Helicity Injection Research Articles

Effects of Injected Current Streams on MHD Equilibrium Reconstruction of Local Helicity Injection Plasmas in a Spherical Tokamak

Open field line currents are intrinsic to DC helicity injection plasma startup and pose a challenge for inferring the plasma equilibrium with standard reconstruction analysis. Local helicity injection (LHI) is a type of DC helicity injection which uses small, modular current sources to drive force-free current along helical field lines to produce tokamak plasmas. MHD modeling and magnetic measurements during LHI indicate the injected current streams remain coherent as helical structures on the outboard edge of a core toroidal plasma that is tokamak-like in a toroidally averaged sense. To extract core plasma equilibrium properties, external magnetic diagnostics corrected for contributions from the injected current streams are fitted by a standard Grad-Shafranov equilibrium code. An iterative approach for estimating and subtracting the stream contributions from the diagnostic signals is described and applied to a model equilibrium database to reduce systematic errors introduced by the streams. Convergence is usually attained with 2 to 4 iterations, with derived equilibrium parameters matching the prescribed axisymmetric core values to within estimated experimental uncertainties. Accurate recovery of core parameters occurs when the ratio of the net toroidal windup current from the streams to the core plasma current is less than 0.2, which is typically satisfied in most experiments.

Implementation of an impurity diagnostic suite on the Pegasus-III experiment.

A suite of diagnostics used to assess impurity content and dynamics has been updated, upgraded, and installed on the Pegasus-III Experiment. Typical plasma parameters during local helicity injection start-up are τshot ∼ 10 ms, ne ∼ 1 × 1019 m-3, and Te ∼ 50eV. The deployed diagnostics are compatible with this modest temperature and density regime and provide species identification, source localization, and estimation of radiation losses. Impurity species are determined by recording time-evolving, single line-of-sight spectra at 1.25 kfps using a SPRED (Survey, Poor Resolution, Extended Domain) vacuum ultraviolet spectrometer. SPRED is equipped with 450 g/mm grating, giving a spectral resolution of 0.33nm and a spectral range from ∼10 to 110nm, useful to identify light impurity species in this temperature and density range. An absolutely calibrated spectrometer that collects light from the plasma at Rtan = 15.9cm and Δt ≥ 2ms is used as a visible survey spectrometer and for continuum measurements. The radiated power from the plasma is estimated with a photodiode-based diagnostic. Two 16-channel absolute extreme ultraviolet diode arrays are placed behind pinhole apertures, resulting in 32 lines of sight at Z = 0, with a spatial resolution of 2-3cm and a time response of 60kHz. A photometrically calibrated collinear Dα/near infrared filtered photodiode-based system measures the Dα emission and around 1040nm. All these instruments have been designed to suppress electromagnetic interference from megawatt-class switching power supplies.

Magnetic helicity evolution during active region emergence and subsequent flare productivity

Aims. Solar active regions (ARs), which are formed by flux emergence, serve as the primary sources of solar eruptions. However, the specific physical mechanism that governs the emergence process and its relationship with flare productivity remains to be thoroughly understood. Methods. We examined 136 emerging ARs, focusing on the evolution of their magnetic helicity and magnetic energy during the emergence phase. Based on the relation between helicity accumulation and magnetic flux evolution, we categorized the samples and investigated their flare productivity. Results. The emerging ARs we studied can be categorized into three types, Type-I, Type-II, and Type-III, and they account for 52.2%, 25%, and 22.8% of the total number in our sample, respectively. Type-I ARs exhibit a synchronous increase in both the magnetic flux and magnetic helicity, while the magnetic helicity in Type-II ARs displays a lag in increasing behind the magnetic flux. Type-III ARs show obvious helicity injections of opposite signs. Significantly, 90% of the flare-productive ARs (flare index ≥ 6) were identified as Type-I ARs, suggesting that this type of AR has a higher potential to become flare productive. In contrast, Type-II and Type-III ARs exhibited a low and moderate likelihood of becoming active, respectively. Our statistical analysis also revealed that Type-I ARs accumulate more magnetic helicity and energy, far beyond what is found in Type-II and Type-III ARs. Moreover, we observed that flare-productive ARs consistently accumulate a significant amount of helicity and energy during their emergence phase. Conclusions. These findings provide valuable insight into the flux emergence phenomena, offering promising possibilities for early-stage predictions of solar eruptions.

Formation of Polar Crown Filament Magnetic Fields by Supergranular Helicity Injection

To understand the magnetic fields of the polar crown filaments (PCFs) at high latitudes near polar regions of the Sun, we perform magnetofrictional numerical simulations on the long-term magnetic evolution of bipolar fields with roughly east–west polarity inversion lines (PILs) in a 3D spherical wedge domain near polar regions. The Coriolis-effect-induced vortical motions at the boundaries of several supergranular cells inject magnetic helicity from the photospheric boundary into the solar atmosphere. Supergranular-scale helicity injection, transfer, and condensation produce strongly sheared magnetic fields. Magnetic reconnections at footpoints of the sheared fields produce magnetic flux ropes (MFRs) with helicity signs consistent with the observed hemispheric helicity rule. The cross-sectional area of MFRs exhibits an uneven distribution, resembling a “foot-node-foot” periodic configuration. Experiments with different tilt directions of PILs indicate that the PCFs preferably form along PILs with the western end close to the polar region. The bending of PILs caused by supergranular flows, forming S-shape (Z-shape) PIL segments, promotes the formation of dextral (sinistral) MFRs. The realistic magnetic models we obtained can serve as starting points for the study of the plasma formation and eruption of PCFs.

Formation of Fan-spine Magnetic Topology through Flux Emergence and Subsequent Jet Production

Fan-spine magnetic structure, as a fundamental three-dimensional topology in magnetic reconnection theory, plays a crucial role in producing solar jets. However, how fan-spine configurations form in the solar atmosphere remains elusive. Using the Chinese Hα Solar Explorer (CHASE) and the Solar Dynamics Observatory, we present a case study on the complete buildup of fan-spine topology driven by flux emergence and the subsequent jet production. Two fan-spine structures and the two associated null points are present. Variations in null-point heights and locations were tracked over time during flux emergence. The north fan-spine structure is found to be created through magnetic reconnection between the newly emerged flux and the background field. Gentle reconnection persistently occurs after formation of the north fan-spine structure, resulting in weak plasma outflows. Subsequently, as flux emergence and magnetic helicity injection continue, the formation and eruption of minifilaments after reconnection at the quasi-separatrix layer between the two nulls trigger three homologous jets. The CHASE observations reveal that the circular flare ribbon, inner bright patch, and remote brightening all exhibit redshifted signatures during these jet ejections. This work unveils the key role of flux emergence in the formation of fan-spine topology, and highlights the importance of minifilaments for subsequent jet production.

Filament Eruption from Active Region 13283 Leading to a Fast Halo-CME and an Intense Geomagnetic Storm on 2023 April 23

Using multi-instrument and multiwavelength observations, we studied a coronal mass ejection (CME) that led to an intense geomagnetic storm on 2023 April 23. The eruption occurred on April 21 in solar active region (AR) 13283 near the disk center. The AR was in its decay stage, with fragmented polarities and a preexisting long filament channel a few days before the eruption. The study of the magnetic field evolution suggests that the flux rope (filament) was built up by monotonous helicity accumulation over several days, and furthermore, converging and canceling fluxes led to a change in helicity injection, resulting in an unstable nature of the magnetic flux rope (MFR) and its further eruption. Importantly, the CME morphology revealed that the MFR apex underwent a rotation of up to 56° in clockwise direction owing to its positive helicity. The CME decelerates in the field of view (FOV) of the Large Angle Spectrometric Coronagraph and has a plane-of-sky velocity of 1226 km s−1 at 20 R ⊙. In the FOV of the Heliospheric Imager, the lateral expansion of the CME is tracked better than the earthward motion. This implies that the arrival time is difficult to assess. The in situ arrival of the interplanetary CME shock was at 07:30 UT on April 23, and a geomagnetic storm commenced at 08:30 UT. The flux rope fitting to the in situ magnetic field observations reveals that the magnetic cloud flux rope orientation is consistent with its near-Sun orientation, which has a strong negative Bz-component. The analysis of this study indicates that the near-Sun rotation of the filament during its eruption to the CME is the key to the negative Bz-component and consequently the intense geomagnetic storm.

Demonstration of transient CHI startup using a floating biased electrode configuration

Results from the successful solenoid-free plasma startup using the method of transient coaxial helicity injection (transient CHI) in the QUEST spherical tokamak (ST) are reported. Unlike previous applications of CHI on HIT-II and on NSTX which required two toroidal insulating breaks to the vacuum vessel, QUEST uses a first of its kind, floating single biased electrode configuration, which does not use such a vacuum break. Instead, the CHI electrode is simply insulated from the outer lower divertor plate support structure. This configuration is much more suitable for implementation in a fusion reactor than the previous configurations. Transient CHI generated toroidal currents of 135 kA were obtained. The toroidal current during the formation of a closed flux configuration was over 50 kA. These results bode well for the application of transient CHI in a new generation of compact high-field STs and tokamaks in which the space for the central solenoid is very restricted.

A flow control method for the leakage reduction of annular gas seals using the helical gas curtain generator

Advanced sealing technology is crucial for turbine efficiency. To reduce seal leakage, the flow control method based on gas injection is an attractive solution. The traditional flow control methods primarily focus on blocking the axial leakage flow, neglecting the helical flow effect of the leakage flow at the design stage. In this paper, a novel flow control method is proposed for the leakage reduction of annular gas seals using the helical gas curtain generator (HGCG). The helical flow effect of the leakage flow is first considered for further sealing enhancement. The design conception of the HGCG involves connecting seal cavities and teeth tips using the specially designed pad with helical injection slots. Driven by the intrinsic pressure difference between the connected areas, helical gas curtains are passively generated, producing multi-row fluidic barriers against the helical leakage flow at each teeth tip. Numerical investigation of a labyrinth seal with HGCGs shows that the proposed method effectively enforces flow resistance at teeth tips, resulting in an extra dissipation effect. The performance of the HGCG is enhanced as the helical flow effect increases. The maximum leakage reduction ratio reaches 59.2%, over eight times higher than that of the traditional air curtain technology (ACT). In addition, the HGCG increases the effective damping coefficient by 45% compared to the ACT. The proposed HGCG presents significant superiority in sealing improvement and rotordynamic stability enhancement.

A versatile power supply system for the central solenoid of the QUEST spherical tokamak

Here, a versatile power supply system, consisting of two current sources, insulated-gate bipolar transistor stacks, and a capacitor bank, is proposed for application in various operations of the QUEST spherical tokamak. The central solenoid coil in this system can carrying a maximum current and voltage of ±8 kA and ±375 V, respectively. In plasma start-up experiments based on ohmic heating through double flux swing, a plasma current of over 100 kA is stably obtained. Moreover, the feedback control of the thyristor power supply is utilized to control the plasma current, plasma shape, and position. The oscillating central-solenoid current applies a bipolar loop voltage at constant time intervals, and the capacitor bank discharge facilitates a co-axial helicity injection as well as amplifies the toroidal current.

Temporal Behaviors of Magnetic Helicity Injections by Self and Mutual Sunspot Rotations

Magnetic helicity is a physical parameter used to quantify the complexity of magnetic fields, providing an indication of the energy state in the coronal magnetic structure. We investigate the temporal evolution of magnetic helicity and its relationship to the occurrence of a variety of flares in the solar active region NOAA 12297, which was well observed using the Solar Dynamics Observatory/Helioseismic and Magnetic Imager in 2015 March. The active region produced many M-class flares and an X-class flare in two distinctive areas, both of which had a similar magnetic evolution, i.e., the opposite polarity of an emerging flux developed beside a preexisting sunspot, but exhibited flares with different magnitudes and frequencies. We derived the spatiotemporal evolution of the magnetic helicity injections and evaluated how spinning and braiding helicity injections evolved with time in the two areas. In one area, we observed a remarkable evolution, in which a negative spinning helicity injection in the preexisting sunspot increased in a positive helicity system, followed by the occurrence of the X-class flare. The negative helicity injection was clearly caused by the flux emergence that developed along the outer edge of the preexisting sunspot. The other area showed positive braiding helicity injections, with spinning helicity injections fluctuating concurrently with flux emergence, changing their signs several times, i.e., variable energy, and helicity input. The observed temporal behaviors of the helicity injections may explain different types of flare occurrences in the regions.

Nonconservative Cascades in a Shell Model of Turbulence

Developed turbulent flows in which the intervention of external forces is fundamentally important at scales where the inertial range should exist are quite common. Then the cascade processes are not conservative any more and, therefore, it is necessary to adequately describe the external forces acting in the whole range of scales. If the work of these forces has a power law scaling, then one can assume that the integral of motion changes and the preserving value becomes a quadratic quantity, which includes the dependence on the scale. We develop this idea within the framework of shell models of turbulence. We show that, in terms of nonconservative cascades, one can describe various situations, including (as a particular case) the Obukhov – Bolgiano scaling proposed for turbulence in a stratified medium and for helical turbulence with a helicity injection distributed along the spectrum.

Digital Control and Power Systems for the Pegasus-III Experiment

The PEGASUS-III experiment is a solenoid-free, low aspect ratio spherical tokamak that will serve as a dedicated U.S. platform for comparative nonsolenoidal tokamak plasma startup studies. Approximately 175 megavolt-ampere (MVA) of reconfigured and expanded programmable power systems, 7 MJ of new stored energy, and new digital control and protection systems for the facility are being commissioned to support PEGASUS-III upgrades. These include: increased toroidal field (0.15–0.6 T); new divertor and poloidal field coils; increased pulse length; local and coaxial helicity injectors for solenoid-free plasma initiation; radio frequency (RF) systems for heating and current drive; and a diagnostic neutral beam (DNB). A new real-time digital control system implements 16 proportional-integral differential (PID) feedback controllers with 25 kHz loop rates to control the electromagnets and helicity injectors. The poloidal field coils, helicity injector arc currents, and toroidal field are driven by 36 3.6 MVA (4 kA, 900 V) insulated-gate bipolar transistor (IGBT) buck converters. Helicity injector bias voltage and current will be provided by a set of four 10.8 MVA multi-level buck converters (MLBCs). Each is comprised of an 1800 V integrated gate-commutated thyristor (IGCT) stage and a ±900 V IGBT stage in series, providing controllable <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I_{\mathrm {inj}}\le 4$ </tex-math></inline-formula> kA at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\mathrm {inj}}\le 2.7$ </tex-math></inline-formula> kV. A field programmable gate array (FPGA)-based digital fault protection system multiplexes controller commands to individual power semiconductors in these supplies, monitors their operational status, and executes shutdown sequences within <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10~\mu \text{s}$ </tex-math></inline-formula> of fault detection. An 80 kV, 4 A zero-voltage-switching (ZVS) resonant converter with < 1% output ripple is under development for the DNB and is being evaluated as a topology to drive RF sources.

Design Considerations for the Implementation of a High-Field-Side Transient CHI System on QUEST

Transient coaxial helicity injection (T-CHI), a method first developed on the small helicity injected torus (HIT-II) experiment and then validated on the much larger National Spherical Torus Experiment (NSTX) device, is a method to initiate an inductive-like tokamak plasma discharge without reliance on the central solenoid. A CHI discharge is initiated by driving current along magnetic flux that connects the inner and outer divertor plates on one end of the tokamak. To permit this, on both HIT-II and NSTX, toroidal ceramic insulators were used to electrically separate the inner and outer vessel components. The use of such large toroidal vacuum insulators may not be easy to implement in reactors. To address this issue, the Q-shu University experiments with steady-state spherical tokamak (ST) (QUEST) is developing a reactor-relevant CHI configuration in which one of the divertor plates is electrically insulated from the rest of the vessel. The first application of T-CHI on QUEST biased the CHI electrode to the outer vessel. While the CHI discharges could be easily generated, it was found that as the discharge filled the vessel, the separation distance between the injector magnetic flux footprints widened, a condition that is not favorable for the generation of closed flux surfaces. Biasing the electrode to the inner wall is a configuration similar to that used on NSTX and HIT-II, but initial testing in this configuration has proved to be challenging. The design described here overcomes the present limitation by locating the CHI electrode much closer to the CHI injector flux coil and using an NSTX-like gas injection manifold to enable high-field-side T-CHI startup on QUEST. The concepts described in this article should also benefit the future implementation of T-CHI systems in other tokamaks and spherical tokamaks.

The New PEGASUS-III Experiment

Developing attractive means of initiating current without using magnetic induction from a central solenoid is a critical scientific and technical challenge facing the spherical tokamak (ST). The PEGASUS program has focused on developing the physics basis and predictive models for nonsolenoidal tokamak startup using local helicity injection (LHI) and has demonstrated startup to ~0.2 MA in a low-field, near-unity aspect ratio ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$A$ </tex-math></inline-formula> ) ST. The PEGASUS facility is being upgraded into a solenoid-free ST called PEGASUS-III. Major features include: increased toroidal field (TF) to 0.6 T for up to 100 ms, improved shape control, and retaining the low-aspect ratio geometry of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$A \sim 1.2$ </tex-math></inline-formula> . This TF directly supports the new mission to expand the breadth of solenoid-free research on the facility with multiple reactor-relevant techniques. The implemented TF upgrade is comprised of a new center rod, outer TF coil system, torque plate assemblies, and TF interconnects between the center rod and outer conductors. The center rod is comprised of 24 water-cooled, insulated wedge conductors inside a new inner vacuum wall. There is no ohmic solenoid, which allows additional TF conductor for the 48-kA/turn TF current. The outer TF conductors are 12 pairs of air-cooled, reinforced Al plate conductors. Pairwise crossed conductor links connect the outer C-plates to the central rod conductor to eliminate the need for a toroidal compensation wind-back coil. Torque plate assemblies on top and bottom mechanically secure both the outer C-conductor and the interconnecting links, counteract torsional magnetic loads during a pulse, and provide compliance for vertical displacement. The assembly accommodates magnetic and thermal forces, limiting the axial excursion of the central assembly to <1 mm.

A Comparison of Global Magnetofrictional Simulations of the 2015 March 20 Solar Eclipse

The solar corona exhibits a wide range of phenomena, from highly non-potential objects such as solar filaments to near-potential structures such as the open magnetic flux. For any global model to be useful in space weather applications, the model must on a single day reproduce all of these phenomena in the same simulation, using the same set of coronal physics and parameters. The purpose of the present paper is to evaluate whether the evolving magnetofrictional model can achieve this goal. Twenty-eight separate simulations are analyzed, where each tries to reproduce both highly non-potential and near-potential phenomena observed in the solar corona on the same day. This day is chosen to be 2015 March 20, the date of the solar eclipse. The study evaluates how the cadence of bipole data, ideal or nonideal coronal physics, and the variety of helicity injection mechanisms affect the accuracy of the simulations. Results show that significantly better agreement arises when using Advective Flux Transport (AFT) synoptic maps to drive the simulations, as compared to 27-day Carrington rotation synoptic maps. Using the nonideal effect of hyperdiffusion leads to the worst agreement with all coronal phenomena. Alternatively, when running either ideal or ohmic diffusion simulations with helicity condensation or bipoles with a self-helicity, a good agreement with both on-disk and limb structures can be found. This suggests that future studies aiming to simulate the corona and reproduce multiple phenomena on a given day should use data products such as AFT and avoid using the nonideal physics of hyperdiffusion.

Transient CHI System Design Studies for Pegasus-III

Transient coaxial helicity injection (transient CHI), first developed on the Helicity Injected Torus-II (HIT-II) and later on the National Spherical Torus Experiment (NSTX) for implementing solenoid-free plasma current startup capability in a spherical tokamak (ST), is now planned to be tested on the Pegasus-III ST using a novel double-biased configuration. Such a configuration is likely needed for transient CHI deployment in a reactor. The transient CHI system optimization will be studied on Pegasus-III to enable startup toroidal persisting currents at the limits permitted by the external poloidal field coils. A transient CHI discharge is generated by driving injector current along magnetic field lines that connect the inner and outer divertor plates on one end of the ST. Simulations using the Tokamak Simulation Code are used to assess the transient CHI toroidal current generation potential and electrode gap location on the Pegasus-III. While past transient CHI systems have used high-voltage, oil-filled capacitors for driving the injector current, for improved safety, Pegasus-III will use a high-current capacitor bank based on low-voltage electrolytic capacitors. The designed and fabricated system is capable of over 32 kA. The modular design features permit the system to be upgraded to higher currents, as needed, to meet the future needs of the Pegasus-III facility.

Nature of helicity injection in non-erupting solar active regions

ABSTRACT Using time-sequence vector magnetic field and coronal observations from Solar Dynamics Observatory, we report the observations of the magnetic field evolution and coronal activity in four emerging active regions (ARs). The ARs emerge with leading polarity being the same as for the majority of ARs in a hemisphere of solar cycle 24. After emergence, the magnetic polarities separate each other without building a sheared polarity inversion line. In all four ARs, the magnetic fields are driven by foot point motions such that the sign of the helicity injection (dH/dt) in the first half of the evolution is changed to the opposite sign in the later part of the observation time. This successive injection of opposite helicity is also consistent with the sign of mean force–free twist parameter (αav). Further, the EUV light curves off the ARs in 94 Å and GOES X-ray flux reveal flaring activity below C-class magnitude. Importantly, the white-light coronagraph images in conjunction with the AR images in Atmospheric Imaging Assembly (AIA) 94 Å delineate the absence of associated Coronal Mass ejections (CMEs) with the studied ARs. These observations imply that the ARs with successive injection of opposite sign magnetic helicity are not favourable to twisted flux rope formation with excess coronal helicity, and therefore are unable to launch CMEs, according to recent reports. This study provides the characteristics of helicity flux evolution in the ARs referring to the conservative property of magnetic helicity and more such studies would help to quantify the eruptive capability of a given AR.

Formation of Quiescent Prominence Magnetic Fields by Supergranulations

To understand the formation of quiescent solar prominences, the origin of their magnetic field structures, i.e., magnetic flux ropes (MFRs), must be revealed. We use three-dimensional magnetofriction simulations in a spherical subdomain to investigate the role of typical supergranular motions in the long-term formation of a prominence magnetic field. Time-dependent horizontal supergranular motions with and without the effect of Coriolis force are simulated on the solar surface via Voronoi tessellation. The vortical motions by the Coriolis effect at boundaries of supergranules inject magnetic helicity into the corona. The helicity is transferred and accumulated along the polarity inversion line (PIL) as a strongly sheared magnetic field via helicity condensation. The diverging motions of supergranules converge opposite magnetic polarities at the PIL and drive the magnetic reconnection between footpoints of the sheared magnetic arcades to form an MFR. The magnetic network, negative-helicity MFR in the northern hemisphere, and fragmented-to-continuous formation process of magnetic dip regions are in agreement with observations. Although diverging supergranulations, differential rotation, and meridional flows are included, the simulation without the Coriolis effect cannot produce an MFR or sheared arcades to host a prominence. Therefore, Coriolis force is a key factor for helicity injection and the formation of magnetic structures of quiescent solar prominences.

Magnetic helicity and energy of emerging solar active regions and their erruptivity

Aims.We investigate the role of the accumulation of magnetic helicity and magnetic energy in the generation of coronal mass ejections (CMEs) from emerging solar active regions (ARs).Methods.Using vector magnetic field data obtained by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we calculated the magnetic helicity and magnetic energy injection rates as well as the resulting accumulated budgets in 52 emerging ARs from the start time of magnetic flux emergence until they reached a heliographic longitude of 45° West (W45).Results.Seven of the ARs produced CMEs, but 45 did not. In a statistical sense, the eruptive ARs accumulate larger budgets of magnetic helicity and energy than the noneruptive ARs over intervals that start from the flux emergence start time and end (i) at the end of the flux emergence phase and (ii) when the AR produces its first CME or crosses W45, whichever occurs first. We found magnetic helicity and energy thresholds of 9 × 1041Mx2and 2 × 1032erg. When these thresholds were crossed, ARs are likely to erupt. In terms of accumulated magnetic helicity and energy budgets, the segregation of the eruptive from the noneruptive ARs is violated in one case when an AR erupts early in its emergence phase and in six cases in which noneruptive ARs exhibit large magnetic helicity and energy budgets. Decay index calculations may indicate that these ARs did not erupt because the overlying magnetic field provided a stronger or more extended confinement than in eruptive ARs.Conclusions.Our results indicate that emerging ARs tend to produce CMEs when they accumulate significant budgets of both magnetic helicity and energy. Any study of their eruptive potential should consider magnetic helicity together with magnetic energy.

Effect of injected flux and current temporal phasing on self-organization in the HIT-SI3 experiment

The HIT-SI3 device at the University of Washington uses three oscillating inductive helicity injectors to form and sustain spheromak plasma equilibria. By adjusting the temporal phase of the injector waveforms with respect to each other, the toroidal spectrum of the imposed perturbations can be controlled. Using a recently implemented GPU-based control system, the available mode spectra were explored experimentally by scanning the space of relative injector phasing. In this space, significant variation in the toroidal mode spectrum (n = 1, 2, 3) of the perturbations was observed. Additionally, variation in characteristics of driven equilibria was also observed, including a ≈30% range in toroidal current gain (Iϕ/IInj). Experimental results are compared with both a composite-equilibria and nonlinear dynamic model, including extended MHD simulations using the NIMROD code and composite Taylor state equilibria computed using the PSI-Tet code. Qualitative agreement is seen with the nonlinear models, but not with composite-equilibria models, suggesting the use of nonlinear models to better capture observed plasma dynamics and provide predictive use for future experiments.