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

A multi-chord, two-color interferometer using Hilbert transform phase detection for measuring electron density in spheromak plasmas

A new, four-chord, CO₂/He-Ne heterodyne interferometer has been designed and built for measuring line-averaged plasma density in the HIT-SI3 and subsequent HIT-SIU sustained spheromak devices. The two-color system successfully eliminates vibration-induced errors caused by mirrors that are secured to the vacuum chamber and is able to resolve electron densities n_e in the full operating range of 10¹⁸-10²⁰ m^-3 in both experiments with an integrated error of 4.7 × 10¹⁷ m^-2. Data are presented from high toroidal current plasma discharges, showing the time evolution of electron densities n_e and j_ϕ/n_e along multiple chords.

Two-color interferometer for study of dense low-ionized plasma on the target in high-power pulse linear accelerator

A two-color infrared interferometer has been developed for the investigation of high-density weakly ionized tantalum plasma in x-ray complexes based on linear induction accelerators (1.6 kA electron beam current, 4.6 MeV energy, and 100 ns pulse duration). Simultaneous probing at two different wavelengths makes it possible to independently measure the density of neutral and electron components. The interferometer uses wavelength values of 1.064 µm (Nd:YAG laser) and 10.6 µm (CO₂ laser). To attenuate the effect of sample beam refraction in inhomogeneous plasma, the interferometer used a refraction suppression scheme composed of spherical mirrors focusing the object beam into the region occupied by the plasma. In addition, the power of the sample beam transmitted through the plasma was controlled in order to analyze whether there was a distortion of the interference pattern because of strong sample beam refraction and absorption in the plasma cloud. To calibrate the initial phase shift of the probe radiation, a movable mirror mounted on a piezoelectric element and oscillating according to a harmonic law with amplitude greater than the laser wavelengths was used. In initial experiments, the parameters of target plasma registered by this interferometer are as follows: the linear density of neutrals reached 1.5 · 10¹⁷ cm^-2, and the degree of ionization was of the order of 10^-2. The target plasma expansion velocity is determined as ∼6 km/s.

A combined millimeter wave and CO2 interferometer on the C-2W Jet plasma

A two wavelength tangentially viewing multi-chord interferometer has been built for the Jet plasma of the C-2W experiment at TAE Technologies. A novel 1 mm wavelength interferometer has been developed to be used simultaneously with a CO₂ laser interferometer to provide full coverage of the Jet plasma and the translating field-reversed configuration (FRC) plasma before merging. With CO₂ and millimeter wave sources, the interferometer proposes to cover a combined dynamic range of line integrated density of more than 1000 although the CO₂ interferometer sub-system is not yet operational. Sited at the axial location of the mirror field of C-2W, the interferometer will play a pivotal role in assessing the FRC before merging and the operation of the inner and outer divertors and particle outflow. The performance of the millimeter wave interferometer and recent measurements is discussed.

Suppressed ion-scale turbulence in a hot high-β 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.

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.

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.

Overview of C-2 field-reversed configuration experiment plasma diagnostics

A comprehensive diagnostic suite for field-reversed configuration (FRC) plasmas has been developed and installed on the C-2 device at Tri Alpha Energy to investigate the dynamics of FRC formation as well as to understand key FRC physics properties, e.g., confinement and stability, throughout a discharge. C-2 is a unique, large compact-toroid merging device that produces FRC plasmas partially sustained for up to ∼5 ms by neutral-beam (NB) injection and end-on plasma-guns for stability control. Fundamental C-2 FRC properties are diagnosed by magnetics, interferometry, Thomson scattering, spectroscopy, bolometry, reflectometry, and NB-related fast-ion/neutral diagnostics. These diagnostics (totaling >50 systems) are essential to support the primary goal of developing a deep understanding of NB-driven FRCs.

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.

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.

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.

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.

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.