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This article covers the in-vessel design of the SPARC interferometry diagnostic system, highlighting unique aspects of the systems design and port plug integration in preparation for "day-1" plasma operations as a critical diagnostic for density feedback control. An early decision for the diagnostic was to deploy two lasers in the infrared wavelength spectrum, allowing the system to have a higher optical throughput. The optimization of the in-vessel geometry for the diagnostic follows a similar approach, focusing on de-risking possible damage to the plasma facing optical components by moving them further from the plasma with an orientation that provides a greater possibility for protective features to be added. The inclusion of in-vessel optical assemblies requires detailed design efforts of custom all-metal parts, designed to remain functional when subjected to harsh operational conditions, in many cases for the entire SPARC lifetime. The details presented here were included in the design to ensure that the assemblies can not only withstand a major electromagnetic disruption or thermal event but also maintain good stability through normal operations. The design also addresses more nuanced effects, such as the transient heat loading of the plasma facing mirrors. Through the utilization of modeling and design tools, these effects were brought into the design and simulation workflow, further reducing uncertainty as the system moves toward system commissioning. [ABSTRACT FROM AUTHOR]

A four-channel compact neutral particle analyzer (CNPA) based on operating small Si diode detectors in pulse-height analysis (PHA) mode is used to measure energetic hydrogen minority ions with energies between ∼50 and 350 keV stemming from ion-cyclotron range-of-frequency heated D(H) Alcator C-Mod plasmas with both active and passive charge exchange (CX). First core minority ion distribution results from Alcator C-Mod discharges and a detailed description of the diagnostic are presented. The diagnostic employs integrated electronics and fast digitization of the shaping amplifier voltage. The digitized data are stored for postshot PHA, which removes the constraints of real-time PHA and allows for improved performance via elimination of base line shift effects and potentially relieving pileup through Gaussian fitting routines. The CNPA is insensitive to the large gamma and neutron background in Alcator C-Mod discharges but is susceptible to the plasma’s soft x-ray flux. The soft x-ray flux limits the CNPA energy resolution to ∼15–20 keV. A simple model is used to interpret the active CNPA data which permits rapid estimates of the core hydrogen minority temperatures and anisotropy with a time resolution of ∼100 ms. Hydrogenlike boron is identified as an important electron donor for the CX signal. [ABSTRACT FROM AUTHOR]

An acousto-optic rf spectrometer (AOS) has been designed and constructed for use on the CONSTANCE B and TARA mirror experiments at MIT. The AOS is an analog preprocessor of data that can come from a variety of sources, including rf probes and microwave/laser scattering experiments. The output of the AOS is the Fourier transform amplitude of the input in a parallel format suitable for computer acquisition. The spectrometer, having a bandwidth of 500 MHz and a resolution of 1 MHz, together with its receiver, is capable of looking at fluctuations anywhere in the frequency range of a few MHz up to 4 GHz. Power levels of a few nanowatts can be detected. Complete 500-MHz-wide spectra can be processed and acquired at the rate of 1 every 10 μs. The optical components, receiver, and data-acquisition interface will be discussed, and initial data taken on the CONSTANCE B experiment will be shown. [ABSTRACT FROM AUTHOR]

Focuses on the use of reflectometry on the International Thermonuclear Experimental Reactor to measure the density profile in the main plasma and the divertor and to provide measurements of the plasma position and shape supplementary to the magnetic diagnostics. Systems required to access the full plasma profile; Measurements of the scrape-off layer; Real-time calibration technique of the whole transmission line.

An algorithm has been developed for fast analysis of a sparse multichord interferometer array, yielding peak density, Gaussian width, radial offset, ellipticity, and instability amplitude and frequency from line-integrated density measurements. A nonlinear least-squares fit is performed over many short time intervals, with independent variables reduced to one dimension. Assumptions of plasma rigidity and slowly evolving rotation rate are necessary to trade temporal resolution (on instability timescales) for spatial resolution across the moving plasma column. Results from the Tara tandem mirror experiment are shown. [ABSTRACT FROM AUTHOR]

Techniques developed for the alignment of high-energy accelerators have been applied to the alignment of the Tara tandem mirror magnetic confinement device. Tools used were: a transit/laser surveyor’s system for establishing an invariant reference; optical scattering from ferromagnetic crystallites for establishing magnetic centers in the quadrupole anchor/transition modules; an electron-optical circle-generating wand for alignment of the solenoidal plug and central cell modules; and four differently configured electron emissive probes, including a 40-beam flux mapping e gun, for testing the alignment of the coils under vacuum. Procedures are outlined, and results are given which show that the magnetic axes of the individual coils in the Tara set have been made colinear with each other and with the reference to within ±1.0 mm over the length of the machine between the anchor midplanes. [ABSTRACT FROM AUTHOR]

High densities with sharp, fast rising profiles are expected during pellet injection in Alcator CMOD. Consideration of interferometer systems other than far infrared will be needed to avoid refractive effects and make profile measurements during pellet injection possible. CO[sub 2] wavelengths are proving to be the most interesting from the standpoint of density measurements, with the necessary vibration subtraction provided by a coaxial He:Ne system. We will describe the two-color interferometer aa it is presently designed and present results from simulations made of the system to determine noise levels, minimum measurable density, and optimal number and location of chords. Ray tracing results both with and without pellet injection will be compared. Possible fringe counting schemes will also be discussed. [ABSTRACT FROM AUTHOR]

A Thomson scattering experiment is under construction for the central cell of the Tara tandem mirror. This experiment has been designed to measure electron temperature and density at densities as low as 3 × 10[sup 11] cm[sup -3]. The measurement is made at a single time point per shot using a Holobeam 10-J Q-switched ruby laser, folded back for two passes through the scattering volume. Detection is at 90° with f/5 optics. The optical table is mounted on linear bearings with a 10-in. stroke, so that the radius of the scattering volume may be changed from shot to shot. Novel features of the experiment include a high-throughput (> 25%) commercial triple-grating polychrometer using holographic gratings, and a directly coupled ten-channel microchannel plate photodetector array. [ABSTRACT FROM AUTHOR]

Designs a multichannel infrared polarimeter system for measurement of the plasma current profile in the Alcator C-Mod tokamak. Utilization of a new carbon dioxide laser in the system; Application of the electro-optic polarization-modulation technique.

An E B end-loss ion spectrometer from the Livermore TMX-U tandem mirror experiment was installed on Tara for high-resolution ion spectroscopy. This diagnostic contains parallel electric and magnetic fields, separating the masses and energies of the ions over 128 collector plates. The ion energy distribution nominally yields confining potentials and parallel ion temperatures. Additional experiments have diagnosed the resonance position of the central cell ion cyclotron heating, rf enhanced losses of high-energy sloshing ions in the axicell ("plug"), and observation of MHD instabilities at higher time resolution (20 kHz). [ABSTRACT FROM AUTHOR]

An endloss spectrograph has been designed and is being fabricated for the Tara tandem mirror. This spectrograph is a double parallel-plate electrostatic analyzer about 60 cm in length with a 30° entrance angle; the high-voltage plate is a transparent wire cloth grid which allows higher energy particles to pass through for detection on a symmetric ground plane. The analyzer has three detection planes: first, the normal focal plane for low-energy detection at high resolution (0.1-4 keV); second, the entrance grounded plate for mid energies (4-6 keV); and third, the symmetric ground plane for coarse resolution at higher energies (8-20 keV). The analyzer is defocusing on the symmetric plane. This spectrograph will be able to measure simultaneously the endloss potential cutoff and ions from the neutral beams; in conjunction with a loss cone neutral beam source the resolution is sufficient (60 eV) to measure the plasma potential. [ABSTRACT FROM AUTHOR]

The TARA Tandem Mirror Experiment has recently begun operation. The set of diagnostics available at this time is discussed. The following diagnostics are now in use: diamagnetic loops, a multichord microwave interferometer, Langmuir and emissive probes, pick-up loops, and secondary-emission detectors. End-loss diagnostics include net current detector arrays, Faraday cup arrays, swept particle analyzer arrays, and calorimetry. Light-emission measurements are made in the visible and VUV regions. A multichord fiber-optic array for plasma position detection is also used. In addition, a three-channel charge exchange analyzer, a hard x-ray system, and fast pressure gauges are available. [ABSTRACT FROM AUTHOR]

The SPARC tokamak is a high-field, Bt0 ∼12 T, medium-sized, R0 = 1.85 m, tokamak that is presently under construction in Devens, MA, led by Commonwealth Fusion Systems. It will be used to de-risk the high-field tokamak path to a fusion power plant and demonstrate the commercial viability of fusion energy. SPARC's first campaign plan is to achieve Qfus > 1 using an ICRF-heated, <10 MW, high current, Ip ∼ 8.5 MA, L-mode fueled by D-T gas injection, and its second campaign will investigate H-mode operations in D-D. To facilitate plasma control and scientific learning, a targeted set of ∼50 plasma diagnostics are being designed and built for operation during these campaigns. While nearly all diagnostics are based on established techniques, the pace of deployment, relative to the first plasma, and the harshness of the thermal, electromagnetic, and radiation environment are unprecedented for medium-sized tokamaks. An overview of the SPARC diagnostic set is given, providing context to further details communicated by the SPARC team in companion publications that are system-specific. The system engineering philosophy for SPARC diagnostics is outlined, and the design and engineering verification process for components inside and outside the primary vacuum boundary are described. Diagnostics are mounted directly to the vacuum vessel as well as housed within a series of eight midplane and 24 off-midplane replaceable port plugs. With limited exceptions, signal conditioning, digitization electronics and cameras as well as lasers and microwave sources are localized to a series of five Diagnostic Lab spaces, totaling ∼350 m2, located >15 m from the center of the tokamak, on the other side of a 2.4 m concrete shielding wall. A series of 31 large-scale penetrations have been included in the SPARC Tokamak Hall to facilitate integration of early campaign diagnostics and to provide upgradability., (© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC) license (https://creativecommons.org/licenses/by-nc/4.0/).)