitation of localized upper hybrid waves are analyzed theoretically. Progress in the development of the neutral particle analysis, gamma-ray spectrometry and divertor Thomson scattering combined with laser-induced fluorescence diagnostics for ITER is discussed. The status of the new Globus-M2 spherical tokamak is reported.

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