39 Search Results

Multiple spectroscopy diagnostics have been fielded on the FuZE-Q sheared-flow-stabilized Z-pinch to measure plasma impurities, flow velocity, temperature, and density. The spectroscopy diagnostics currently cover the wavelengths from extreme ultraviolet (EUV) in the 5–40 nm (30–250 eV) range, ion Doppler spectroscopy (IDS) in the 225–233 nm range, to compact broadband spectroscopy (UV/VIS) in the 220–1050 nm range. Spectral databases and collisional-radiative (CR) modeling have been used to identify impurities and estimate plasma parameters. Many emission lines from deuterium (D), carbon (C), and oxygen (O) have been identified, at this time. Current line-emission spectroscopy is sensitive to relatively low temperatures characteristicmore » of the edge plasma, outside the fusion core. Temperature estimates using O as an EUV spectroscopic tracer have been compared to IDS estimates using C. EUV and IDS spectra find evidence of an ensemble of edge plasma temperatures in device commissioning shots. Modeling line pairs involving B-like, Be-like, Li-like, and He-like O yielded electron temperatures from 16 to 155 eV. This profile is comparable to ion temperatures observed by IDS from Doppler broadening of Be-like C (34 eV) and He-like C (298 eV). Deposition and debris from within FuZE-Q have been analyzed with scanning electron microscope (SEM) energy dispersive X-ray spectroscopy (EDS) and find evidence of C, O, Si, W, Al, Cu, Fe, and Cr. Synthetic spectra from CR modeling, informed by plasma numerical simulations, can be compared to experimental spectra and thereby benchmark the calculations. Measurements of the edge plasma provide insight into plasma-wall interactions. Impurity identification is vital for radiative power and scientific Q calculations.« less

A diagnostic for extreme ultraviolet spectroscopy was fielded on the sheared-flow-stabilized (SFS) fusion Z-pinch experiment (FuZE-Q) for the first time. The spectrometer collected time-gated plasma emission spectra in the 5–40 nm wavelength (30–250 eV) range for impurity identification, radiative power studies, and for plasma temperature and density measurements. The unique implementation of the diagnostic included fast (10 ns risetime) pulsed high voltage electronics and a multi-stage differential pumping system that allowed the vacuum-coupled spectrometer to collect three independently timed spectra per FuZE-Q shot while also protecting sensitive internal components. Analysis of line emission identifies oxygen (N-, C-, B-, Be-, Li-,more » and He-like O), peaking in intensity shortly after maximum current (>500 kA). This work provides a foundation for future high energy spectroscopy experiments on SFS Z-pinch devices.« less

At the W7-X stellarator, the bolometer system has measured an intensive radiation zone in the inner plasma region (at a normalized radius $$ρ$$ ~ 0.3–0.4) in the hydrogen plasma generated by electron cyclotron resonance heating; it differs from the normal plasma radiation distribution with an edge-localized emission zone. Spectroscopic diagnostics have recorded high-Z elements such as iron. This phenomenon happens in the plasma phases after gas supply turn-off, which results in all impurity relevant diagnostic signals increasing for several seconds. Despite the enhancement of the core radiation, the plasma energy confinement is improved. A transport analysis shows that this impuritymore » radiation behavior is associated with a low diffusion coefficient ($$D$$~ 0.02 m² s^-1) and a reversal of the convection around the radial position of the emission peak, which, under normal conditions, separates the zones of outward convection in the central (|$$V$$| ~ 0.1 m s^-1) and inward convection in the outer region (|$$V$$| ~ 0.3 m s^-1). An impurity accumulation around this radial position has been identified. The transport coefficients obtained are comparable with the theoretical predictions of collisional impurity transport. In the plasma phases studied, both impurity and energy confinement are enhanced. The mechanism responsible for the improvement is believed to be a reduction of micro-instabilities associated with the observed steepening of the density profile, initiated by a low edge plasma density (<1.0 × 10¹⁹ m^-3) after switching off the gas fueling. The normalized temperature and density gradients fulfil the condition for the suppression of ITG turbulence.« less

Wendelstein 7-X (W7-X) (Greifswald, Germany) is an advanced stellarator, which uses the modular coil concept to realize a magnetic configuration optimized for fusion-relevant plasma properties. The magnet system of the machine allows a variation of the rotational transform (iota) at the boundary. In the latest W7-X operational phase a dedicated configuration scan has been performed varying the rotational transform between magnetic configurations with iota = 5/4 and iota = 5/5 at the boundary. This paper presents an overview of the experiments and of the main results with respect to confinement and stability. The main observation is an increase of themore » plasma energy in several intermediate configurations of the scan when the 5/5-islands are close to the plasma boundary but still inside the last-closed-flux-surface. In addition, these configurations showed marked MHD-activity with a crashing behavior related to the 5/5-islands. The corresponding mode amplitude was correlated with the size of the internal 5/5 islands.« less

Wendelstein 7-X (W7-X), the largest advanced stellarator, is built to demonstrate high power, high performance quasi-continuous operation. Therefore, in the recent campaign, experiments were performed to prepare for long pulse operation, addressing three critical issues: the development of stable detachment, control of the heat and particle exhaust, and the impact of leading edges on plasma performance. The heat and particle exhaust in W7-X is realized with the help of an island divertor, which utilizes large magnetic islands at the plasma boundary. This concept shows very efficient heat flux spreading and favourable scaling with input power. Experiments performed to overload leadingmore » edges showed that the island divertor yields good impurity screening. A highlight of the recent campaign was a robust detachment scenario, which allowed reducing power loads even by a factor of ten. At the same time, neutral pressures at the pumping gap entrance yielded the particle removal rate close to the values required for stable density control in steady-state operation.« less

In this work, a Serpent 2 neutronics model of the Wendelstein 7-X (W7-X) stellarator is prepared, and an response function for the Scintillating-Fibre neutron detector (SciFi) is calculated using the model. The neutronics model includes the simplified geometry for the key components of the stellarator itself as well as the torus hall. The objective of the model is to assess the 14.1 MeV neutron flux from deuteron-triton fusions in W7-X, where the neutrons are modelled only until they have slowed down to 1 MeV energy. The key messages of this article are: demonstration of unstructured mesh geometry usage for stellarators,more » W7-X in particular; technical documentation of the model and first insights in fast neutron behaviour in W7-X, especially related to the SciFi: the model indicates that the superconducting coils are the strongest scatterers and block neutrons from large parts of the plasma. The back-scattering from e.g. massive steel support structures is found to be small. The SciFi will detect neutrons from an extended plasma volume in contrast to having an effective line-of-sight.« less

In the Wendelstein 7-X stellarator, the main locations of particle sources are expected to be the carbon divertors, baffles and graphite heat shield first wall. In this paper, the heat shield is implemented in EMC3-EIRENE to understand the expected areas and magnitudes of the recycling flux to this component. It is found that in the simulation the heat shield is not a significant source of recycling neutrals. The areas of simulated recycling flux are shown to correlate well with footprints of plasma-wetting seen in post-experimental campaign in-vessel inspection photos. EMC3-EIRENE reconstruction of line-integrated H-alpha measurements at the heat shield indicatemore » that the majority of emission does not come from local recycling neutrals. Rather, the H-alpha signals at the heat shield are dominated by ionization of neutrals which have leaked from the divertor/baffle region into the midplane. The magnitude of the H-alpha line emission from the synthetic reconstruction is consistent with the experiment, indicating that a large overestimation of heat shield recycling would occur if these measurements were assumed to be from local recycling sources. In the future, it may be possible to obtain some information of local recycling from the heat shield since it was found that the majority of the recycling flux occurs on two well-localized areas.« less

The island divertor concept is an innovative and promising idea to handle heat and particle exhaust in stellarators. At the Wendelstein 7-X (W7-X) stellarator, this divertor concept plays a central role in the device mission to demonstrate reactor relevant plasma confinement for steady-state time scales of up to 30 minutes in the high-performance campaign (OP2) starting in 2022. During the recently concluded first campaign with the inertially cooled island divertor, a large step in the experimental qualification of this divertor concept has been made. In discharges heated with electron cylotron resonance heating of 5-6 MW, central densities in the rangemore » of 0.7-1.2 × 10²⁰ m^-3 have been reached in combination with full divertor heat flux detachment. Also, significant neutral gas pressures and neutral compression ratios were shown for the first time in combination with reduced divertor particle flux. The divertor heat loads drop by an order of magnitude from >5 MW m^-2 to below 0.5 MW m-2 with increasing density, and substantial compression of neutrals reaching neutral pressure in the sub-divertor volume of >6.0 × 10^-4 mbar was seen. These elevated neutral pressure levels can be obtained and maintained with an up to 80% reduction of the particle fluxes onto the divertor target tiles. This discharge scenario was held stably detached for up to 28 seconds, which is equivalent to several hundred energy confinement times τ_E and longer than the time scales for current relaxation. No impurity accumulation was seen at constant Z_eff ≈ 1.5 and the stored energy stayed constant at levels of W_dia >600 kJ. The level of neutral pressure and compression reached in this scenario extrapolates well to the steady-state particle exhaust requirements for high-performance steady-state operation in OP2, in which the fully actively cooled high-heat-flux divertor will be available. Finally, an overview of this recently discovered divertor regime is given and the status of the physics understanding based on modeling of these regimes with the EMC3-EIRENE code is presented.« less

Wendelstein 7-X (W7-X) is a superconducting, long-pulse, optimized modular stellarator, presently operating at the Max Planck Institute for Plasma Physics, in Greifswald, Germany. After a lengthy design and construction period, it achieved first plasma on December 10, 2015, and has since completed three operational campaigns. The purpose of W7-X is to demonstrate the confinement and heat handling capabilities, in long pulse (30 minutes, … basically steady-state) operation, of an optimized stellarator with magnetic island divertors, first using normal hydrogen, and then later with deuterium. The machine has five-fold periodicity, and was built in five modules, fully enclosed in a close-fittingmore » cryostat. In Figure 1, you can see some of the 150 ports at the outside of the cryostat, with a human for scale. The torus hall and machine is licensed for producing up to 3·10¹⁹ DD neutrons per year.« less

The tokamak has been the primary magnetic fusion confinement platform in the USA since the Model C Stellarator was converted to the Symmetric Tokamak in 1970. The tokamak concept holds records for the best fusion performance parameters of any MFE concept, to-date. Presently, the only mid-scale operating US DOE magnetic fusion confinement facility, is the DIII-D tokamak at General Atomics in San Diego (at least until NSTX-U repairs are completed). But it has been more than thirty years since DIII-D was first put into operation. There is not a single new next-generation DOE-funded mid-scale (or larger) magnetic fusion confinement facilitymore » planned in the USA for the next 10 years. When and if there is a next generation new US domestic confinement machine funded by DOE, it should not be a tokamak.« less