Internal Reconnection Events Research Articles

Precise measurements of self-absorbed rising reverse shock emission from gamma-ray burst 221009A

The deaths of massive stars are sometimes accompanied by the launch of highly relativistic and collimated jets. If the jet is pointed towards Earth, we observe a ‘prompt’ gamma-ray burst due to internal shocks or magnetic reconnection events within the jet, followed by a long-lived broadband synchrotron afterglow as the jet interacts with the circumburst material. While there is solid observational evidence that emission from multiple shocks contributes to the afterglow signature, detailed studies of the reverse shock, which travels back into the explosion ejecta, are hampered by a lack of early-time observations, particularly in the radio band. We present rapid follow-up radio observations of the exceptionally bright gamma-ray burst GRB 221009A that reveal in detail, both temporally and in frequency space, an optically thick rising component from the reverse shock. From this, we are able to constrain the size, Lorentz factor and internal energy of the outflow while providing accurate predictions for the location of the peak frequency of the reverse shock in the first few hours after the burst. These observations challenge standard gamma-ray burst models describing reverse shock emission.

Observation of MHD-correlated blobs during internal reconnection events in VEST

Internal reconnection events (IREs), one of the relaxation events driven by internal magnetohydrodynamic (MHD) instabilities in fusion plasmas, are accompanied by a strongly MHD-correlated blob at the edge in the Versatile Experiment Spherical Torus spherical tokamak. The MHD-correlated blob plays a significant role in the onset and the strength of IREs. Various techniques analyzing visible camera images show correlated waveforms between blobs and magnetic fluctuations, and they produce visualized images of corotating structures of the MHD modes and the MHD-correlated blobs. In the images, a phase drag in the rotations of the two structures initially appears and vanishes on the verge of IREs. IREs maintaining the phase drag, however, leads to a less violent impact in terms of current decrease and magnetic field bursting. In addition, the MHD-correlated blobs are followed by the increasing degree of nonlinear interactions between the internal MHD mode and high-frequency broadband fluctuations (>60 kHz) at the edge. These results suggest that boundary plasmas can impact internally driven relaxation events via MHD-correlated edge phenomena.

Acceleration of ion rotation during internal reconnection events in the versatile experiment spherical torus (VEST)

Acceleration of ion rotation is observed during internal reconnection events (IREs) in the versatile experiment spherical torus. Two IRE discharges with opposite torques, i.e. acceleration or deceleration of ohmic plasmas with intrinsic, counter-I p rotation, can be generated using different wall conditionings method. When an IRE occurs, acceleration and deceleration of impurity ion rotation as well as well-known ion heating are observed globally via ion Doppler spectroscopy with multiple channels including a single channel with a high temporal resolution of 0.2 ms. Interestingly, ion heating is observed earlier than the ion rotation acceleration, indicating that this phenomenon has a different mechanism from usual magnetic reconnection. We present several possible mechanisms for the rotation acceleration. For several reasons, the ions are thought to be accelerated by a neoclassical toroidal viscosity (NTV) torque rather than mechanisms such as the reconnection outflow and toroidal electric field induced by the current profile change. A simple 0D momentum balance model with NTV torque in the 1/ν collisionality regime agrees well with the experimental results. Furthermore, a positive correlation between ion temperature and plasma rotation is identified, whereby the NTV torque S NTV and ion temperature T i are related via .

Radial profile measurement with an improved 1 kHz Thomson scattering system on Versatile Experiment Spherical Torus.

A Thomson scattering (TS) system has been utilized to measure the electron temperature and density of the core region of Versatile Experiment Spherical Torus (VEST). Recently, the laser injection system is successfully upgraded adopting the burst laser with the repetition rate of 1 kHz and the energy of 2J. Furthermore, improved collection optics with additional polychromators and a 32-channel fast digitizer are prepared to observe the fast time evolution of radial profiles. This improvement is essential to study fast phenomena such as internal reconnection event (IRE). We increase the TS signal and reduce the stray light by introducing new filters having better optical properties such as high optical density at 1064nm, transmission, and reflectance. Moreover, the optimum reverse bias voltages are newly set to make the system operational independent of the background radiation. As a result, 1 kHz radial profiles of the core electron temperature and density are measured for the first time, showing characteristics of IREs in VEST.

Measurements of edge plasma parameters during internal reconnection events in the TST-2 spherical tokamak

Measurements of edge plasma parameters such as current density, electron density, and electron temperature were performed during internal reconnection events in TST-2 Ohmic plasmas. The measured current density consists of two components: a slowly varying component and a spiky bipolar component. The magnitude of the slowly varying component is comparable to the mean current density averaged over the poloidal cross section, and it seems to represent the global transport from the core to the edge. The spiky bipolar component is about an order of magnitude larger than the slowly varying component, but the spatial structure seems to be localized and its effect on plasma confinement is not catastrophic.

Nonlinear Energy Cascading in Turbulence during the Internal Reconnection Event at the Sino-United Spherical Tokamak**Supported by the National Basic Research Program of China under Grant No 2012CB934303, the Natural Science Foundation of Shanghai under Grant No 15ZR1403300 and the National Natural Science Foundation of China under Grant No 11275051.

The characteristics of the energy transfer and nonlinear coupling among edge electromagnetic turbulence in thermal quench sub-period of the internal reconnection event (IRE) are studied at the sino-united spherical tokamak device using multiple Langmuir and magnetic probe arrays. The wavelet bispectral analysis and the modified Kim method are applied to investigate linear growth/damping and nonlinear energy transfer rates, along with multi-field turbulence interactions. The results show a multi-field nonlinear energy transfer from electrostatic to magnetic turbulence that results in two-mode coupling in magnetic turbulence, which may play a crucial role to trigger the IRE.

Investigation of Some MHD Events in the SUNIST Spherical Tokamak

Internal reconnection event (IRE), which is characterized by a perturbation in plasma current Ip, loop voltage Vloop, Hα radiation and magnetic perturbation dBp, was observed in the SUNIST experiment. It is found that, the fluctuation before IREs is characterized by a structure of m = 2/n = 1, then changes to m = 4/n = 1 during the IREs; and, after IREs, the mode number changes to m = 3/n = 1. An analysis in the evolution of equilibrium parameters during IREs shows that a positive spike appears in the evolution of the plasma's elongation and a negative spike in the poloidal beta of plasma. A collapse in the pressure profile, corresponding to the occurrence of IREs, is also found.

Comment on ‘Plasma current spikes due to internal reconnection during tokamak disruptions’

In tokamak disruptions, a brief increase in the total plasma current—an Ip spike—frequently precedes the current quench. The Ip spike is commonly attributed to an internal reconnection event that lowers the internal inductance. Plasma current spikes seen in four fixed-boundary MHD disruption simulations support this explanation and are discussed in terms of the evolution of poloidal flux and poloidal magnetic energy.

Probes for measuring fluctuation-induced Maxwell and Reynolds stresses in the edge of the Madison Symmetric Torus reversed field pinch

Several probes have been constructed to measure fluctuation-induced Maxwell and Reynolds stresses in the edge of the Madison Symmetric Torus reversed field pinch (RFP). The magnetic probe is composed of six magnetic pickup coil triplets. The triplets are separated spatially, which allows for local measurements of the Maxwell stress. To measure the plasma flow components for evaluation of the Reynolds stress, we employ a combination of an optical probe [Kuritsyn et al., Rev. Sci. Indrum. 77, 10F112 (2006)] and a Mach probe. The optical probe measures the radial ion flow locally using Doppler spectroscopy. The Mach probe consists of four current collectors biased negatively with respect to a reference tip and allows for measurements of the poloidal and toroidal components of the bulk plasma flow. The stresses are observed to play an important role in the momentum balance in the RFP edge during internal reconnection events.

Temporal Evolution of the Pressure Profile and Mode Behavior during Internal Reconnection Events in the MAST Spherical Tokamak

Mode behaviors including non-linear development and pressure profiles during internal reconnection events are clarified in the MAST spherical tokamak. In a q0 > 1 discharge, the tearing mode is a trigger; however, non-linearity of modes is not confirmed. On the other hand, in a q0 < 1 discharge, the harmonics of a m/n = 4/1 mode of ∼22 kHz are confirmed. Method for identification of poloidal mode in ST configuration is also given.

Ion heating during magnetic relaxation in the helicity injected torus-II experiment

Ion doppler spectroscopy (IDS) is applied to the helicity injected torus (HIT-II) spherical torus to measure impurity ion temperature and flows. [A. J. Redd et al., Phys. Plasmas 9, 2006 (2002)] The IDS instrument employs a 16-channel photomultiplier and can track temperature and velocity continuously through a discharge. Data for the coaxial helicity injection (CHI), transformer, and combined current drive configurations are presented. Ion temperatures for transformer-driven discharges are typically equal to or somewhat lower than electron temperatures measured by Thomson scattering. Internal reconnection events in transformer-driven discharges cause rapid ion heating. The CHI discharges exhibit anomalously high ion temperatures &amp;gt;250eV, which are an order of magnitude higher than Thomson measurements, indicating ion heating through magnetic relaxation. The CHI discharges that exhibit current and poloidal flux buildup after bubble burst show sustained ion heating during current drive.

Ion temperature increase during MHD events on the TST-2 spherical tokamak

Various types of MHD events including internal reconnection events are studied on the TST-2 spherical tokamak. In weak MHD events no positive current spike was observed, but in strong MHD events with positive current spikes, a rapid and significant impurity ion temperature increase was observed. The decrease in the poloidal magnetic energy is the most probable energy source for ion heating. The plasma current shows a stepwise change. The magnitude of this step correlates with the temperature increase and is found to be a good indicator of the strength of each event.

Nonlinear simulation studies of tokamaks and STs

The multilevel physics, massively parallel plasma simulation code, M3D has been used to study spherical toris (STs) and tokamaks. The magnitude of outboard shift of density profiles relative to electron temperature profiles seen in NSTX under strong toroidal flow is explained. Internal reconnection events in ST discharges can be classified depending on the crash mechanism, just as in tokamak discharges; a sawtooth crash, disruption due to stochasticity, or high-β disruption. Toroidal shear flow can reduce linear growth of internal kink. It has a strong stabilizing effect nonlinearly and causes mode saturation if its profile is maintained, e.g. through a fast momentum source. Normally, however, the flow profile itself flattens during the reconnection process, allowing a complete reconnection to occur. In some cases, the maximum density and pressure spontaneously occur inside the island and cause mode saturation. Gyrokinetic hot particle/MHD hybrid studies of NSTX show the effects of fluid compression on a fast-ion driven n = 1 mode. MHD studies of recent tokamak experiments with a central current hole indicate that the current clamping is due to sawtooth-like crashes, but with n = 0.

Phenomenology of internal reconnections in the National Spherical Torus Experiment

The behavior of large scale magnetohydrodynamic modes was investigated in the National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)] during reconnection events using combined analysis of magnetic probe signals, and soft x-ray data. The comparison of mode dynamics during precursor and disruption stages in T-11M (V. S. Vlasenkov et al., in Proceedings of the 6th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Berchtesgaden, 6–13 October 1976, p. 95) (small circular plasma), Tokamak Fusion Test Reactor [R. J. Hawryluk et al., Plasma Phys. Controlled Fusion 33, 1509 (1991)] (large circular plasma), and NSTX (large spherical plasma) was done. The analysis shows that the sequence of events of minor and major internal reconnection events (IRE) in NSTX is essentially similar to that for disruptions in moderate aspect ratio tokamaks. The main feature of disruption dynamics apparently affected by the small aspect ratio in NSTX appears in the relatively slow thermal quench event (5–10 times longer compared to ordinary tokamaks), which precedes the major IRE. The coincidence of the electron and neutron quench times during the major IRE leads us to the conclusion that the fast ions and hot electrons leave the center of a plasma column simultaneously, i.e., convectively.

Ion Acceleration during Reconnection in MAST

Spontaneous acceleration of ions to suprathermal energies is observed during magnetic reconnection in the Mega-Ampere Spherical Tokamak (MAST). A high-energy tail is observed in the ion-distribution function following each internal reconnection event in Ohmic discharges. This phenomenon is explained in terms of runaway ion acceleration in the electric field induced by the reconnection.

Excitation of axisymmetric Alfvénic modes in Ohmic tokamak discharges

Magnetohydrodynamic (MHD)mode activity in the Alfvén frequency range has been detected in the absence of energetic ions during discharges in several conventional tokamaks and spherical tokamaks, including the Tokamak Fusion Test Reactor (TFTR) and the Mega-Amp Spherical Tokamak (MAST). In TFTR the dominant toroidal mode number n was found to be zero; this is also the case in MAST discharges for which mode number information is available. The observed properties of these modes are shown to be consistent with global Alfvén eigenmodes (GAEs). Although they appear to have little or no effect on plasma performance in present-day devices, the fact that they are frequently observed in MAST Ohmic discharges suggests that they could be used as a diagnostic of plasma equilibrium parameters. In principle, they couldalso provide the basis for a plasma heating scheme. A possible mechanism for the excitation of the Alfvén eigenmodes in the absence of fast ions is suggestedby two-fluid simulations of various tokamaks, in which high-frequency mode activity is found to be correlated with relatively long-timescale MHD events in the plasma, such as internal reconnection events (IREs) or edge localized modes (ELMs). A simple analytical model describing the excitation of Alfvénicmodes by either IREs or ELMs is proposed. The coupling of low- and high-frequency MHD is predicted to be strongest for radially-extended modes: this is consistent with the low mode numbers of the activity observed in TFTR and MAST.

Initial results from the TST-2 spherical tokamak

A new spherical tokamak, TST-2, was constructed at the University of Tokyo and started operation in September 1999. Reliable plasma initiation is achieved with typically 1 kW of ECH power at 2.45 GHz. Plasma currents of up to 90 kA andtoroidal fields of up to 0.2 T have been achieved during the initialexperimental campaign. The ion temperature is typically 100 eV. Internal reconnection events are often observed. The internalmagnetic field measured at r/a = 2/3 indicated growth of fluctuationsup to the fourth harmonic, suggesting the existence of modes with severaldifferent mode numbers. In the presence of a toroidal field and avertical field with positive curvature, non-inductively driven currentsof the order of 1 kA were observed with 1 kW of ECH power. The driven current increased with decreasing filling pressure, down to 4 × 10-4Pa. A study of high harmonic fast wave excitationand propagation has begun. Initial results indicate highly efficientwave launching. The spatial distribution of the RF magnetic field wasqualitatively consistent with the result of a full-wave calculation.

Multilayer mirror and foil filter AXUV diode arrays on CDX-U spherical torus

Recent upgrades to CDX-U spherical torus diagnostics include two 10-channel AXUV diode arrays. The multilayer mirror (MLM) array measures the λ150 O VI brightness profile in the poloidal plane using the Mo/B4C synthetic multilayer structures as dispersive elements. The foil filter array has a tangential view and is equipped with interchangeable clear aperture, beryllium and titanium filters. This allows measurements of radiated power, O VI or C V radial distributions, respectively. The O VI and C V emissivity and the radiated power profiles are highly peaked. A Neoclassical impurity accumulation mechanism is considered as an explanation. For radiated power measurements in the Te⩽100 eV plasmas, photon energy dependent corrections must be used in order to account for nonlinear AXUV sensitivity in the range Ephot⩽20 eV. The arrays are also used for characterization of resistive MHD phenomena, such as the low m modes, saw-tooth oscillations and internal reconnection events. Based on the successful operation of the diagnostics, a new ultra soft x-ray multilayer mirror diode AXUV diode array monitoring the 34 Å emissivity distribution of C VI will be built and installed on the National Spherical Torus Experiment.

Operation of the ultrasoft x-ray system on NSTX (abstract)

The ultrasoft x-ray imaging system on National Spherical Torus Experiment (NSTX) became operational and provided the first data in the filtered diode slow bow tie configuration. Using different band pass filters on each of three arrays allows an approximate spectroscopic estimate of the plasma impurity content, as well as of the electron temperature. Magnetohydrodynamics (MHD) activity from different plasma regions is also observed. The soft x-ray emission profiles are well behaved until an Internal Reconnection Event occurs. Examples of NSTX MHD phenomena seen in the ultrasoft x-ray emission under different operational regimes will be presented. From a technical point of view, we point out that the industrial PC based data acquisition system was not adversely affected by stray magnetic fields due to its close proximity to the NSTX device. Also, the surface barrier diodes withstood baking to 100 °C relatively well.

Dynamics of spherical tokamak plasma on the internal reconnection event

Nonlinear magnetohydrodynamic (MHD) simulations are executed to investigate the dynamical behavior of the relaxation phenomenon observed in spherical tokamak (ST) plasma that is known as the Internal Reconnection Event (IRE). The simulation results successfully reproduce several key features of IRE, and the physical mechanisms are revealed. A sudden collapse of the pressure profile takes place as a result of a nonlinear time development of a pressure-driven instability. A magnetic reconnection induced between the internal and the external magnetic field is found to play a crucial role in determining the nature of the overall process, namely, the rapid expulsion of the plasma heat energy due to the pressure imbalance along the reconnected field lines, and the large distortion in the overall shape. The resultant deformations in overall shape of the plasma are in good agreement with the experimental observations.