R. Kaita

Plasma ion temperature measurements via charge-exchange recombination radiation

Spatially and temporally resolved plasma ion temperatures can be determined by measuring the Doppler‐broadened line profiles of transitions excited by charge‐exchange recombination reactions between fast hydrogen atoms and fully ionized low‐Z ions. Plasma rotation velocity profiles can also be obtained. A sample result from the PDX tokamak using He+ radiation is presented, and expected line intensities for model cases for PDX and TFTR are calculated.

Impurity levels and power loading in the pdx tokamak with high power neutral beam injection

Abstract The PDX tokamak provides an experimental facility for the direct comparison of various impurity control techniques under reactor-like conditions. Four neutral beam lines inject > 6 MW for 300 ms. Carbon rail limiter discharges have been used to test the effectiveness of perpendicular injection, but non-disruptive full power operation for > 100 ms is difficult without extensive conditioning. Initial tests of a toroidal bumper limiter indicate reduced power loading and roughly similar impurity levels compared to the carbon rail limiter discharges. Poloidal divertor discharges with up to 5 MW of injected power are cleaner than similar circular discharges, and the power is deposited in a remote divertor chamber. High density divertor operation indicates a reduction of impurity flow velocity in the divertor and enhanced recycling in the divertor region during neutral injection.

Initial results from the scoop limiter experiment in PDX

Abstract A particle scoop limiter with a graphite face backed by a 50 liter volume for collecting particles was used in PDX. Experiments were performed to test its particle control and power handling capabilities with up to 5 MW of D° power injected into D+ plasmas. Line average plasma densities of up to 8 × 1013 cm−3 and currents up to 450 kA were obtained. Plasma densities in the scoop channels greater than 2 × 1013 cm−3 and neutral densities in the scoop volume greater than 5 × 1014 cm−3 were observed. There is evidence that recycling may have occurred in the scoop channels for several discharges with large line-averaged plasma density. At beam powers up to 2.5 MW, energy confinement times above 40 ms were deduced from magnetics measurements and from transport analysis. Pressures in the vacuum vessel were in the 10 −5 Torr range, and recycling source neutral densities in the central plasma were low.

New electron cyclotron emission diagnostic for measurement of temperature based upon the electron Bernstein wave

Most magnetically confined plasma devices cannot take advantage of standard electron cyclotron emission (ECE) diagnostics to measure temperature. They either operate at high density relative to their magnetic field (e.g., ωp≫Ωc in spherical tokamaks) or they do not have sufficient density and temperature to reach the blackbody condition (τ>2). The standard ECE technique measures the electromagnetic waves emanating from the plasma. Here we propose to measure electron Bernstein waves (EBW) to ascertain the local electron temperature in these plasmas. The optical thickness of EBW is extremely high because it is an electrostatic wave with a large ki. For example, the National Spherical Torus Experiment (NSTX) will have an optical thickness τ≈3000 and CDX-U will have τ≈300. One can reach the blackbody condition with a plasma density ≈1011u2009cm−3 and Te≈1u2009eV. This makes it attractive to most plasma devices. The serious issue with using EBW is the wave accessibility for the emission measurement. Simple accessibili...

Electron Bernstein wave electron temperature profile diagnostic (invited)

Electron cyclotron emission (ECE) has been employed as a standard electron temperature profile diagnostic on many tokamaks and stellarators, but most magnetically confined plasma devices cannot take advantage of standard ECE diagnostics to measure temperature. They are either “overdense,” operating at high density relative to the magnetic field (e.g., ωpe≫Ωce in a spherical torus) or they have insufficient density and temperature to reach the blackbody condition (τ>2). Electron Bernstein waves (EBWs) are electrostatic waves that can propagate in overdense plasmas and have a high optical thickness at the electron cyclotron resonance layers as a result of their large kperp. In this article we report on measurements of EBW emission on the CDX-U spherical torus, where B0∼2u200akG, 〈ne〉∼1013u200acm−3 and Te≈10–200u200aeV. Results are presented for electromagnetic measurements of EBW emission, mode converted near the plasma edge. The EBW emission was absolutely calibrated and compared to the electron temperature profile measured by a multipoint Thomson scattering diagnostic. Depending on the plasma conditions, the mode-converted EBW radiation temperature was found to be ⩽Te and the emission source was determined to be radially localized at the electron cyclotron resonance layer. A Langmuir triple probe and a 140 GHz interferometer were employed to measure changes in the edge density profile in the vicinity of the upper hybrid resonance where the mode conversion of the EBWs is expected to occur. Initial results suggest EBW emission and EBW heating are viable concepts for plasmas where ωpe≫Ωce.Electron cyclotron emission (ECE) has been employed as a standard electron temperature profile diagnostic on many tokamaks and stellarators, but most magnetically confined plasma devices cannot take advantage of standard ECE diagnostics to measure temperature. They are either “overdense,” operating at high density relative to the magnetic field (e.g., ωpe≫Ωce in a spherical torus) or they have insufficient density and temperature to reach the blackbody condition (τ>2). Electron Bernstein waves (EBWs) are electrostatic waves that can propagate in overdense plasmas and have a high optical thickness at the electron cyclotron resonance layers as a result of their large kperp. In this article we report on measurements of EBW emission on the CDX-U spherical torus, where B0∼2u200akG, 〈ne〉∼1013u200acm−3 and Te≈10–200u200aeV. Results are presented for electromagnetic measurements of EBW emission, mode converted near the plasma edge. The EBW emission was absolutely calibrated and compared to the electron temperature profile me...

Spectroscopic imaging diagnostics for burning plasma experiments

Spectroscopic imaging of plasma emission profiles from a few electron volts to tens of kilo-electron volts enables basic diagnostics in present day tokamaks. For the more difficult burning plasma conditions, light extraction and detection techniques, as well as instrument designs need to be investigated. As an alternative to light extraction with reflective optics, we discuss normal incidence, transmissive-diffractive optics (e.g., transmission gratings), which might withstand plasma exposure with less degradation of optical properties. Metallic multilayer reflectors are also of interest for light extraction. Although a shift of the diffraction peak might occur, instrument designs that accommodate such shifts are possible. As imaging detectors we consider “optical” arrays based on conversion of the short-wavelength light into visible light followed by transport of the visible signal with hollow lightguides. The proposed approaches to light extraction and detection could enable radiation resistant diagnostics.

Plasma transport control and self-sustaining fusion reactor

The possibility of a high-performance/low-cost fusion reactor concept which can simultaneously satisfy (1) high beta, (2) high bootstrap fractio (self-sustaining) and (3) high confinement is discussed. In CDX-U, a tokamak configuration was created and sustained solely by internally generated bootstrap currents, in which a seed current is created through a nonclassical current diffusion process. Recent theoretical studies of MHD stability limits in spherical tori [e.g. the National Spherical Torus Experiment (NSTX)] produced a promising regime with stable beta of 45% and bootstrap current fraction of . Since the bootstrap current is generated by the pressure gradient, to satisfy the needed current profile for MHD stable high beta regimes, it is essential to develop a means to control the pressure profile. It is suggested that the most efficient approach for pressure profile control is through the creation of transport barriers (localized regions of low plasma transport) in the plasma. As a tool for creating the core transport barrier, poloidal-sheared-flow generation by ion Bernstein waves (IBW) near the wave absorption region appears to be promising. In PBX-M, application of IBW power produced a high-quality internal transport barrier where the ion energy and particle transport became neoclassical in the barrier region. The observation is consistent with the IBW-induced-poloidal-sheared-flow model. An experiment is planned on TFTR to demonstrate this concept with D - T reactor-grade plasmas. For edge transport control, a method based on electron ripple injection (ERI), driven by electron cyclotron heating (ECH), is being developed on CDX-U. It is estimated that both the IBW and ERI methods can create a transport barrier in reactor-grade plasmas (e.g. ITER) with a relatively small amount of power .

High energy and particle confinement times in PDX scoop discharges

Abstract Scoop limited discharges in PDX with neutral beam heating achieved energy and particle confinement times higher than those inferred from similar poloidal rail limiter discharges. We present transport simulations using TRANSP and BALDUR and neutral simulations using DEGAS. Large neutral densities and ionization rates localized near the scoop are inferred. Thermal neutral ionization increases the edge electron density substantially, resulting in a flattened density profile. Fueling from the scoop limiter is deeper than from simple limiters.

Fast-wave ion-cyclotron heating in the Princeton Large Torus

Recent experimental results for ICRF heating in PLT are presented. For the two-ion regime in D-H or D-/sup 3/He plasmas minority H and /sup 3/He ions are found to absorb the rf power and transfer it to the deuterons and electrons in accordance with Fokker-Planck theory. The deuteron heating rate is approx. 3 eV x 10/sup 13/ cm/sup -3//kW for H and approx. 6 eV x 10/sup 13/ cm/sup -3//kW for /sup 3/He minorities. Neutron fluxes of approx. 3 x 10/sup 11/ sec/sup -1/ corresponding to a T/sub d/ approx. 2 keV (..delta..T/sub d/ approx. 1.2 keV) have been produced with P/sub rf/ approx. = 620 kW at anti n/sub e/ approx. = 2.9 x 10/sup 13/ cm/sup -3/. Neutron energy spectra and mass sensitive charge exchange spectra indicate Maxwellian deuteron distributions. In addition, D-/sup 3/He fusion reaction rates greater than or equal to 10/sup 12/ sec/sup -1/ have been produced by the energetic /sup 3/He ions. For the second harmonic regime, initial heating results for an H plasma at P/sub rf/ approx. = 140 kW are consistent with the Fokker-Planck theory and the bulk heating rate is comparable to that of D heating in the D-H minority regime.

High throughput measurements of soft x-ray impurity emission using a multilayer mirror telescope

A 4in. multilayer mirror telescope has been tested on National Spherical Torus Experiment (NSTX) for high throughput measurements of the beam excited soft x-ray impurity emission. The design is aimed at imaging low-k turbulent fluctuations in the plasma core. The test device used curved and planar Mo∕Si mirrors to focus with ≈15% optical transmission and few angstrom bandwidths, the 135A Lyα line from injected Li III atoms, or the n=2–4 line from intrinsic C VI ions. As test detectors we used 1cm2 absolute extreme ultraviolet diodes, equipped with 400kHz bandwidth, low noise preamplifiers. With the available view on NSTX the telescope successfully detected small impurity density fluctuations associated with 1∕1 modes rotating at midradius, indicating that a high signal to noise ratio and cost effective core turbulence diagnostic is feasible based on this concept.