A high-power long lifetime beam dump for the Thomson scattering diagnostic system in the XuanLong-50 experiment.

The Energy iNNovation's XuanLong-50 is a spherical torus experiment with up to 10 s plasma operation duration. A 3 J/50 Hz pulsed laser is used in the Thomson scattering diagnostic system that is developed to measure the time evolutions of plasma electron temperature and density profiles. The expected laser pulse number is about 7.5 × 106/year with a power load of 150 W. To meet at least 1-year lifetime requirement, a Chevron type beam dump with polished molybdenum plates is designed and fabricated, which absorbs the laser beam energy in a 3D structure to reduce the laser fluence deposited on the material surface. To prevent the backscattered stray light from interfering with the Thomson scattering measurements, a 7.5 m beam path with folding mirrors is set between the beam dump and the plasma scattering volumes. Details of the beam dump design procedure including the laser beam profile control, multi-pulse laser damage threshold, heat dissipation, Zemax modeling, folding mirror selection, and beam path enclosure are presented together with the testing results.

A novel polychromator calibration method for Thomson scattering diagnostics.

Polychromators are most frequently used in Thomson scattering (TS) diagnostics to analyze the scattered light spectrum and intensity so that the plasma electron temperature (Te) and density (ne) can be derived. For Te measurements, the spectral response of the polychromator channels and the relative spectral responsivities need to be calibrated. The spectral response is calibrated with a bromine tungsten lamp and a monochromator in a conventional way. A novel method for calibrating the relative spectral responsivities of the polychromators is described in detail. A broadband pulsed Light Emission Diode (LED) is used, which has a spectral irradiance similar to that of the TS spectrum, and the LED can be driven in pulse mode with the pulse width similar to the TS signal pulse width of about 10-20 ns full width at half maximum. This new method allows for the calibration to be done after the polychromator is fully installed, and in situ system calibration can be easily performed, showing the advantages of accuracy, simplicity, efficiency, and flexibility. For ne measurements, absolute sensitivity calibration is done by Rayleigh scattering with argon gas. Formulas for calculating the plasma density from the calibration data and the polychromator signals from the off-laser wavelength channels are presented.

Thomson scattering diagnostic system for the XuanLong-50 experiment.

A 15-point Thomson scattering diagnostic system is developed for ENN's spherical torus experiment XuanLong-50 (EXL-50). A BeamTech laser with 3 J/pulse (1064 nm wavelength) at 50 Hz repetition rate is chosen for measurements during EXL-50 plasma operations. To enable measurements at low density (∼0.5 × 1018 m-3) plasma operations, the opto-mechanical subsystems are carefully designed to maximize the collection and transmission of the scattered light and to minimize the stray light level. In addition, the high bandwidth trans-impedance amplifiers and segmented high speed waveform digitizers allow for the application of muti-pulse averaging to further improve the signal-to-noise ratio. Details of the diagnostic system are described and initial experimental results are presented.

Development of a three-wave far-infrared laser interferometry and polarimetry diagnostic system for the C-2W field-reversed configuration plasmas.

Great advancements in modern field-reversed configuration (FRC) experiments motivated the development of a 14-chord three-wave far infrared (FIR) laser interferometry and polarimetry diagnostic system, which can provide simultaneous high temporal resolution measurements of density and Faraday rotation profiles with high accuracy. The unique challenges facing FIR diagnostics in high beta FRC plasmas are the extremely small (<0.5°) Faraday rotation angles, and severe laser beam refraction effects due to high density gradient and choice of long wavelength. The diagnostic system design and development are described with methods to overcome the challenges, and initial experimental data are also presented.

Suppressed ion-scale turbulence in a hot high-ß plasma.

An economic magnetic fusion reactor favours a high ratio of plasma kinetic pressure to magnetic pressure in a well-confined, hot plasma with low thermal losses across the confining magnetic field. Field-reversed configuration (FRC) plasmas are potentially attractive as a reactor concept, achieving high plasma pressure in a simple axisymmetric geometry. Here, we show that FRC plasmas have unique, beneficial microstability properties that differ from typical regimes in toroidal confinement devices. Ion-scale fluctuations are found to be absent or strongly suppressed in the plasma core, mainly due to the large FRC ion orbits, resulting in near-classical thermal ion confinement. In the surrounding boundary layer plasma, ion- and electron-scale turbulence is observed once a critical pressure gradient is exceeded. The critical gradient increases in the presence of sheared plasma flow induced via electrostatic biasing, opening the prospect of active boundary and transport control in view of reactor requirements.

Improved density profile measurements in the C-2U advanced beam-driven Field-Reversed Configuration (FRC) plasmas.

In the prior C-2 experiment, electron density was measured using a two-color 6-chord CO2/HeNe interferometer. Analysis shows that high-frequency common mode phase noise can be reduced by a factor of 3 by constructing a reference chord. In the system upgrade from C-2 to C-2U a 4-chord far-infrared laser interferometer was developed, which demonstrated superior sensitivity (1 × 1016 m-2 at >1 MHz bandwidth) and solved the under spatial sampling issue of the C-2 interferometer system. Improved density-profile measurement results are presented in this paper, including evidence of fast-ion modified density profile and stabilization of the n = 1 plasma wobble mode.

High sensitivity far infrared laser diagnostics for the C-2U advanced beam-driven field-reversed configuration plasmas.

A high sensitivity multi-channel far infrared laser diagnostics with switchable interferometry and polarimetry operation modes for the advanced neutral beam-driven C-2U field-reversed configuration (FRC) plasmas is described. The interferometer achieved superior resolution of 1 × 1016 m-2 at >1.5 MHz bandwidth, illustrated by measurement of small amplitude high frequency fluctuations. The polarimetry achieved 0.04° instrument resolution and 0.1° actual resolution in the challenging high density gradient environment with >0.5 MHz bandwidth, making it suitable for weak internal magnetic field measurements in the C-2U plasmas, where the maximum Faraday rotation angle is less than 1°. The polarimetry resolution data is analyzed, and high resolution Faraday rotation data in C-2U is presented together with direct evidences of field reversal in FRC magnetic structure obtained for the first time by a non-perturbative method.

Far infrared laser polarimetry and far forward scattering diagnostics for the C-2 field reversed configuration plasmas.

A two-chord far infrared (FIR) laser polarimeter for high speed sub-degree Faraday rotation measurements in the C-2 field reversed configuration experiment is described. It is based on high power proprietary FIR lasers with line width of about 330 Hz. The exceptionally low intrinsic instrument phase error is characterized with figures of merit. Significant toroidal magnetic field with rich dynamics is observed. Simultaneously obtained density fluctuation spectra by far forward scattering are presented.

Multi-channel Doppler backscattering measurements in the C-2 field reversed configuration.

A versatile heterodyne Doppler Backscattering (DBS) system is used to measure density fluctuation levels (in the wavenumber range kρs ≤ 50), and the toroidal E × B flow velocity in the C-2 Field-Reversed Configuration (FRC). Six tunable frequencies in three waveguide bands (26 GHz ≤ f ≤ 90 GHz) are launched using monostatic beam optics, via a quasi-optical beam combiner/polarizer and an adjustable parabolic focusing mirror (inside the vacuum enclosure) achieving Gaussian beam spot sizes of 3-5.5 cm at the X/O-mode cutoff. The DBS system covers plasma densities of 0.8 × 10(13) ≤ ne ≤ 1 × 10(14) cm(-3), and provides access to the FRC core (up to the field null) and across the FRC separatrix into the scrape-off layer plasma.

Measurements of neutral density profiles using a deuterium Balmer-alpha diagnostic in the C-2 FRC plasma.

In C-2 field-reversed configuration (FRC) device, low neutral density outside the FRC separatrix is required to minimize the charge exchange loss of fast particles. Titanium gettering is used in C-2 to reduce the wall recycling and keep the neutral density low in plasma edge. The measurements of neutral density radial profile are desirable to understand the plasma recycling and the effects of titanium gettering. These measurements are also needed to study the interaction of neutral beams with FRC plasma and confinement of fast ions. Diagnostic based on absolute deuterium Balmer-alpha (D-alpha) radiation measurements is developed and deployed on C-2 device to measure the radial profile of neutral density. Simultaneous measurements of electron density and temperature are done using CO(2) interferometer, Thomson scattering, and triple probes diagnostics along with absolute D-alpha radiation. Abel inversion was performed to get the time dependent radial profile of the local D-alpha emission density. Neutral density profiles are obtained under different machine conditions of titanium deposition.

Internal magnetic field measurement on C-2 field-reversed configuration plasmas.

A long-lived field-reversed configuration (FRC) plasma has been produced in the C-2 device by dynamically colliding and merging two oppositely directed, highly supersonic compact toroids (CTs). The reversed-field structure of the translated CTs and final merged-FRC state have been directly verified by probing the internal magnetic field structure using a multi-channel magnetic probe array near the midplane of the C-2 confinement chamber. Each of the two translated CTs exhibits significant toroidal fields (B(t)) with opposite helicity, and a relatively large B(t) remains inside the separatrix after merging.

Electron density and temperature profile diagnostics for C-2 field reversed configuration plasmas.

The 9-point Thomson scattering diagnostic system for the C-2 field reversed configuration plasmas is improved and the measured electron temperature profiles are consistent with theoretical expectations. Rayleigh scattering revealed a finite line width of the ruby laser emission, which complicates density calibration. Taking advantage of the plasma wobble motion, density profile reconstruction accuracy from the 6-chord two-color CO(2)∕HeNe interferometer data is improved.

Field reversed configuration confinement enhancement through edge biasing and neutral beam injection.

Field reversed configurations (FRCs) with high confinement are obtained in the C-2 device by combining plasma gun edge biasing and neutral beam injection. The plasma gun creates an inward radial electric field that counters the usual FRC spin-up. The n = 2 rotational instability is stabilized without applying quadrupole magnetic fields. The FRCs are nearly axisymmetric, which enables fast ion confinement. The plasma gun also produces E × B shear in the FRC edge layer, which may explain the observed improved particle transport. The FRC confinement times are improved by factors 2 to 4, and the plasma lifetimes are extended from 1 to up to 4 ms.

Development of a nine spatial point, multipulse Thomson scattering diagnostic.

A Thomson scattering diagnostic has been developed for the C-2 field-reversed configuration device. Based on a multipulse ruby laser, the system measures the electron temperature at nine spatial points. These points are chosen from 22 selectable positions covering r≈1-41 cm. Twin collection lenses couple the scattered photons to nine optical fiber pairs. Extra fiber lengths delay the signals from different spatial points relative to each other, allowing up to three points to be analyzed by a single polychromator. The polychromator, using compact photomultipliers as detectors, has six spectral channels covering the range of 685-725 nm and is able to estimate electron temperatures of ≈10-200 eV. The photomultiplier output signals are recorded by digital storage oscilloscopes integrated with the main MDSplus database, with temperature and error estimates generated automatically at the conclusion of each plasma discharge.

Two-chord interferometry using 3.39 µm He-Ne laser on a flux-coil-generated FRC.

A two-chord λ(IR)∼3.39 µm He-Ne laser interferometer system was developed for a flux-coil-generated field-reversed configuration to estimate the electron density and the total temperature of the field-reversed configuration (FRC) plasma. This two-chord heterodyne interferometer system consists of a single ∼2 mW infrared He-Ne laser, a visible (λ(vis)∼632.8 nm) He-Ne laser for the alignment, a 40 MHz acousto-optic modulator, photodetectors, and quadrature phase detectors. Initial measurement was performed and the measured average electron densities were 2-10×10(19) m(-3) at two different radial positions in the midplane. A time shift in density was observed as the FRC expands radially. The time evolution of the line-averaged density agrees with the density estimated from the in situ internal magnetic probes, based on a rigid-rotor profile model.

Two-color CO2/HeNe laser interferometer for C-2 experiment.

A six-channel two-color interferometer has been developed for plasma electron density measurements in the C-2 field reversed configuration experiment. A CO(2) laser is utilized as the main probe beams, while copropagating visible HeNe laser beams are mainly sensitive to vibration. Density measurements in C-2 plasmas have shown that this is a reliable turn-key system. The maximum residual phase noise after vibration compensation is less than ±5°, corresponding to a line integral density of 3×10(18) m(-2). The time resolution for routine operation is 2 µs.

Doppler spectroscopy and D-alpha emission diagnostics for the C-2 FRC plasma.

Two Doppler spectroscopy diagnostics with complementary capabilities are developed to measure the ion temperatures and velocities of FRC plasmas in the C-2 device. First, the multichord ion doppler diagnostic can simultaneously measure 15 chords of the plasma using an image intensified camera. Second, a single-chord fast-response ion Doppler diagnostic provides much higher faster time response by using a 16-channel photo-multiplier tube array. To study the neutral density of deuterium under different wall and plasma conditions, a highly sensitive eight-channel D-alpha diagnostic has been developed and calibrated for absolute radiance measurements. These spectroscopic diagnostics capabilities, combined with other plasma diagnostics, are helping to understand and improve the field reversed configuration plasmas in the C-2 device.

Dynamic formation of a hot field reversed configuration with improved confinement by supersonic merging of two colliding high-ß compact toroids.

A hot stable field-reversed configuration (FRC) has been produced in the C-2 experiment by colliding and merging two high-ß plasmoids preformed by the dynamic version of field-reversed θ-pinch technology. The merging process exhibits the highest poloidal flux amplification obtained in a magnetic confinement system (over tenfold increase). Most of the kinetic energy is converted into thermal energy with total temperature (T{i}+T{e}) exceeding 0.5 keV. The final FRC state exhibits a record FRC lifetime with flux confinement approaching classical values. These findings should have significant implications for fusion research and the physics of magnetic reconnection.

Nonambipolar magnetic-fluctuation-induced particle transport and plasma flow in the MST reversed-field pinch.

First direct measurements of nonambipolar magnetic fluctuation-induced charge transport in the interior of a high-temperature plasma are reported. Global resistive tearing modes drive the charge transport which is measured in the vicinity of the resonant surface for the dominant core resonant mode. Finite charge transport has two important consequences. First, it generates a potential well along with locally strong electric field and electric field shear at the resonant surface. Second, this electric field induces a spontaneous E x B driven zonal flow.

Measurement of the Hall dynamo effect during magnetic reconnection in a high-temperature plasma.

The fluctuation-induced Hall electromotive force, [deltaJ x deltaB]/nee, is experimentally measured in the high-temperature interior of a reversed-field pinch plasma by a fast Faraday rotation diagnostic. It is found that the Hall dynamo effect is significant, redistributing (flattening) the equilibrium core current near the resonant surface during a reconnection event. These results imply that effects beyond single-fluid MHD are important for the dynamo and magnetic reconnection.