Overview of C-2 field-reversed configuration experiment plasma diagnostics H. Gota, M. C. Thompson, M. Tuszewski, and M. W. Binderbauer
Citation: Review of Scientific Instruments 85, 11D836 (2014); View online: https://doi.org/10.1063/1.4884616 View Table of Contents: http://aip.scitation.org/toc/rsi/85/11 Published by the American Institute of Physics
Articles you may be interested in A high performance field-reversed configuration Physics of Plasmas 22, 056110 (2015); 10.1063/1.4920950 Recent breakthroughs on C-2U: Norman’s legacy AIP Conference Proceedings 1721, 030003 (2016); 10.1063/1.4944019 Review of field-reversed configurations Physics of Plasmas 18, 070501 (2011); 10.1063/1.3613680 Diagnostic suite of the C-2U advanced beam-driven field-reversed configuration plasma experiment Review of Scientific Instruments 87, 11D435 (2016); 10.1063/1.4960730 A new high performance field reversed configuration operating regime in the C-2 device Physics of Plasmas 19, 056108 (2012); 10.1063/1.3694677 Two-color /HeNe laser interferometer for C-2 experiment Review of Scientific Instruments 81, 10D516 (2010); 10.1063/1.3478983
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D836 (2014)
Overview of C-2 field-reversed configuration experiment plasma diagnosticsa) H. Gota,b) M. C. Thompson, M. Tuszewski, and M. W. Binderbauer Tri Alpha Energy, Inc., Rancho Santa Margarita, California 92688, USA
(Presented 3 June 2014; received 31 May 2014; accepted 9 June 2014; published online 4 August 2014) 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. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4884616] I. INTRODUCTION
A field-reversed configuration (FRC) is an extremely high-beta (β = 2μ0 p/Be 2 ∼ 90%; here, p is the average plasma pressure and Be is the external magnetic field) compact toroid (CT) with predominantly poloidal axisymmetric magnetic field and little or no self-generated toroidal magnetic field.1, 2 The FRC consists of a torus of closed-field-lines inside a separatrix and an annular edge layer on the openfield-lines just outside the separatrix. The edge layer coalesces into jets beyond the FRC length, providing a natural divertor, which may allow extraction of energy without restriction. C-2 is a large θ -pinch, CT-merging system, built at Tri Alpha Energy to form relatively high flux, high temperature FRC plasmas.3, 4 Studying aspects of FRC sustainment by neutral-beam (NB) injection and fueling is the main goal of C-2 experiments. The C-2 device, as illustrated in Fig. 1, consists of a central confinement region surrounded by two θ pinch formation sources and two divertors. The C-2 confinement chamber is made of stainless steel that serves as a flux conserver on the timescale of the experiment. The formation tubes are made of quartz. A set of DC-magnets generates a quasi-static axial magnetic field, Bz , throughout the device. The typical field is Bz ∼ 0.1 T in the confinement region with an end-mirror ratio of ∼3.5. There are magnetic mirror plugs in between the formation and divertor sections on each side that can produce a strong magnetic field up to ∼2 T. The mirror plugs play an important role in controlling the open-fieldline plasma confinement. The C-2 device has more than 50 diagnostic systems installed on the confinement vessel, formation sections, and divertor regions to investigate the FRC plasma behavior as well as to characterize the machine operating state. Signals and data from individual diagnostics are transferred to a) Contributed paper, published as part of the Proceedings of the 20th
Topical Conference on High-Temperature Plasma Diagnostics, Atlanta, Georgia, USA, June 2014. b) Author to whom correspondence should be addressed. Electronic mail: [email protected]. 0034-6748/2014/85(11)/11D836/4/$30.00
a data-acquisition system that acquires over 1000 channels on every C-2 discharge. The acquired raw data are generally post-processed into plasma parameters and then stored on databases such as MDS+ and MySQL for further data analysis. On typical C-2 discharges, about half a gigabyte of data is generated after each shot including analysis movies and computations. Fundamental FRC plasma parameters and properties are diagnosed by magnetics, interferometry, Thomson scattering, and spectroscopy on the C-2 confinement vessel midplane. In addition, a bolometry-based tomographic tool, multi-chord Bremsstrahlung/Dα -line monitors, and Mirnov probes provide global FRC characteristics, such as shape, position, and some information on low order magnetohydrodynamic (MHD) instabilities. NB-related fast-ion/neutral diagnostics on C-2 are essential to study the effect of NB injection into FRC plasmas and support our primary goal of NB-driven FRCs. A complete list of C-2 diagnostics is provided in Table I. In this paper, C-2 operating condition and FRC plasmas are described in Sec. II, and the comprehensive diagnostic suite for C-2 FRC plasmas is summarized and described by diagnostic categories in Sec. III.
II. C-2 FRC PLASMAS
In C-2, a typical long-lived FRC plasma state has the following properties: radius ∼0.4 m, length ∼3 m, rigid-rotor poloidal flux up to ∼10 mWb, total temperature (Ti + Te ) up to ∼1 keV, electron density ∼3 × 1019 m−3 , and external axial magnetic field ∼0.1 T. Significant progress has recently been made.5, 6 Stable plasmas with lifetimes up to ∼5 ms have been achieved by combining effects of NB injection and edgeplasma biasing from end-on plasma-guns. We refer to these long-lived stable plasma discharges as the high performance FRC (HPF) regime. The C-2 NBs (20 keV hydrogen, ∼4 MW) are injected tangentially to the FRC current (co-injection), with an average radial impact parameter of 0.19 m which per-
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mits current drive. The fast ions, created primarily by charge exchange, have betatron orbits that add to the FRC azimuthal current, and a sufficiently large fast ion population may improve FRC stability and confinement properties significantly. C-2 FRC parameters, obtained through dynamic formation and CT-merging, are suitable for NB capture (shine-through and first orbit losses < 15%) and for fast ion confinement (slowing down time ∼ FRC lifetime). Two plasma-guns are mounted inside of each divertor and produce a hot (Te ∼ 30– 50 eV, Ti ∼ 100 eV) tenuous (∼1018 m−3 ) plasma stream. The gun also creates an inward radial electric field (Er < 0) that counters the usual FRC spin-up in the ion diamagnetic direction and mitigates the toroidal mode n = 2 rotational instability without applying quadrupole magnetic fields. Hence, NBs are injected into near-axisymmetric FRC discharges. The plasma-gun also produces E × B velocity shear just outside of FRC separatrix, yielding improved FRC confinement properties and stability. Better plasma centering (less n = 1 wobble motion) is also obtained presumably from line-tying to the gun electrodes.
field. For FRC formation in C-2 the typical negative-bias and main-reversal fields are −0.05 T and 0.4 T, respectively. Each fast magnetic signal is passively integrated (RC time constant ∼3 ms) and transferred to a 16 MS/s data-acquisition system. Magnetic probe arrays in the confinement vessel are one of the core C-2 diagnostic systems and provide important information on the FRC global performance such as shape and stability as well as on many key plasma parameters.7 There are 44 internal/external-vacuum B-dot probes mounted on the confinement vessel. Together, these probes measure the threedimensional magnetic fields (Bz , Bθ , Br ) and the excluded-flux profile of the FRC, which approximates the separatrix shape. These B-dot probe signals are processed with active integrator circuits (RC ∼ 100 μs) and then acquired at 0.5 MS/s. In addition, azimuthal arrays of B-dot probes, also known as Mirnov probes, can detect FRC global instabilities with toroidal mode number up to 3. These Mirnov probe signals are acquired directly at 60 MS/s so that they can also detect fast magnetic field fluctuations associated with NB fast particle buildup. Internal magnetic probe arrays were developed and inserted directly into the FRC plasma at the C-2 midplane.8 One of the probe arrays consists of 16 sets of Bz and Bθ , and it covers r = 0–0.45 m (vessel-wall inner-radius ∼0.7 m). Although the internal probe perturbed the FRC performance significantly, the magnetic-field-reversal inside the separatrix was successfully measured and the FRC structure of the merged final state was verified.
III. DIAGNOSTIC SUITE ON C-2
B. Interferometer and polarimeter
FIG. 1. Schematic of the C-2 device, illustrating FRC formation and neutralbeam injection.
A. Magnetic diagnostics
In the two θ -pinch formation sections 17 sets of magnetic probes and flux loops are mounted outside of each quartztube to measure the axial magnetic field and the loop voltage from the corresponding θ -pinch coil straps. This allows for monitoring individual coil discharges to dynamically form and translate FRC/CT plasmoids into the confinement vessel. C-2’s pulsed-power system has ∼100 kJ of the stored energy in each formation section for the main reversal field, and its rise-time is about 4 μs and then crowbarred at the peak of the
Laser and microwave interferometry provides nonperturbing measurements of the plasma electron density. In the C-2 midplane, a 6-channel two-color (CO2 laser at λ ∼ 10.6 μm with a visible He-Ne laser) interferometer system is routinely used to measure line-integrated electron densities along each beam path (impact parameters at r ∼ 3, 15, 20, 25, 30, 35 cm) and to obtain the radial density profiles of the FRC plasma using an Abel inversion technique.9 The Abel inverted density profile evolution of typical HPF plasma discharge is largely flat in the early phase and develops a hollowness of the profile later in the FRC lifetime.
TABLE I. Diagnostic suite on C-2 FRC plasmas. Shape/position North/south formation Bz probes North/south formation flux loops Confinement Bz probes Confinement Bθ probes Mirnov probe arrays Internal B probe arrays Polarimeter CCD end cameras X-ray imaging cameras Bremsstrahlung arrays Video camera
Densities/fluctuation
Temperatures/radiation
Fast ions/neutrals
CO2 interferometer Visible He-Ne interferometer IR He-Ne interferometer Dispersion interferometers Microwave interferometers Cut-off reflectometer Fluctuation reflectometer Fluctuation imaging Triple probe Mach probe Rake probe Gundestrup probe Particle flux probe Plasma potential probe
Thomson scattering Soft X-ray detector Line ratio spectrometer He-jet Mach nozzle Fast Doppler spectrometer High resolution Doppler Survey spectrometers IR spectrometer Impurity line monitors VUV spectrometer He-3 neutron counters Ion energy analyzer Bolometer arrays End-on bolometer
Electrostatic NPA Electromagnetic NPAs Neutral particle bolometer arrays Pyro bolometers SEE detectors ZnS scintillator Plastic scintillators Beam Hα analyzer Modulated H beam Dα monitors Dα /Dβ intensity ratios Pellet CCD camera Fast ionization gauges RGAs
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A far infrared (FIR) laser polarimetry system has recently been developed on C-2, which allows for non-perturbing magnetic field measurements.10 Two FIR laser beams (impact parameters of r ∼ 20 and 30 cm) are set up from the side of the machine, like CO2 interferometer chords, and the system measures the Faraday rotation angles in the FRC plasma. We have successfully estimated the internal toroidal magnetic fields during the FRC lifetime. The FIR laser can also be used as in interferometer mode; it has advantages in low-density regimes because of its long wavelength and can detect a fluctuation along the beam path. C. Thomson scattering system
The C-2 Thomson scattering (TS) system measures the electron temperature of the FRC plasma at 9 spatial points per laser pulse.11 The ruby laser is rated up to 10 J when operated in a single-pulse mode and can produce up to 4 pulses within ∼0.8 ms period. The TS collection optics and polychromators that have 7 spectral channels each were custom designed and built at Tri Alpha Energy. The C-2 TS system can also provide the FRC electron density profile at the same spatial points. The absolute intensity response of the system is calibrated with Rayleigh scattering of argon gas from 0.2 to 5.0 Torr, where the Rayleigh scattering signal is comparable to the Thomson scattering signal at electron densities from 1.6 × 1013 to 4.0 × 1014 cm−3 .
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A survey spectrometer is a useful tool to monitor the impurity spectrum and concentration during C-2 discharges. There are several types of survey spectrometers mounted on C-2; e.g., visible range, highly resolved Dα /Hα region, and IR region. These impurity monitors are very important to understand background impurity levels as well as to evaluate the effect of C-2 vessel wall conditioning such as a titanium gettering and lithium evaporation. A vacuum ultraviolet (VUV) spectrometer has recently been developed and installed on C-2 to measure the composition/intensity of high-energy photons, emitted from the plasma beyond the visible spectrum. This provides key information on what filter set would be best for a bolometry in the soft X-ray region. F. Bolometers
Sets of bolometer arrays are mounted inside of the C-2 confinement vessel to measure the total flux of electromagnetic and neutral particle energy emitted from the plasma. The systems include an extensive array of absolute extreme ultraviolet (AXUV) photodiodes and pyro-electric crystals. The side-on 16-channel AXUV photodiode arrays with pinholes provide spatially resolved measurements, and a total of 144 channels/sensors cover almost the entire FRC volume, which allows for 3D reconstruction. The data set from the bolometers also indicates FRC global motion/instability and estimates the total radiated power from the FRC plasma.
D. Reflectometers
A microwave reflectometry/Doppler Backscattering (DBS) system has been used to measure the toroidal wavenumber spectrum and the radial correlation length of density fluctuations in C-2 FRC plasmas.12 A 6-channel frequency-tunable heterodyne system is coupled to beam optics and a steerable parabolic focusing mirror that launches the beam into the FRC. The DBS system covers plasma densities of (0.8–10) × 1013 cm−3 and provides density fluctuation amplitudes as well as E × B flow measurements in the FRC core and scrape-off-layer (SOL) regions. Fluctuation levels are substantial in the SOL while they are very low in the core, particularly in the HPF regime. A dedicated, fast swept profile reflectometer was also developed for measurements of the edge density profile in the range of (0.3–2.2) × 1013 cm−3 with a maximum time-resolution of 2.5 μs. E. Spectrometers
Two ion Doppler spectroscopy systems13 are used to measure the ion temperature and velocity of C-2 FRC plasmas; (i) the multi-chord ion Doppler (MCID) system can measure 15-chordal views of FRCs with a CMOS camera, and (ii) the single-chord fast-response ion Doppler (FRID) system provides a high temporal resolution using a 16 channel photomultiplier tube (PMT) array. For high temperature FRC core plasmas, an Oxygen-V emission line (λ ∼ 278.1 nm) is typically selected on these spectrometers to estimate the FRCcore ion temperature radial profile and time evolution. For the FRC edge/SOL region a lower excitation energy level of oxygen or other impurity lines are chosen.
G. Electric (Langmuir) probes
Several types of Langmuir probes have been implemented to characterize the open-field-line plasma properties such as SOL temperature, density, and potential as well as jet (plasma exhaust) flow.14 The Langmuir probes include linear arrays of triple probes, a Rake probe (linear array of single-tipped swept probe), a multi-faced Gundestrup probe, a particle-flux probe, an ion-sensitive (baffle) probe, and a combination of Mach/triple probe. They are all interchangeable designs and mounted on linear actuators allowing for programmatic radial scans of the various plasma parameters over the course of several C-2 discharges. H. Neutral-beam related diagnostics
A neutral particle analyzer (NPA) measures the energy distribution of charge exchange neutrals escaping the plasma. In C-2 there are two kinds: electro-magnetic and electro-static NPAs mounted at the same cross-section as the NB injectors. The typical C-2 NBs are 20 keV hydrogen fueled but can be deuterium fueled and modulated for actively measuring a localized charge-exchange target for confined fast protons.15 A neutral particle bolometer (NPB) measures the fastneutral flux resulting from charge-exchanged fast-ions. The custom built NPB array has 16-channel AXUV photodiodes with a ∼40 nm tungsten coating to attenuate photon energies in the visible to extreme ultraviolet range, so that only fastneutral particles will be detected.16 Neutron and proton detectors measure the particles produced from nuclear reactions to help understand fast particle
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confinement in the plasma. This includes slow He-3 counters, fast scintillation detectors mounted just outside the vacuum chamber for neutron detection, and an in-vacuum high-energy proton detector.17 Charge-exchange recombination spectroscopy (CHERS) is used with the modulated NBs to measure the ion temperature and velocity. CHERS provides measurements of the ion energy distribution that are resolved in both space and time. Secondary electron emission (SEE) detectors are mounted on each NB-dump target plate to monitor NB injection performance and coupling to the FRC plasma; typical NB shine-through losses are 10%–15% early in time but glow as the FRC plasma shrinks.
ACKNOWLEDGMENTS
The authors would like to acknowledge the rest of the TAE staff for their dedicated diagnostic system development, maintenance, data analysis, and useful discussions.
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Tuszewski, Nucl. Fusion 28, 2033 (1988). C. Steinhauer, Phys. Plasmas 18, 070501 (2011). 3 M. Binderbauer, H. Y. Guo et al., Phys. Rev. Lett. 105, 045003 (2010). 4 H. Y. Guo, M. Binderbauer et al., Phys. Plasmas 18, 056110 (2011). 5 M. Tuszewski, A. Smirnov et al., Phys. Plasmas 19, 056108 (2012). 6 M. Tuszewski, A. Smirnov et al., Phys. Rev. Lett. 108, 255008 (2012). 7 M. C. Thompson, A. D. Van Drie et al., Rev. Sci. Instrum. 83, 10D709 (2012). 8 H. Gota, M. C. Thompson et al., Rev. Sci. Instrum. 83, 10D706 (2012). 9 O. Gornostaeva, B. H. Deng et al., Rev. Sci. Instrum. 81, 10D516 (2010). 10 B. H. Deng et al., “Far infrared laser polarimetry and far forward scattering diagnostics for the C-2 field reversed configuration plasmas,” Rev. Sci. Instrum. (these proceedings). 11 F. Glass, B. H. Deng et al., Rev. Sci. Instrum. 81, 10D506 (2010). 12 L. Schmitz et al., “Multi-Channel Doppler Backscattering Measurements in the C-2 Field Reversed Configuration,” Rev. Sci. Instrum. (these proceedings). 13 D. K. Gupta, E. Paganini et al., Rev. Sci. Instrum. 81, 10D737 (2010). 14 T. Roche et al., “Langmuir probe diagnostic suite in the C-2 field-reversed configuration,” Rev. Sci. Instrum. (these proceedings). 15 S. Korepanov, A. Smirnov et al., Rev. Sci. Instrum. 83, 10D720 (2012). 16 R. Clary, A. Smirnov et al., Rev. Sci. Instrum. 83, 10D713 (2012). 17 R. Magee et al., “Fusion proton diagnostic for the C-2 field reversed configuration experiment,” Rev. Sci. Instrum. (these proceedings). 2 L.
Citation: Review of Scientific Instruments 85, 11D836 (2014); View online: https://doi.org/10.1063/1.4884616 View Table of Contents: http://aip.scitation.org/toc/rsi/85/11 Published by the American Institute of Physics
Articles you may be interested in A high performance field-reversed configuration Physics of Plasmas 22, 056110 (2015); 10.1063/1.4920950 Recent breakthroughs on C-2U: Norman’s legacy AIP Conference Proceedings 1721, 030003 (2016); 10.1063/1.4944019 Review of field-reversed configurations Physics of Plasmas 18, 070501 (2011); 10.1063/1.3613680 Diagnostic suite of the C-2U advanced beam-driven field-reversed configuration plasma experiment Review of Scientific Instruments 87, 11D435 (2016); 10.1063/1.4960730 A new high performance field reversed configuration operating regime in the C-2 device Physics of Plasmas 19, 056108 (2012); 10.1063/1.3694677 Two-color /HeNe laser interferometer for C-2 experiment Review of Scientific Instruments 81, 10D516 (2010); 10.1063/1.3478983
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D836 (2014)
Overview of C-2 field-reversed configuration experiment plasma diagnosticsa) H. Gota,b) M. C. Thompson, M. Tuszewski, and M. W. Binderbauer Tri Alpha Energy, Inc., Rancho Santa Margarita, California 92688, USA
(Presented 3 June 2014; received 31 May 2014; accepted 9 June 2014; published online 4 August 2014) 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. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4884616] I. INTRODUCTION
A field-reversed configuration (FRC) is an extremely high-beta (β = 2μ0 p/Be 2 ∼ 90%; here, p is the average plasma pressure and Be is the external magnetic field) compact toroid (CT) with predominantly poloidal axisymmetric magnetic field and little or no self-generated toroidal magnetic field.1, 2 The FRC consists of a torus of closed-field-lines inside a separatrix and an annular edge layer on the openfield-lines just outside the separatrix. The edge layer coalesces into jets beyond the FRC length, providing a natural divertor, which may allow extraction of energy without restriction. C-2 is a large θ -pinch, CT-merging system, built at Tri Alpha Energy to form relatively high flux, high temperature FRC plasmas.3, 4 Studying aspects of FRC sustainment by neutral-beam (NB) injection and fueling is the main goal of C-2 experiments. The C-2 device, as illustrated in Fig. 1, consists of a central confinement region surrounded by two θ pinch formation sources and two divertors. The C-2 confinement chamber is made of stainless steel that serves as a flux conserver on the timescale of the experiment. The formation tubes are made of quartz. A set of DC-magnets generates a quasi-static axial magnetic field, Bz , throughout the device. The typical field is Bz ∼ 0.1 T in the confinement region with an end-mirror ratio of ∼3.5. There are magnetic mirror plugs in between the formation and divertor sections on each side that can produce a strong magnetic field up to ∼2 T. The mirror plugs play an important role in controlling the open-fieldline plasma confinement. The C-2 device has more than 50 diagnostic systems installed on the confinement vessel, formation sections, and divertor regions to investigate the FRC plasma behavior as well as to characterize the machine operating state. Signals and data from individual diagnostics are transferred to a) Contributed paper, published as part of the Proceedings of the 20th
Topical Conference on High-Temperature Plasma Diagnostics, Atlanta, Georgia, USA, June 2014. b) Author to whom correspondence should be addressed. Electronic mail: [email protected]. 0034-6748/2014/85(11)/11D836/4/$30.00
a data-acquisition system that acquires over 1000 channels on every C-2 discharge. The acquired raw data are generally post-processed into plasma parameters and then stored on databases such as MDS+ and MySQL for further data analysis. On typical C-2 discharges, about half a gigabyte of data is generated after each shot including analysis movies and computations. Fundamental FRC plasma parameters and properties are diagnosed by magnetics, interferometry, Thomson scattering, and spectroscopy on the C-2 confinement vessel midplane. In addition, a bolometry-based tomographic tool, multi-chord Bremsstrahlung/Dα -line monitors, and Mirnov probes provide global FRC characteristics, such as shape, position, and some information on low order magnetohydrodynamic (MHD) instabilities. NB-related fast-ion/neutral diagnostics on C-2 are essential to study the effect of NB injection into FRC plasmas and support our primary goal of NB-driven FRCs. A complete list of C-2 diagnostics is provided in Table I. In this paper, C-2 operating condition and FRC plasmas are described in Sec. II, and the comprehensive diagnostic suite for C-2 FRC plasmas is summarized and described by diagnostic categories in Sec. III.
II. C-2 FRC PLASMAS
In C-2, a typical long-lived FRC plasma state has the following properties: radius ∼0.4 m, length ∼3 m, rigid-rotor poloidal flux up to ∼10 mWb, total temperature (Ti + Te ) up to ∼1 keV, electron density ∼3 × 1019 m−3 , and external axial magnetic field ∼0.1 T. Significant progress has recently been made.5, 6 Stable plasmas with lifetimes up to ∼5 ms have been achieved by combining effects of NB injection and edgeplasma biasing from end-on plasma-guns. We refer to these long-lived stable plasma discharges as the high performance FRC (HPF) regime. The C-2 NBs (20 keV hydrogen, ∼4 MW) are injected tangentially to the FRC current (co-injection), with an average radial impact parameter of 0.19 m which per-
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mits current drive. The fast ions, created primarily by charge exchange, have betatron orbits that add to the FRC azimuthal current, and a sufficiently large fast ion population may improve FRC stability and confinement properties significantly. C-2 FRC parameters, obtained through dynamic formation and CT-merging, are suitable for NB capture (shine-through and first orbit losses < 15%) and for fast ion confinement (slowing down time ∼ FRC lifetime). Two plasma-guns are mounted inside of each divertor and produce a hot (Te ∼ 30– 50 eV, Ti ∼ 100 eV) tenuous (∼1018 m−3 ) plasma stream. The gun also creates an inward radial electric field (Er < 0) that counters the usual FRC spin-up in the ion diamagnetic direction and mitigates the toroidal mode n = 2 rotational instability without applying quadrupole magnetic fields. Hence, NBs are injected into near-axisymmetric FRC discharges. The plasma-gun also produces E × B velocity shear just outside of FRC separatrix, yielding improved FRC confinement properties and stability. Better plasma centering (less n = 1 wobble motion) is also obtained presumably from line-tying to the gun electrodes.
field. For FRC formation in C-2 the typical negative-bias and main-reversal fields are −0.05 T and 0.4 T, respectively. Each fast magnetic signal is passively integrated (RC time constant ∼3 ms) and transferred to a 16 MS/s data-acquisition system. Magnetic probe arrays in the confinement vessel are one of the core C-2 diagnostic systems and provide important information on the FRC global performance such as shape and stability as well as on many key plasma parameters.7 There are 44 internal/external-vacuum B-dot probes mounted on the confinement vessel. Together, these probes measure the threedimensional magnetic fields (Bz , Bθ , Br ) and the excluded-flux profile of the FRC, which approximates the separatrix shape. These B-dot probe signals are processed with active integrator circuits (RC ∼ 100 μs) and then acquired at 0.5 MS/s. In addition, azimuthal arrays of B-dot probes, also known as Mirnov probes, can detect FRC global instabilities with toroidal mode number up to 3. These Mirnov probe signals are acquired directly at 60 MS/s so that they can also detect fast magnetic field fluctuations associated with NB fast particle buildup. Internal magnetic probe arrays were developed and inserted directly into the FRC plasma at the C-2 midplane.8 One of the probe arrays consists of 16 sets of Bz and Bθ , and it covers r = 0–0.45 m (vessel-wall inner-radius ∼0.7 m). Although the internal probe perturbed the FRC performance significantly, the magnetic-field-reversal inside the separatrix was successfully measured and the FRC structure of the merged final state was verified.
III. DIAGNOSTIC SUITE ON C-2
B. Interferometer and polarimeter
FIG. 1. Schematic of the C-2 device, illustrating FRC formation and neutralbeam injection.
A. Magnetic diagnostics
In the two θ -pinch formation sections 17 sets of magnetic probes and flux loops are mounted outside of each quartztube to measure the axial magnetic field and the loop voltage from the corresponding θ -pinch coil straps. This allows for monitoring individual coil discharges to dynamically form and translate FRC/CT plasmoids into the confinement vessel. C-2’s pulsed-power system has ∼100 kJ of the stored energy in each formation section for the main reversal field, and its rise-time is about 4 μs and then crowbarred at the peak of the
Laser and microwave interferometry provides nonperturbing measurements of the plasma electron density. In the C-2 midplane, a 6-channel two-color (CO2 laser at λ ∼ 10.6 μm with a visible He-Ne laser) interferometer system is routinely used to measure line-integrated electron densities along each beam path (impact parameters at r ∼ 3, 15, 20, 25, 30, 35 cm) and to obtain the radial density profiles of the FRC plasma using an Abel inversion technique.9 The Abel inverted density profile evolution of typical HPF plasma discharge is largely flat in the early phase and develops a hollowness of the profile later in the FRC lifetime.
TABLE I. Diagnostic suite on C-2 FRC plasmas. Shape/position North/south formation Bz probes North/south formation flux loops Confinement Bz probes Confinement Bθ probes Mirnov probe arrays Internal B probe arrays Polarimeter CCD end cameras X-ray imaging cameras Bremsstrahlung arrays Video camera
Densities/fluctuation
Temperatures/radiation
Fast ions/neutrals
CO2 interferometer Visible He-Ne interferometer IR He-Ne interferometer Dispersion interferometers Microwave interferometers Cut-off reflectometer Fluctuation reflectometer Fluctuation imaging Triple probe Mach probe Rake probe Gundestrup probe Particle flux probe Plasma potential probe
Thomson scattering Soft X-ray detector Line ratio spectrometer He-jet Mach nozzle Fast Doppler spectrometer High resolution Doppler Survey spectrometers IR spectrometer Impurity line monitors VUV spectrometer He-3 neutron counters Ion energy analyzer Bolometer arrays End-on bolometer
Electrostatic NPA Electromagnetic NPAs Neutral particle bolometer arrays Pyro bolometers SEE detectors ZnS scintillator Plastic scintillators Beam Hα analyzer Modulated H beam Dα monitors Dα /Dβ intensity ratios Pellet CCD camera Fast ionization gauges RGAs
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A far infrared (FIR) laser polarimetry system has recently been developed on C-2, which allows for non-perturbing magnetic field measurements.10 Two FIR laser beams (impact parameters of r ∼ 20 and 30 cm) are set up from the side of the machine, like CO2 interferometer chords, and the system measures the Faraday rotation angles in the FRC plasma. We have successfully estimated the internal toroidal magnetic fields during the FRC lifetime. The FIR laser can also be used as in interferometer mode; it has advantages in low-density regimes because of its long wavelength and can detect a fluctuation along the beam path. C. Thomson scattering system
The C-2 Thomson scattering (TS) system measures the electron temperature of the FRC plasma at 9 spatial points per laser pulse.11 The ruby laser is rated up to 10 J when operated in a single-pulse mode and can produce up to 4 pulses within ∼0.8 ms period. The TS collection optics and polychromators that have 7 spectral channels each were custom designed and built at Tri Alpha Energy. The C-2 TS system can also provide the FRC electron density profile at the same spatial points. The absolute intensity response of the system is calibrated with Rayleigh scattering of argon gas from 0.2 to 5.0 Torr, where the Rayleigh scattering signal is comparable to the Thomson scattering signal at electron densities from 1.6 × 1013 to 4.0 × 1014 cm−3 .
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A survey spectrometer is a useful tool to monitor the impurity spectrum and concentration during C-2 discharges. There are several types of survey spectrometers mounted on C-2; e.g., visible range, highly resolved Dα /Hα region, and IR region. These impurity monitors are very important to understand background impurity levels as well as to evaluate the effect of C-2 vessel wall conditioning such as a titanium gettering and lithium evaporation. A vacuum ultraviolet (VUV) spectrometer has recently been developed and installed on C-2 to measure the composition/intensity of high-energy photons, emitted from the plasma beyond the visible spectrum. This provides key information on what filter set would be best for a bolometry in the soft X-ray region. F. Bolometers
Sets of bolometer arrays are mounted inside of the C-2 confinement vessel to measure the total flux of electromagnetic and neutral particle energy emitted from the plasma. The systems include an extensive array of absolute extreme ultraviolet (AXUV) photodiodes and pyro-electric crystals. The side-on 16-channel AXUV photodiode arrays with pinholes provide spatially resolved measurements, and a total of 144 channels/sensors cover almost the entire FRC volume, which allows for 3D reconstruction. The data set from the bolometers also indicates FRC global motion/instability and estimates the total radiated power from the FRC plasma.
D. Reflectometers
A microwave reflectometry/Doppler Backscattering (DBS) system has been used to measure the toroidal wavenumber spectrum and the radial correlation length of density fluctuations in C-2 FRC plasmas.12 A 6-channel frequency-tunable heterodyne system is coupled to beam optics and a steerable parabolic focusing mirror that launches the beam into the FRC. The DBS system covers plasma densities of (0.8–10) × 1013 cm−3 and provides density fluctuation amplitudes as well as E × B flow measurements in the FRC core and scrape-off-layer (SOL) regions. Fluctuation levels are substantial in the SOL while they are very low in the core, particularly in the HPF regime. A dedicated, fast swept profile reflectometer was also developed for measurements of the edge density profile in the range of (0.3–2.2) × 1013 cm−3 with a maximum time-resolution of 2.5 μs. E. Spectrometers
Two ion Doppler spectroscopy systems13 are used to measure the ion temperature and velocity of C-2 FRC plasmas; (i) the multi-chord ion Doppler (MCID) system can measure 15-chordal views of FRCs with a CMOS camera, and (ii) the single-chord fast-response ion Doppler (FRID) system provides a high temporal resolution using a 16 channel photomultiplier tube (PMT) array. For high temperature FRC core plasmas, an Oxygen-V emission line (λ ∼ 278.1 nm) is typically selected on these spectrometers to estimate the FRCcore ion temperature radial profile and time evolution. For the FRC edge/SOL region a lower excitation energy level of oxygen or other impurity lines are chosen.
G. Electric (Langmuir) probes
Several types of Langmuir probes have been implemented to characterize the open-field-line plasma properties such as SOL temperature, density, and potential as well as jet (plasma exhaust) flow.14 The Langmuir probes include linear arrays of triple probes, a Rake probe (linear array of single-tipped swept probe), a multi-faced Gundestrup probe, a particle-flux probe, an ion-sensitive (baffle) probe, and a combination of Mach/triple probe. They are all interchangeable designs and mounted on linear actuators allowing for programmatic radial scans of the various plasma parameters over the course of several C-2 discharges. H. Neutral-beam related diagnostics
A neutral particle analyzer (NPA) measures the energy distribution of charge exchange neutrals escaping the plasma. In C-2 there are two kinds: electro-magnetic and electro-static NPAs mounted at the same cross-section as the NB injectors. The typical C-2 NBs are 20 keV hydrogen fueled but can be deuterium fueled and modulated for actively measuring a localized charge-exchange target for confined fast protons.15 A neutral particle bolometer (NPB) measures the fastneutral flux resulting from charge-exchanged fast-ions. The custom built NPB array has 16-channel AXUV photodiodes with a ∼40 nm tungsten coating to attenuate photon energies in the visible to extreme ultraviolet range, so that only fastneutral particles will be detected.16 Neutron and proton detectors measure the particles produced from nuclear reactions to help understand fast particle
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confinement in the plasma. This includes slow He-3 counters, fast scintillation detectors mounted just outside the vacuum chamber for neutron detection, and an in-vacuum high-energy proton detector.17 Charge-exchange recombination spectroscopy (CHERS) is used with the modulated NBs to measure the ion temperature and velocity. CHERS provides measurements of the ion energy distribution that are resolved in both space and time. Secondary electron emission (SEE) detectors are mounted on each NB-dump target plate to monitor NB injection performance and coupling to the FRC plasma; typical NB shine-through losses are 10%–15% early in time but glow as the FRC plasma shrinks.
ACKNOWLEDGMENTS
The authors would like to acknowledge the rest of the TAE staff for their dedicated diagnostic system development, maintenance, data analysis, and useful discussions.
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