A long-standing enigma in plasma transport has been resolved by modeling of cold-pulse experiments conducted on the Alcator C-Mod tokamak. Controlled edge cooling of fusion plasmas triggers core ...electron heating on time scales faster than an energy confinement time, which has long been interpreted as strong evidence of nonlocal transport. This Letter shows that the steady-state profiles, the cold-pulse rise time, and disappearance at higher density as measured in these experiments are successfully captured by a recent local quasilinear turbulent transport model, demonstrating that the existence of nonlocal transport phenomena is not necessary for explaining the behavior and time scales of cold-pulse experiments in tokamak plasmas.
Overview of the SPARC tokamak Creely, A. J.; Greenwald, M. J.; Ballinger, S. B. ...
Journal of plasma physics,
10/2020, Letnik:
86, Številka:
5
Journal Article
Recenzirano
Odprti dostop
The SPARC tokamak is a critical next step towards commercial fusion energy. SPARC is designed as a high-field ($B_0 = 12.2$ T), compact ($R_0 = 1.85$ m, $a = 0.57$ m), superconducting, D-T tokamak ...with the goal of producing fusion gain $Q>2$ from a magnetically confined fusion plasma for the first time. Currently under design, SPARC will continue the high-field path of the Alcator series of tokamaks, utilizing new magnets based on rare earth barium copper oxide high-temperature superconductors to achieve high performance in a compact device. The goal of $Q>2$ is achievable with conservative physics assumptions ($H_{98,y2} = 0.7$) and, with the nominal assumption of $H_{98,y2} = 1$, SPARC is projected to attain $Q \approx 11$ and $P_{\textrm {fusion}} \approx 140$ MW. SPARC will therefore constitute a unique platform for burning plasma physics research with high density ($\langle n_{e} \rangle \approx 3 \times 10^{20}\ \textrm {m}^{-3}$), high temperature ($\langle T_e \rangle \approx 7$ keV) and high power density ($P_{\textrm {fusion}}/V_{\textrm {plasma}} \approx 7\ \textrm {MW}\,\textrm {m}^{-3}$) relevant to fusion power plants. SPARC's place in the path to commercial fusion energy, its parameters and the current status of SPARC design work are presented. This work also describes the basis for global performance projections and summarizes some of the physics analysis that is presented in greater detail in the companion articles of this collection.
The database of the nine radars of the Super Dual Auroral Radar Network (SuperDARN) in the northern hemisphere has been analyzed for information on factors that influence the convection of plasma in ...the high‐latitude ionosphere. The velocity measurements were collected over the period 1998–2002. The data were first used to derive a new statistical model of convection that improves upon the earlier one‐radar model of Ruohoniemi and Greenwald (1996) in its specification of the dependence of the convection pattern on the magnitude and direction of the IMF in the GSM Y‐Z plane. We then derived average patterns for secondary sortings by season, year, and radar. Such dependencies as emerged were most clearly seen by contrasting the results for By+ and By−. The seasonal effect in the convection pattern is found to have similarities to that of the sign of By. In particular, the combination of By+/summer (By−/winter) reinforces the tendency of the By sign factor to sculpt the dusk and dawn cells into more round/crescent (crescent/round) shapes and to shift the crescent cell across the midnight MLT meridian. However, these combinations are associated with lower estimates of the total cross polar cap potential drop, ΦPC, while the nonreinforcing combinations produce elevated ΦPC, especially By−/summer. There is an overall tendency for ΦPC to increase from winter to summer, although the pure seasonal effect on the potential drop is weaker than that of the By−sign/season factor. We did not find pronounced differences among the patterns derived for the 5 individual years, which spanned the most recent interval of solar cycle maximum. Sorting by radar, we found few differences among the patterns for By+, but for By−, variations emerged that are consistent with a possible dependence on universal time (UT). The impacts of season and UT on convection in the high‐latitude ionosphere thus depends on the IMF, especially the sign of By. We speculate that variability in the ionospheric conductivity has a greater effect on magnetosphere‐ionosphere coupling under By− conditions.
We present a global view of large‐scale ionospheric disturbances during the main phase of a major geomagnetic storm. We find that the low‐latitude, auroral, and polar latitude regions are coupled by ...processes that redistribute thermal plasma throughout the system. For the large geomagnetic storm on 20 November 2003, we examine data from the high‐latitude incoherent scatter radars at Millstone Hill, Sondrestrom, and EISCAT Tromso, with SuperDARN HF radar observations of the high‐latitude convection pattern and DMSP observations of in situ plasma parameters in the topside ionosphere. We combine these with north polar maps of stormtime plumes of enhanced total electron content (TEC) derived from a network of GPS receivers. The polar tongue of ionization (TOI) is seen to be a continuous stream of dense cold plasma entrained in the global convection pattern. The dayside source of the TOI is the plume of storm enhanced density (SED) transported from low latitudes in the postnoon sector by the subauroral disturbance electric field. Convection carries this material through the dayside cusp and across the polar cap to the nightside where the auroral F region is significantly enhanced by the SED material. The three incoherent scatter radars provided full altitude profiles of plasma density, temperatures, and vertical velocity as the TOI plume crossed their different positions, under the cusp, in the center of the polar cap, and at the midnight oval/polar cap boundary. Greatly elevated F peak density (>1.5E12 m−3) and low electron and ion temperatures (∼2500 K at the F peak altitude) characterize the SED/TOI plasma observed at all points along its high‐latitude trajectory. For this event, SED/TOI F region TEC (150–1000 km) was ∼50 TECu both in the cusp and in the center of the polar cap. Large, upward directed fluxes of O+ (>1.E14 m−2 s−1) were observed in the topside ionosphere from the SED/TOI plume within the cusp.
Medium‐scale traveling ionospheric disturbances (MSTIDs) are wave‐like ionospheric perturbations routinely observed by high‐frequency radars. We focus on a class of MSTIDs observed during the winter ...daytime at high latitudes and midlatitudes. The source of these MSTIDs remains uncertain, with the two primary candidates being space weather and lower atmospheric processes. We surveyed observations from four high‐latitude and six midlatitude Super Dual Auroral Radar Network radars in the North American sector from November to May 2012 to 2015. The MSTIDs observed have horizontal wavelengths between ∼150 and 650 km and horizontal velocities between ∼75 and 325 m s−1. In local fall and winter seasons the majority of MSTIDs propagated equatorward, with bearings ranging from ∼125° to 225° geographic azimuth. No clear correlation with space weather activity as parameterized by AE and SYM‐H could be identified. Rather, MSTID observations were found to have a strong correlation with polar vortex dynamics on two timescales. First, a seasonal timescale follows the annual development and decay of the polar vortex. Second, a shorter 2–4 week timescale again corresponds to synoptic polar vortex variability, including stratospheric warmings. Additionally, statistical analysis shows that MSTIDs are more likely during periods of strong polar vortex. Direct comparison of the MSTID observations with stratospheric zonal winds suggests that a wind filtering mechanism may be responsible for the strong correlation. Collectively, these observations suggest that polar atmospheric processes, rather than space weather activity, are primarily responsible for controlling the occurrence of high‐latitude and midlatitude winter daytime MSTIDs.
Key Points
A climatology of MSTIDs over North America is presented
AGW‐MSTIDs are not correlated with space weather
AGW‐MSTIDs are associated with polar atmospheric processes
Multi-center, prospective, cohort study.
To assess the validity and reliability of the Spinal Cord Independence Measure (SCIM III) in measuring functional ability in persons with spinal cord injury ...(SCI).
Inpatient rehabilitation hospitals in the United States (US).
Functional ability was measured with the SCIM III during the first week of admittance into inpatient acute rehabilitation and within one week of discharge from the same rehabilitation program. Motor and sensory neurologic impairment was measured with the American Spinal Injury Association Impairment Scale. The Functional Independence Measure (FIM), the default functional measure currently used in most US hospitals, was used as a comparison standard for the SCIM III. Statistical analyses were used to test the validity and reliability of the SCIM III.
Total agreement between raters was above 70% on most SCIM III tasks and all κ-coefficients were statistically significant (P<0.001). The coefficients of Pearson correlation between the paired raters were above 0.81 and intraclass correlation coefficients were above 0.81. Cronbach's-α was above 0.7, with the exception of the respiration task. The coefficient of Pearson correlation between the FIM and SCIM III was 0.8 (P<0.001). For the respiration and sphincter management subscale, the SCIM III was more responsive to change, than the FIM (P<0.0001).
Overall, the SCIM III is a reliable and valid measure of functional change in SCI. However, improved scoring instructions and a few modifications to the scoring categories may reduce variability between raters and enhance clinical utility.
The Super Dual Auroral Radar Network (SuperDARN) has been operating as an international co-operative organization for over 10 years. The network has now grown so that the fields of view of its 18 ...radars cover the majority of the northern and southern hemisphere polar ionospheres. SuperDARN has been successful in addressing a wide range of scientific questions concerning processes in the magnetosphere, ionosphere, thermosphere, and mesosphere, as well as general plasma physics questions. We commence this paper with a historical introduction to SuperDARN. Following this, we review the science performed by SuperDARN over the last 10 years covering the areas of ionospheric convection, field-aligned currents, magnetic reconnection, substorms, MHD waves, the neutral atmosphere, and E-region ionospheric irregularities. In addition, we provide an up-to-date description of the current network, as well as the analysis techniques available for use with the data from the radars. We conclude the paper with a discussion of the future of SuperDARN, its expansion, and new science opportunities.
Background Endoscopic ablation to treat Barrett's esophagus (BE) with high-grade dysplasia (HGD) is associated with a decreased incidence of esophageal adenocarcinoma. Endoscopic spray cryotherapy ...(CRYO) demonstrates promising preliminary data. Objective To assess the safety and efficacy of CRYO in BE with HGD. Design Multicenter, retrospective cohort study. Setting Nine academic and community centers; treatment period, 2007 to 2009. Patients Subjects with HGD confirmed by 2 pathologists. Previous EMR was allowed if residual HGD remained. Interventions CRYO with follow-up biopsies. Main Outcome Measurements Complete eradication of HGD with persistent low-grade dysplasia, complete eradication of all dysplasia with persistent nondysplastic intestinal metaplasia, and complete eradication of all intestinal metaplasia. Results Ninety-eight subjects (mean age 65.4 years, 83% male) with BE and HGD (mean length 5.3 cm) underwent 333 treatments (mean 3.4 treatments per subject). There were no esophageal perforations. Strictures developed in 3 subjects. Two subjects reported severe chest pain managed with oral narcotics. One subject was hospitalized for bright red blood per rectum. Sixty subjects had completed all planned CRYO treatments and were included in the efficacy analysis. Fifty-eight subjects (97%) had complete eradication of HGD, 52 (87%) had complete eradication of all dysplasia with persistent nondysplastic intestinal metaplasia, and 34 (57%) had complete eradication of all intestinal metaplasia. Subsquamous BE was found in 2 subjects (3%). Limitations Nonrandomized, retrospective study with no control group, short follow-up (10.5 months), lack of centralized pathology, and use of surrogate outcome for decreased cancer risk. Conclusions CRYO is a safe and well-tolerated therapy for BE and HGD. Short-term results suggest that CRYO is highly effective in eradicating HGD.
We present simultaneous measurements of flow velocities inside a subauroral polarization stream (SAPS) made by six midlatitude high‐frequency SuperDARN radars. The instantaneous observations cover ...three hours of universal time and six hours of magnetic local time (MLT). From velocity variations across the field‐of‐view of the radars we infer the local 2D flow direction at three different longitudes. We find that the local flow direction inside the SAPS channel is remarkably constant over the course of the event. The flow speed, however, shows significant temporal and spatial variations. After correcting for the radar look direction we are able to accurately determine the dependence of the SAPS velocity on magnetic local time. We find that the SAPS velocity variation with magnetic local time is best described by an exponential function. The average velocity at 00 MLT was 1.2 km/s and it decreased with a spatial e‐folding scale of two hours of MLT toward the dawn sector. We speculate that the longitudinal distribution of pressure gradients in the ring current is responsible for this dependence and find these observations in good agreement with results from ring current models. Using TEC measurements we find that the high westward velocities of the SAPS are ‐ as expected ‐ located in a region of low TEC values, indicating low ionospheric conductivities.
Key Points
Instantaneous longitudinal velocity variation of SAPS is presented
Longitudinal velocity variation is found to be exponential
The longitudinal velocity variation agrees well with model predictions
PDF
