Elevated Electron Temperature Coincident with Observed Fusion Reactions in a Sheared-Flow-Stabilized Z Pinch

The sheared-flow-stabilized Z pinch concept has been studied extensively and is able to produce fusion-relevant plasma parameters along with neutron production over several microseconds. We present here elevated electron temperature results spatially and temporally coincident with the plasma neutron source. An optical Thomson scattering apparatus designed for the FuZE device measures temperatures in the range of 1-3 keV on the axis of the device, 20 cm downstream of the nose cone. The 17-fiber system measures the radial profiles of the electron temperature. Scanning the laser time with respect to the neutron pulse time over a series of discharges allows the reconstruction of the T_{e} temporal response, confirming that the electron temperature peaks simultaneously with the neutron output, as well as the pinch current and inductive voltage generated within the plasma. Comparison to spectroscopic ion temperature measurements suggests a plasma in thermal equilibrium. The elevated T_{e} confirms the presence of a plasma assembled on axis, and indicates limited radiative losses, demonstrating a basis for scaling this device toward net gain fusion conditions.

Radial localization of electron temperature pedestal and ELM-like events using ECE measurements at Wendelstein 7-X

A magnetic configuration scan was performed at Wendelstein 7-X stellarator by varying the rotational transform to analyze the plasma confinement for magnetic configurations with different edge magnetic island locations and sizes. For the magnetic configurations, where the 5/5 island chain was moved inside the last closed flux surface, it was observed with electron cyclotron emission measurements that an electron temperature, Te, pedestal develops in the plasma buildup phase and followed by the edge localized mode (ELM)-like crashes. From the mapping of the island to the plasma radius from HINT equilibrium, it was found that the Te pedestal is formed at the island location on the high field side of the plasma. The ELM-like crashes occur at the location of the pedestal and the transport barrier is broken typically with an energy loss of 3-4% during a single ELM-like event. The frequency and the amplitude of the ELM-like crashes were observed to be changing with island size, plasma heating power and density. Additionally during the plasma decay, after the heating was switched-off, a transition to degraded plasma confinement state was observed with changed Te profile gradients, faster decay rate of diamagnetic energy, and increased H-alpha levels.

Towards a new image processing system at Wendelstein 7-X: From spatial calibration to characterization of thermal events

Wendelstein 7-X (W7-X) is the most advanced fusion experiment in the stellarator line and is aimed at proving that the stellarator concept is suitable for a fusion reactor. One of the most important issues for fusion reactors is the monitoring of plasma facing components when exposed to very high heat loads, through the use of visible and infrared (IR) cameras. In this paper, a new image processing system for the analysis of the strike lines on the inboard limiters from the first W7-X experimental campaign is presented. This system builds a model of the IR cameras through the use of spatial calibration techniques, helping to characterize the strike lines by using the information given by real spatial coordinates of each pixel. The characterization of the strike lines is made in terms of position, size, and shape, after projecting the camera image in a 2D grid which tries to preserve the curvilinear surface distances between points. The description of the strike-line shape is made by means of the Fourier Descriptors.

A divertor scraper observation system for the Wendelstein 7-X stellarator

Two graphite divertor elements called scrapers have been installed on the Wendelstein 7-X stellarator in the throat of the magnetic island divertor. To diagnose one, we have designed, built, calibrated, and installed a new infrared/visible imaging endoscope system to enable detailed observations of the plasma interactions and heat loads at one of the scrapers and the nearby divertor surfaces. The new system uses a shuttered pinhole-protected pair of 90° off-axis 228 mm focal length aluminum parabolic mirrors, and two flat turning metal mirrors, to send light to a sapphire vacuum window 1.6 meters away, beyond which we have co-located telephoto lens-based infrared and visible cameras. The back-to-back off-axis parabolas serve to cancel out most aberrations, enabling the use of off-the-shelf commercial optics outside of the vessel. For the infrared, we use a 3-5 μm 1-megapixel FLIR SC8303HD camera and for the visible, a 5-megapixel CMOS PCO 5.5 edge camera. A short 1-m quartz pickoff fiber is used to send 200-1100 nm light to a compact spectrometer, also located in the same iron shield box as the cameras. The camera field of view covers the 700 mm length of the scraper, and includes locations monitored by thermocouples and Langmuir probes embedded in some of the scraper tiles. Predicted and actual optical test performances of the overall system are compared.

Overview of diagnostic performance and results for the first operation phase in Wendelstein 7-X (invited)

Wendelstein 7-X, a superconducting optimized stellarator built in Greifswald/Germany, started its first plasmas with the last closed flux surface (LCFS) defined by 5 uncooled graphite limiters in December 2015. At the end of the 10 weeks long experimental campaign (OP1.1) more than 20 independent diagnostic systems were in operation, allowing detailed studies of many interesting plasma phenomena. For example, fast neutral gas manometers supported by video cameras (including one fast-frame camera with frame rates of tens of kHz) as well as visible cameras with different interference filters, with field of views covering all ten half-modules of the stellarator, discovered a MARFE-like radiation zone on the inboard side of machine module 4. This structure is presumably triggered by an inadvertent plasma-wall interaction in module 4 resulting in a high impurity influx that terminates some discharges by radiation cooling. The main plasma parameters achieved in OP1.1 exceeded predicted values in discharges of a length reaching 6 s. Although OP1.1 is characterized by short pulses, many of the diagnostics are already designed for quasi-steady state operation of 30 min discharges heated at 10 MW of ECRH. An overview of diagnostic performance for OP1.1 is given, including some highlights from the physics campaigns.

A high resolution IR/visible imaging system for the W7-X limiter

A high-resolution imaging system, consisting of megapixel mid-IR and visible cameras along the same line of sight, has been prepared for the new W7-X stellarator and was operated during Operational Period 1.1 to view one of the five inboard graphite limiters. The radial line of sight, through a large diameter (184 mm clear aperture) uncoated sapphire window, couples a direct viewing 1344 × 784 pixel FLIR SC8303HD camera. A germanium beam-splitter sends visible light to a 1024 × 1024 pixel Allied Vision Technologies Prosilica GX1050 color camera. Both achieve sub-millimeter resolution on the 161 mm wide, inertially cooled, segmented graphite tiles. The IR and visible cameras are controlled via optical fibers over full Camera Link and dual GigE Ethernet (2 Gbit/s data rates) interfaces, respectively. While they are mounted outside the cryostat at a distance of 3.2 m from the limiter, they are close to a large magnetic trim coil and require soft iron shielding. We have taken IR data at 125 Hz to 1.25 kHz frame rates and seen that surface temperature increases in excess of 350 °C, especially on leading edges or defect hot spots. The IR camera sees heat-load stripe patterns on the limiter and has been used to infer limiter power fluxes (∼1-4.5 MW/m²), during the ECRH heating phase. IR images have also been used calorimetrically between shots to measure equilibrated bulk tile temperature, and hence tile energy inputs (in the range of 30 kJ/tile with 0.6 MW, 6 s heating pulses). Small UFO's can be seen and tracked by the FLIR camera in some discharges. The calibrated visible color camera (100 Hz frame rate) has also been equipped with narrow band C-III and H-alpha filters, to compare with other diagnostics, and is used for absolute particle flux determination from the limiter surface. Sometimes, but not always, hot-spots in the IR are also seen to be bright in C-III light.

Spectroscopic imaging of limiter heat and particle fluxes and the resulting impurity sources during Wendelstein 7-X startup plasmas

A combined IR and visible camera system [G. A. Wurden et al., "A high resolution IR/visible imaging system for the W7-X limiter," Rev. Sci. Instrum. (these proceedings)] and a filterscope system [R. J. Colchin et al., Rev. Sci. Instrum. 74, 2068 (2003)] were implemented together to obtain spectroscopic data of limiter and first wall recycling and impurity sources during Wendelstein 7-X startup plasmas. Both systems together provided excellent temporal and spatial spectroscopic resolution of limiter 3. Narrowband interference filters in front of the camera yielded C-III and H_α photon flux, and the filterscope system provided H_α, H_β, He-I, He-II, C-II, and visible bremsstrahlung data. The filterscopes made additional measurements of several points on the W7-X vacuum vessel to yield wall recycling fluxes. The resulting photon flux from both the visible camera and filterscopes can then be compared to an EMC3-EIRENE synthetic diagnostic [H. Frerichs et al., "Synthetic plasma edge diagnostics for EMC3-EIRENE, highlighted for Wendelstein 7-X," Rev. Sci. Instrum. (these proceedings)] to infer both a limiter particle flux and wall particle flux, both of which will ultimately be used to infer the complete particle balance and particle confinement time τ_P.

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.

A New Vision for Fusion Energy Research: Fusion Rocket Engines for Planetary Defense

We argue that it is essential for the fusion energy program to identify an imagination-capturing critical mission by developing a unique product which could command the marketplace. We lay out the logic that this product is a fusion rocket engine, to enable a rapid response capable of deflecting an incoming comet, to prevent its impact on the planet Earth, in defense of our population, infrastructure, and civilization. As a side benefit, deep space solar system exploration, with greater speed and orders-of-magnitude greater payload mass would also be possible.

An in situ runaway electron diagnostic for DIII-D

We are designing a new diagnostic based on laser inverse Compton scattering to study the dynamics of runaway electron formation during killer-pellet triggered disruptions in DIII-D, and their subsequent loss. We can improve the expected S/N ratio by using a high-intensity short-pulse laser combined with gated x-ray imagers. With 80 ps sampling, time-of-flight spatial resolution within the laser chord can be obtained. We will measure the time-resolved spatial profile and energy distribution of the runaway electrons while they are in the core of the tokamak plasma.

Bright laser-driven neutron source based on the relativistic transparency of solids

Neutrons are unique particles to probe samples in many fields of research ranging from biology to material sciences to engineering and security applications. Access to bright, pulsed sources is currently limited to large accelerator facilities and there has been a growing need for compact sources over the recent years. Short pulse laser driven neutron sources could be a compact and relatively cheap way to produce neutrons with energies in excess of 10 MeV. For more than a decade experiments have tried to obtain neutron numbers sufficient for applications. Our recent experiments demonstrated an ion acceleration mechanism based on the concept of relativistic transparency. Using this new mechanism, we produced an intense beam of high energy (up to 170 MeV) deuterons directed into a Be converter to produce a forward peaked neutron flux with a record yield, on the order of 10(10)  n/sr. We present results comparing the two acceleration mechanisms and the first short pulse laser generated neutron radiograph.

A multi-frame soft x-ray pinhole imaging diagnostic for single-shot applications

For high energy density magnetized target fusion experiments at the Air Force Research Laboratory FRCHX machine, obtaining multi-frame soft x-ray images of the field reversed configuration (FRC) plasma as it is being compressed will provide useful dynamics and symmetry information. However, vacuum hardware will be destroyed during the implosion. We have designed a simple in-vacuum pinhole nosecone attachment, fitting onto a Conflat window, coated with 3.2 mg∕cm(2) of P-47 phosphor, and covered with a thin 50-nm aluminum reflective overcoat, lens-coupled to a multi-frame Hadland Ultra intensified digital camera. We compare visible and soft x-ray axial images of translating (~200 eV) plasmas in the FRX-L and FRCHX machines in Los Alamos and Albuquerque.

Divertor IR thermography on Alcator C-Mod

Alcator C-Mod is a particularly challenging environment for thermography. It presents issues that will similarly face ITER, including low-emissivity metal targets, low-Z surface films, and closed divertor geometry. In order to make measurements of the incident divertor heat flux using IR thermography, the C-Mod divertor has been modified and instrumented. A 6° toroidal sector has been given a 2° toroidal ramp in order to eliminate magnetic field-line shadowing by imperfectly aligned divertor tiles. This sector is viewed from above by a toroidally displaced IR camera and is instrumented with thermocouples and calorimeters. The camera provides time histories of surface temperatures that are used to compute incident heat-flux profiles. The camera sensitivity is calibrated in situ using the embedded thermocouples, thus correcting for changes and nonuniformities in surface emissivity due to surface coatings.

Experimental demonstration of plasma-drag acceleration of a dust cloud to hypervelocities

Simultaneous acceleration of hundreds of dust particles to hypervelocities by collimated plasma flows ejected from a coaxial gun is demonstrated. Graphite and diamond grains with radii between 5 and 30 microm, and flying at speeds up to 3.7 km/s, have been recorded with a high-speed camera. The observations agree well with a model for plasma-drag acceleration of microparticles much larger than the plasma screening length.