Pop-up Langmuir probe diagnostic in the water cooled divertor of Wendelstein 7-X.

The design, development, and successful implementation of pop-up Langmuir probes installed in the water-cooled divertor of W7-X are described. The probes are controlled by drive coils (actuators) installed behind the divertor plates. These drive coils make use of the magnetic field in W7-X to move the probe tips into and out of the plasma. The drive coils were installed in the vacuum vessel after extensively testing the durability of the coils and analyzing the criteria for safe operation. The probe design is carefully tailored for each of the 36 probe tips in order to be suitable for the different magnetic field configurations used in W7-X and ensure that the probes do not present leading edges to the magnetic flux tubes. An electronic bridge circuit is used for measurement to compensate for the effects of signal propagation time on the long cable lengths used. The diagnostic is integrated with the segment control of W7-X for automated operation and control of the diagnostic. The evaluation of the results from the plasma operation is presented after accounting for appropriate sheath expansion for negative bias voltage on the probes.

On the stability of thoriated tungsten cathodes in strong magnetic fields.

Thoriated tungsten cathodes, first studied by Langmuir [Phys. Rev. 22, 357-398 (1923)], are used in many applications as efficient electron emitters. However, neutral pressure gauges with thoriated tungsten cathodes (or ASDEX pressure gauges) are not reliable when operated in the strong magnetic field of fusion devices of several Tesla. We have identified the reason for the bad performance in the Wendelstein 7-X stellarator during the operation of several 100 s. Not only were slow, creeping mechanical deformations of the cathodes observed, but also fast events, such as sudden short circuits. The temperature of the cathode is often much higher (about 2400 K) than the maximum value recommended by Langmuir [Phys. Rev. 22, 357-398 (1923)] (about 1900 K). Our test in a superconducting magnet revealed that for a long-pulse operation of 30 min or more in a 3.1 T field, there is an additional effect. We observed that the cathodes required a very high heating current after 6 h of operation. As a consequence, the possible temperature range of the thoriated tungsten cathodes became very small near to an experimentally determined failure limit. In fusion devices with long-pulse operation or in reactors, new cathode types must be used. We give a brief overview of alternative designs that are currently under development.

Demonstration of reduced neoclassical energy transport in Wendelstein 7-X.

Research on magnetic confinement of high-temperature plasmas has the ultimate goal of harnessing nuclear fusion for the production of electricity. Although the tokamak1 is the leading toroidal magnetic-confinement concept, it is not without shortcomings and the fusion community has therefore also pursued alternative concepts such as the stellarator. Unlike axisymmetric tokamaks, stellarators possess a three-dimensional (3D) magnetic field geometry. The availability of this additional dimension opens up an extensive configuration space for computational optimization of both the field geometry itself and the current-carrying coils that produce it. Such an optimization was undertaken in designing Wendelstein 7-X (W7-X)2, a large helical-axis advanced stellarator (HELIAS), which began operation in 2015 at Greifswald, Germany. A major drawback of 3D magnetic field geometry, however, is that it introduces a strong temperature dependence into the stellarator's non-turbulent 'neoclassical' energy transport. Indeed, such energy losses will become prohibitive in high-temperature reactor plasmas unless a strong reduction of the geometrical factor associated with this transport can be achieved; such a reduction was therefore a principal goal of the design of W7-X. In spite of the modest heating power currently available, W7-X has already been able to achieve high-temperature plasma conditions during its 2017 and 2018 experimental campaigns, producing record values of the fusion triple product for such stellarator plasmas3,4. The triple product of plasma density, ion temperature and energy confinement time is used in fusion research as a figure of merit, as it must attain a certain threshold value before net-energy-producing operation of a reactor becomes possible1,5. Here we demonstrate that such record values provide evidence for reduced neoclassical energy transport in W7-X, as the plasma profiles that produced these results could not have been obtained in stellarators lacking a comparably high level of neoclassical optimization.

Bayesian inference of spatially resolved Zeff profiles from line integrated bremsstrahlung spectra.

In nuclear fusion research, the effective ion charge Zeff, which characterizes the overall content of impurities, can be experimentally derived from the plasma electron-ion bremsstrahlung, given the electron density ne and temperature Te. At Wendelstein 7-X, a multichannel near-infrared spectrometer is installed to collect the plasma bremsstrahlung along 27 lines of sight covering more than half the plasma cross section, which provides information on Zeff over the entire plasma radius. To infer spatially resolved Zeff profiles, a Bayesian model is developed in the Minerva framework. Zeff, ne, and Te profiles are modeled as Gaussian processes, whose smoothness is determined by hyperparameters. These profiles are transformed to fields in Cartesian coordinates, given the poloidal magnetic flux surfaces calculated by the variational moments equilibrium code. Given all these physical quantities, the model predicts line-of-sight integrals of near-infrared bremsstrahlung spectra. The model includes the predictive (forward) models of the interferometer, Thomson scattering system, and visible and near-infrared spectrometers. Given the observations of all these diagnostics, the posterior probability distribution of Zeff profiles is calculated and shown as an inference solution. The smoothness (gradient) of the profiles is optimally chosen by Bayesian Occam's razor. Furthermore, wall reflections can significantly pollute the measurements of the plasma bremsstrahlung, which leads to over-estimation of Zeff values in the edge region. In the first results presented in this work, this problem does not appear, and the posterior samples of Zeff profiles are overall plausible and consistent with Zeff values inferred, given the data from the single-channel visible spectrometer.

First Observation of a Stable Highly Dissipative Divertor Plasma Regime on the Wendelstein 7-X Stellarator.

For the first time, the optimized stellarator Wendelstein 7-X has operated with an island divertor. An operation regime in hydrogen was found in which the total plasma radiation approached the absorbed heating power without noticeable loss of stored energy. The divertor thermography recorded simultaneously a strong reduction of the heat load on all divertor targets, indicating almost complete power detachment. This operation regime was stably sustained over several energy confinement times until the preprogrammed end of the discharge. The plasma radiation is mainly due to oxygen and is located at the plasma edge. This plasma scenario is reproducible and robust at various heating powers, plasma densities, and gas fueling locations. These experimental results show that the island divertor concept actually works and displays good power dissipation potential, producing a promising exhaust concept for the stellarator reactor line.

An ionization pressure gauge with LaB6 emitter for long-term operation in strong magnetic fields.

We report here on a potentially significant improvement in the design of neutral pressure gauges of the so-called ASDEX-type which were first used in the Axially Symmetric Divertor EXperiment (ASDEX). Such gauges are considered state-of-the-art and are in wide use in fusion experiments, but they nonetheless suffer from a relatively high failure rate when operated at high magnetic field strengths for long times. This is therefore a significant concern for long-pulse, high-field experiments such as Wendelstein 7-X (W7-X) and ITER. The new design is much more robust. The improvement is to use a LaB6 crystal instead of a tungsten wire as the thermionic emitter of electrons in the gauge. Such a LaB6 prototype gauge was successfully operated for a total of 60 h in B = 3.1 T, confirming the significantly improved robustness of the new design and qualifying it for near-term operation in W7-X. With the LaB6 crystal, an order of magnitude reduction in heating current is achieved, relative to the tungsten filament based gauges, from 15-20 A to 1-2 A. This reduces the Lorenz forces and the heating power by an order of magnitude also and is presumably the reason for the much improved robustness. The new gauge design, test environment setup at the superconducting magnet, and results from test operation are described.

Synthetic plasma edge diagnostics for EMC3-EIRENE, highlighted for Wendelstein 7-X.

Interpretation of spectroscopic measurements in the edge region of high-temperature plasmas can be a challenge since line of sight integration effects make direct interpretation in terms of quantitative, local emission strengths often impossible. The EMC3-EIRENE code-a 3D fluid edge plasma and kinetic neutral gas transport code-is a suitable tool for full 3D reconstruction of such signals. A versatile synthetic diagnostic module has been developed recently which allows the realistic 3D setup of various plasma edge diagnostics to be captured. We highlight these capabilities with two examples for Wendelstein 7-X (W7-X): a visible camera for the analysis of recycling, and a coherent-imaging system for velocity measurements.

Detecting divertor damage during steady state operation of Wendelstein 7-X from thermographic measurements.

Wendelstein 7-X (W7-X) aims to demonstrate the reactor capability of the stellarator concept, by creating plasmas with pulse lengths of up to 30 min at a heating power of up to 10 MW. The divertor plasma facing components will see convective steady state heat flux densities of up to 10 MW/m(2). These high heat flux target elements are actively cooled and are covered with carbon fibre reinforced carbon (CFC) as plasma facing material. The CFC is bonded to the CuCrZr cooling structure. Over the life time of the experiment this interface may weaken and cracks can occur, greatly reducing the heat conduction between the CFC tile and the cooling structure. Therefore, there is not only the need to monitor the divertor to prevent damage by overheating but also the need to detect these fatigue failures of the interface. A method is presented for an early detection of fatigue failures of the interface layer, solely by using the information delivered by the IR-cameras monitoring the divertor. This was developed and validated through experiments made with high heat flux target elements prior to installation in W7-X.

Status of the diagnostics development for the first operation phase of the stellarator Wendelstein 7-X.

An overview of the diagnostics which are essential for the first operational phase of Wendelstein 7-X and the set of diagnostics expected to be ready for operation at this time are presented. The ongoing investigations of how to cope with high levels of stray Electron Cyclotron Resonance Heating (ECRH) radiation in the ultraviolet (UV)/visible/infrared (IR) optical diagnostics are described.

In situ "artificial plasma" calibration of tokamak magnetic sensors.

A unique in situ calibration technique has been used to spatially calibrate and characterize the extensive new magnetic diagnostic set and close-fitting conducting wall of the High Beta Tokamak-Extended Pulse (HBT-EP) experiment. A new set of 216 Mirnov coils has recently been installed inside the vacuum chamber of the device for high-resolution measurements of magnetohydrodynamic phenomena including the effects of eddy currents in the nearby conducting wall. The spatial positions of these sensors are calibrated by energizing several large in situ calibration coils in turn, and using measurements of the magnetic fields produced by the various coils to solve for each sensor's position. Since the calibration coils are built near the nominal location of the plasma current centroid, the technique is referred to as an "artificial plasma" calibration. The fitting procedure for the sensor positions is described, and results of the spatial calibration are compared with those based on metrology. The time response of the sensors is compared with the evolution of the artificial plasma current to deduce the eddy current contribution to each signal. This is compared with simulations using the VALEN electromagnetic code, and the modeled copper thickness profiles of the HBT-EP conducting wall are adjusted to better match experimental measurements of the eddy current decay. Finally, the multiple coils of the artificial plasma system are also used to directly calibrate a non-uniformly wound Fourier Rogowski coil on HBT-EP.

A high-power spatial filter for Thomson scattering stray light reduction.

The Thomson scattering diagnostic on the High Beta Tokamak-Extended Pulse (HBT-EP) is routinely used to measure electron temperature and density during plasma discharges. Avalanche photodiodes in a five-channel interference filter polychromator measure scattered light from a 6 ns, 800 mJ, 1064 nm Nd:YAG laser pulse. A low cost, high-power spatial filter was designed, tested, and added to the laser beamline in order to reduce stray laser light to levels which are acceptable for accurate Rayleigh calibration. A detailed analysis of the spatial filter design and performance is given. The spatial filter can be easily implemented in an existing Thomson scattering system without the need to disturb the vacuum chamber or significantly change the beamline. Although apertures in the spatial filter suffer substantial damage from the focused beam, with proper design they can last long enough to permit absolute calibration.

Renal water and sodium handling during gradated unilateral ureter obstruction.

OBJECTIVE: Unilateral complete obstruction of the ureter (UUO) is associated with characteristic changes in renal function. To improve the understanding of how urine concentration directly is affected by changes in pelvic pressure, changes in renal salt and water handling along the nephron and collecting duct were examined. MATERIAL AND METHODS: Pelvic pressure was raised stepwise using an adjustable lever inserted in the right ureter. Urine samples were collected from the tip of the catheter by way of an overflow system. Water and sodium handling in the distal and proximal tubules were measured by the lithium clearance technique. Renal blood flow (RBF) was measured with implanted ultrasonic flow probes. Catheters were placed in both renal veins and glomerular filtration rate (GFR) and filtration fraction were calculated using renal extraction of (51)Cr-EDTA independent of urine sampling. RESULTS: The sequence of changes for each parameter is provided. The parameters did not show a uniform pattern from which specific threshold values could be derived. However, the mean value of the following parameters were markedly changed at specific pressures: (1) at 10 cm H(2)O ipsilateral urine output decreased rapidly and distal absolute reabsorption of sodium (DAR(H2O)) decreased, (2) at 20 cm H(2)O GFR started to decline rapidly, (3) at 30 cm H(2)O urine output was impaired, and (4) at 40 cm H(2)O proximal absolute reabsorption of water (PAR(H2O)) showed a decreasing tendency in all pigs together with impairment in tubular sodium handling. Furthermore, free water clearance was slightly impaired (-0.26 +/- 0.15 at baseline and -0.15 +/- 0.08 ml/min at maximum pressure) and ipsilateral RBF decreased from 171.1 +/- 12.4 ml/min at baseline to 136.3 +/- 12.3 ml/min at ureteral pressure of 80 cm H(2)O (p < 0.05). Consistent with that, ipsilateral renal vascular resistance increased with increasing pressure in the renal pelvis. CONCLUSION: Water reabsorption and sodium handling is progressively impaired with increasing pelvic pressure. GFR and RBF are reduced in parallel. The study shows that the kidney responds to ureteral obstruction is unique and individual.

Reduced renal vascular resistance in response to verapamil during gradated ureter obstruction in pigs.

Unilateral ureteral obstruction (UUO) is associated with reductions in ipsilateral renal blood flow (RBF) and glomerular filtration rate (GFR) caused by an active preglomerular vasoconstriction, where angiotensin II (ANGII) may be an important mediator. Calcium-channel blockers preferentially dilate preglomerular vessels and abolish the vasoconstrictor actions of ANGII in preglomerular arterioles of the hydronephrotic rat kidney. In this study, we, therefore, examined the effects of the calcium-channel blocker verapamil (3.65 microg/kg per minute i.v.) on RBF, GFR and renal vascular resistance (RVR) in our pig model with UUO, where ultrasonic flow probes are mounted on each renal artery and catheters placed in the abdominal aorta and both renal veins. Verapamil treatment was associated with a 34% reduction in ipsilateral RBF (from 182.6 +/- 20.5 ml/min to 120.6 +/- 12.2 ml/min, P < 0.001), which was similar to the 27% reduction in ipsilateral RBF in controls (from 194.6 +/- 13.1 ml/min to 140.6 +/- 15.2 ml/min, P < 0.001). Ipsilateral GFR was reduced by 70% in the verapamil-treated pigs (from 29.0 +/- 2.6 to 8.5 +/- 0.9 ml/min, P < 0.001) and by 73% in control animals (from 29.2 +/- 3.1 to 7.6 +/- 2.1 ml/min, p < 0.001). However, the increase in RVR was significantly attenuated in the verapamil-treated pigs. Ipsilateral RVR increased by 19% in the verapamil-treated pigs (from 0.585 +/- 0.076 to 0.726 +/- 0.081 mmHg/min/ml, P < 0.05) compared with a 34% increase in control pigs (from 0.560 +/- 0.056 to 0.854 +/- 0.091 mmHg/min per milliliter, P<0.001), suggesting that an intact calcium-channel may be important for the increase in renal vascular resistance during unilateral ureter obstruction. In conclusion, the present study shows that verapamil is able to modulate the increase in renal vascular resistance in response to increased pelvic pressure.

Fixed minidose warfarin and aspirin alone and in combination vs adjusted-dose warfarin for stroke prevention in atrial fibrillation: Second Copenhagen Atrial Fibrillation, Aspirin, and Anticoagulation Study.

BACKGROUND: Despite the efficacy of warfarin sodium therapy for stroke prevention in atrial fibrillation, many physicians hesitate to prescribe it to elderly patients because of the risk for bleeding complications and because of inconvenience for the patients. METHODS: The Second Copenhagen Atrial Fibrillation, Aspirin, and Anticoagulation Study was a randomized, controlled trial examining the following therapies: warfarin sodium, 1.25 mg/d; warfarin sodium, 1.25 mg/d, plus aspirin, 300 mg/d; and aspirin, 300 mg/d. These were compared with adjusted-dose warfarin therapy (international normalized ratio of prothrombin time [INR], 2.0-3.0). Stroke or a systemic thromboembolic event was the primary outcome event. Transient ischemic attack, acute myocardial infarction, and death were secondary events. Data were handled as survival data, and risk factors were identified using the Cox proportional hazards model. The trial was scheduled for 6 years from May 1, 1993, but due to scientific evidence of inefficiency of low-intensity warfarin plus aspirin therapy from another study, our trial was prematurely terminated on October 2, 1996. RESULTS: We included 677 patients (median age, 74 years). The cumulative primary event rate after 1 year was 5.8% in patients receiving minidose warfarin; 7.2%, warfarin plus aspirin; 3.6%, aspirin; and 2.8%, adjusted-dose warfarin (P = .67). After 3 years, no difference among the groups was seen. Major bleeding events were rare. CONCLUSIONS: Although the difference was insignificant, adjusted-dose warfarin seemed superior to minidose warfarin and to warfarin plus aspirin after 1 year of treatment. The results do not justify a change in the current recommendation of adjusted-dose warfarin (INR, 2.0-3.0) for stroke prevention in atrial fibrillation.

Renal hemodynamic response to gradated ureter obstruction in the pig.

In anesthetized pigs with ultrasonic flow probes mounted on each renal artery and catheters placed in the abdominal aorta and both renal veins, renal blood flow (RBF), glomerular filtration rate (GFR) and filtration fraction (FF) were investigated during stepwise unilateral ureteral obstruction. Elevation of the ureteral pressure in steps of 10 mm Hg to a maximum of 80 mm Hg decreased ipsilateral RBF by 45 +/- 3% from 300 +/- 25 to 168 +/- 20 ml/min (p < 0.01). Contralateral RBF did not change significantly. The mean arterial pressure ws constant during the experimental procedures, suggesting that the decrease of RBF was due to a significant increase in ipsilateral renal vascular resistance. Concomitantly with these changes ipsilateral GFR was reduced by 75% from 40 +/- 2 to 10 +/- 3 ml/min. In the contralateral kidney, GFR was unchanged during the experiment. The renal extraction of 51Cr-EDTA equal to FF increased temporarily from 0.202 +/- 0.013 at start to 0.239 +/- 0.015 (p < 0.05) at 20 mm Hg in the ipsilateral kidney, whereafter it was dramatically reduced to 0.090 +/- 0.024 at maximum pressure. In addition, renal tubular handling of salt and water in the contralateral kidney were investigated using the lithium clearance technique. No significant changes were found. In conclusion, the renal hemodynamic changes during ureteral obstruction are compatible with a predominant preglomerular vasoconstriction. The inter-relationship between obstruction, high pressure and reduction in RBF and GFR is substantiated. Moreover, variation in the reactive mechanisms between species with different kidney activities is established.

[Cutaneous myiasis--an imported case caused by the South American fly larvae Dermatobia hominis]. / Kutan myiasis--et importeret tilfoelde forårsaget af den sydamerikanske fluelarve Dermatobia hominis.

Cutaneous myiasis is the infestation of skin or mucous membranes with larvae of flies. We describe a case caused by Dermatobia hominis, acquired in South America. Cutaneous myiasis should be suspected in a patient with a secreting, non-healing furuncular skin-lesion and relevant travel history. The patient may remember being bitten by insects. Sensation of movement in the lesion, which may be observed, supports the diagnosis. Correct diagnosis will prevent unnecessary treatment with antibiotics, and surgery will almost always prove unnecessary.

Fc-mediated immune precipitation. IV. Antigen dependency and specificity.

The antigen dependency of Fc-mediated immune precipitation was investigated by comparing the immune precipitation of three different protein antigens (bovine serum albumin, BSA, human plasma transferrin, HPT, and human fibrinogen, HFg) using specific intact rabbit IgG antibodies and isomolar solutions of the corresponding F(ab')2 fragments. We found that this precipitin mechanism is highly antigen-dependent both quantitatively and qualitatively. Sucrose density gradient ultracentrifugation was used for the identification of Fc-precipitating immune complexes, and we demonstrated that different types of complexes take part in Fc-mediated immune precipitation in the different antigen-antibody systems. Thus, only small complexes are involved in the HFg system, whereas small as well as large complexes precipitate by this mechanism in the BSA and the HPT systems. The specificity of Fc-mediated immune precipitation was studied by mixing human serum albumin (HSA)-anti-HSA IgG complexes with either HPT-anti-HPT IgG or HFg-anti-HFg IgG complexes. These investigations show that Fc-mediated immune precipitation is a specific process due to the preferential binding of homologous complexes.