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Magnetoencephalography (MEG) is a functional neuroimaging technique that noninvasively detects the brain magnetic field from neuronal activations. Conventional MEG measures brain signals using superconducting quantum interference devices (SQUIDs). SQUID-MEG requires a cryogenic environment involving a bulky non-magnetic Dewar flask and the consumption of liquid helium, which restricts the variability of the sensor array and the gap between the cortical sources and sensors. Recently, miniature optically pumped magnetometers (OPMs) have been developed and commercialized. OPMs do not require cryogenic cooling and can be placed within millimeters from the scalp. In the present study, we arranged six OPM sensors on the temporal area to detect auditory-related brain responses in a two-layer magnetically shielded room. We presented the auditory stimuli of 1 kHz pure-tone bursts with 200 ms duration and obtained the M50 and M100 components of auditory-evoked fields. We delivered the periodic stimuli with a 40 Hz repetition rate and observed the gamma-band power changes and inter-trial phase coherence of auditory steady-state responses at 40 Hz. We found that the OPM sensors have a performance comparable to that of conventional SQUID-MEG sensors, and our results suggest the feasibility of using OPM sensors for functional neuroimaging and brain-computer interface applications., (© 2022. The Author(s).)

The Cable-In-Conduit Conductor (CICC) for the KSTAR buslines is made of NbTi superconducting (SC) strands. A busline consists of several electrical joints, which are the major heat load contributors to the busline cryo-system. In the poloidal field (PF) busline helium circuit, the supercritical helium is fed to the electrical joint of current lead end and comes out to the magnet terminal joint. This helium flow configuration has been verified to maintain the cryogenic stability of the buslines through the KSTAR operation. During the normal operation of the KSTAR PF coil, the heated helium coming out to both the coil and the busline meets at the magnet terminal and exchange heat, but the busline outlet temperature still remained less than magnet outlet temperature. As the buslines for the electrical connection in series of the upper and lower coils for PF1 and PF2 have the helium path through the two terminal joints of magnet, they experience higher temperature than the other buslines mainly due to the larger heat exchange. In this case, the connection buslines are considered to have very low safety margin and have the strong possibility of quenches. [ABSTRACT FROM PUBLISHER]

Korea Superconducting Tokamak Advanced Research (KSTAR) is a superconducting tokamak which adopts Nb3Sn and NbTi as superconductors and it has been operated since 2008. One of main issues is the performance of superconducting magnet system in the real operation. In view of this issue, the KSTAR magnet system shows the sufficient operational capability through the whole campaigns. For example, we have already achieved the maximum Toroidal Field (TF) current and the bipolar pulsed Poloidal Field (PF) operation up to \pm 10 kA. It is very important to operate stably the full superconducting magnet system such as KSTAR and ITER. Each KSTAR magnet consists of Cable-In-Conduit Conductors (CICC) cooled by forced flow of the supercritical helium in cryogenic temperature. The temperature, pressure and mass flow rate of coolant are interacted organically inside of CICC and the behaviors are also complicated during operation. The pressure drop between inlet and outlet of coil, the temperature increasing in accordance with coil current variation, and so on are critical issues related with the flexibility and reliability of superconducting tokamak. [ABSTRACT FROM PUBLISHER]

The helium refrigeration system (HRS) of the Korea Superconducting Tokamak Advanced Research (KSTAR), which was designed to provide an energetic equivalent cooling power of 9 kW at 4.5 K, has been operated successfully during the last three years since 2008. For the operation of KSTAR, all of the cold components, such as the superconducting (SC) magnets, magnet structure, thermal shields, SC buslines, and current leads, were cooled down to the aimed cryogenic temperature by generated helium in the HRS. The supercritical helium (4.5 K and 5.5 bar) of 300 g/s was delivered for cooling of the SC magnet (poloidal field (PF) magnet: 14; toroidal field (TF) magnet: 16) and magnet structure and was circulated by the PF and the TF circulators. Thermal loads due to magnet operation are removed at the thermal damper installed in the distribution box of the HRS. Moreover, the pressure change in the magnet will directly affect the circulators. In particular, the pressure drop of the PF circulator drastically changed due to the PF magnet operation depending on the plasma generation scenario. In the 2010 campaign, the operating range of the PF magnet expanded from \pm4 to \pm10 kA, and the plasma scenario was improved. Accordingly, the mass flow rate of the PF circuit was increased from 350 to 369 g/s in order to achieve cryogenic stability. However, the operating range of the PF circulator exceeded the allowable range during shot #2526 and shot #3878. Nevertheless, the circulator successfully operated without any significant fault. The operating plan of the PF magnet that will be further expanded in the operating range may cause damages to the circulator continuously. Consequently, the influence of PF operating scenarios on the PF circulator was analyzed to investigate the safety margin of the HRS. [ABSTRACT FROM PUBLISHER]

The Korea Superconducting Tokamak Advanced Research (KSTAR) adopted superconducting coils which consist of rectangular cable-in-conduit conductors (CICC) without a central hole for supercritical helium. Most of all, the KSTAR has a unique superconducting magnet system with Nb3Sn and NbTi such as ITER. We expect that the test results of KSTAR experiments can also provide useful information for ITER and the other fusion devices. Up to now, the KSTAR superconducting magnet system is showing a larger flexibility and reliability in operation. For example, the TF magnet system has achieved the current charging of 36.2 kA and PF magnet system has been also charged up to \pm4 kA without difficulties during the last two times KSTAR campaigns. The significant thermo-hydraulic characteristics such as helium flow distribution, pressure drop between the inlet and outlet of coil, the friction factor and the temperature variation according to the current charging operations are introduced in this paper. [ABSTRACT FROM AUTHOR]

Superconducting magnets of the Korea Superconducting Tokamak Advanced Research (KSTAR) are cooled by supercritical helium with 4.5 K, which was supplied and recovered by the 9 kW of the Helium Refrigerator System (HRS). While current is being charged, the supercritical helium expands to both side of the helium inlet and the outlet of the magnets due to the generated AC losses. To maintain the pressure gradient, both the supply and the return pressures of the HRS are increased at the same time and the differential pressure of the HRS was reduced after the event. However, the pressure rising in the magnets may block the helium flow or create reversal flow of the helium. During unipolar experiment of PF1 magnet up to 2 kA with 1 kA/s of ramp-up rate, the mass flow rate was decreased at the PF1 cooling tube (manifold) in the helium distribution system (HDS), whereas the pressure is increased and the temperature is to be increased or decreased according to compression and expansion of the heated helium in the magnets. For the bipolar experiment of PF1 up to \pm2 kA with 1 kA/s ramp-up rate and 2 kA/s ramp-down rate, the conditions in the helium flow were drastically changed, especially the mass flow rate was measured to be maintained at zero for a few second (more than 4 s). This behavior could decisively affect the cryogenic stabilities in the magnet, and may impose a major limit on the long pulse operation of KSTAR. In this paper, we investigated this behavior and analysed by using 1-dimentional thermo-hydraulic code, GANDALF. [ABSTRACT FROM AUTHOR]

The KSTAR superconducting magnetic coils, which are made of cable in-conduit conductor (CICC), maintain a superconducting state with forced-flow supercritical helium (4.5 K, 5.5 bar). During current changing of the superconducting magnetic coils, AC losses are generated in the CICC due to dI/dt, and the heat generated from the loss is removed by high heat capacity supercritical helium. At the same time, reversed flow of the helium occurs due to a rapid increase of the helium temperature and momentary changing of the pressure inside the CICC. This phenomenon has been detected in all of the poloidal field (PF) coils, especially in the upper (U) and lower (L) PF1 ~ PF4 coils. The maximum change of the magnetic field in the PF1UL ~ PF4UL coils is located near the inlet and outlet of the helium cooling channels, and that of the PF5UL ~ 7UL coils is located at the center of the cooling channel. The temperature variation at the helium inlet was always measured to have a time delay after each shot. In the PF1 coil tests, it was measured to have a delay of 26 sec. During the first plasma campaign, this phenomenon was more severe in the case of all PF coils operating together than for a single PF operation. In this paper, we investigated the thermal-hydraulics of this phenomenon. [ABSTRACT FROM AUTHOR]

Abstract: An un-calibrated sensitive 3He detector was used to monitor the KSTAR neutron rate during Ohmically heated discharges. Neutrons were detected for every shot. If the neutrons were from D–D reactions then there should be a dependence on the ion temperature. We obtained ion temperature, electron density, Ohmic heating input power measurements, etc. from standard diagnostics. The study shows no observable dependence on the Ohmic power, ion temperature, or calculated source strength. It appears that the neutrons are from sources other than D–D reactions. The most probable source is high-energy runaway electrons. [Copyright &y& Elsevier]