Plasma Technology (United States)

Abstract At JET, Deuterium-Helium-3 (D- 3 He) experiments using ion cyclotron resonance heating (ICRH) with the three-ion scheme were conducted, involving thermal Deuterium (D), thermal Helium-3 ( 3 He), and a fast D beam injected via Neutral Beam Injection (NBI) and further heated using ICRH. In these plasmas, intense neutron emission at the level of ≈ 10 16 n/s was observed, and a non-thermal neutron spectrum was measured with the JET time of flight neutron spectrometer. Furthermore, the spatial profile of ≈ 16.4 MeV gamma rays originating from the 3 He(D,γ) 5 Li reaction was observed. This is a weak branch (≈ 10 -5 relative probability) of the more common 3 He(D,p)α reaction and it can be used to assess the production of alpha particles in these experiments. In this work we analyse a set of data from nuclear diagnostics in D-D NBI - 3 He experiments at JET and compare them with synthetic diagnostics based on TRANSP simulations of the fast deuterium distribution function, using the so-called "kick operator" to model power transfer from the wave to the beam fast deuterons. The neutron rate predicted by the TRANSP simulations broadly agrees with data measured with fission chambers. The shape of the neutron emission spectrum is also well understood, once finite Larmor radius effects are introduced into calculations. Data from the gamma ray camera are used to infer the alpha birth profile through tomographic reconstructions based on the TREVISO code. These were compared with corresponding calculations starting from TRANSP results and showing an overall good agreement with the experimental tomographies. The validated TRANSP results are finally used to discuss the modifications of the fast deuterium distribution function and of the alpha birth profile in a set of discharges.

Abstract The plasma transport analysis code TRANSP and full-orbit following code LORBIT were used to study fast-ion loss in the Experimental Advanced Superconducting Tokamak (EAST) under varying discharge parameters and heating scenarios, considering the synergistic effect of neutral beam injection (NBI) and ion cyclotron range of frequency (ICRF). To more faithfully simulate fast ion loss processes, the axisymmetric two dimensional poloidal contours of the EAST first wall and the antenna limiters were used as the loss boundary and then stretched toroidally to construct a three dimensional boundary structure. Results indicate that the fast ion loss diminishes as the plasma current (IP) and toroidal magnetic field (Bt) increase. Furthermore, the impact of different magnetic configurations on the fast-ion loss was considered. Increasing the distance between the antenna and the separatrix (R gapout ) can significantly reduced the fast-ion loss. When the toroidal magnetic field was reversed, .i.e., B t > 0, the lower single-null configuration was more effective in mitigating the fast-ion loss than double-null and upper single-null configurations. A significant reduction in the fraction of fast-ion loss was observed when an outer strike point was positioned on the vertical target plate. Finally, the effects of varying NBI energies, injection angles, and ICRF antenna phases were investigated. The results indicate that, within certain limits, fast-ion loss decreases as the NBI injection angle increases, whereas higher NBI injection energies lead to more fast ion escape. In contrast, variations in the ICRF antenna phase exert only a marginal effect on fast-ion loss.

Abstract Neoclassical tearing modes (NTM) are a class of resistive instabilities that arise in tokamaks at rational surfaces and form magnetic islands. These islands flatten the pressure gradient, reducing plasma performance and may lead to disruptions should they grow large enough. Driving current within the island can stabilize an NTM, which has been achieved with electron cyclotron current drive (ECCD) on multiple devices. An alternative to ECCD is lower hybrid current drive (LHCD), which offers the advantages of increased current drive efficiency and reduced system cost. LHCD has been viewed as poorly suited for NTM suppression due to the large spatial extent of the driven current when in the multi-pass absorption regime (as has been the case in all past LHCD experiments). However, the driven current is more localized when in the single pass absorption regime, as is predicted for the DIII-D high field side (HFS) LHCD experiment. This work evaluates the feasibility of NTM suppression with HFS LHCD on DIII-D by predicting the island growth rate for a set of representative DIII-D plasmas via the modified Rutherford equation with and without the application of LHCD. In these plasmas, NTM suppression is achieved at reasonable power levels, even with finite misalignment between LH current and the island. The effect of current condensation was included and found to be most significant at smaller island sizes, assuming an experimentally typical temperature perturbation amplitude of 10%.

In preparation for high field side lower hybrid current drive (HFS LHCD) experiments in DIII-D, the HFS LHCD was to be commissioned and physics experiments commence once the system operated up to 300 kW for 0.5 s. The initial physics experiments sought to characterize coupling, wave propagation, and driven current measurements. In HFS LHCD first campaign, the maximum power was limited to <200 kW due to waveguide pressure leaks limiting the number of available modules and power per module. Here we summarize commissioning progress and initial physics observations. The HFS LHCD coupler was optimized for high qmin, DIII-D discharges where efficient off-axis current at r/a~0.6-0.8 is desired. The coupler n|| spectrum is peaked at 2.7 and is predicted to generate ~0.14 MA/MW coupled for 1.6 MW injected. In preparation, the HFS scrape-off layer density profile was characterized and found to have steeper profiles and lower fluctuation levels than the low field side. Furthermore, the HFS SOL density profile can be accurately predicted using global plasma quantities using machine learning. Thus far, one module has injected ~100 kW for 0.5 s with <5% reflected power. Nonthermal electrons have been observed on lower frequency channels of the electron cyclotron emission radiometer correlated with the LH power indicating core wave absorption. To avoid 30R neutral beam heat flux, a split launcher is proposed to avoid the high heat flux region while maintaining power spectrum and directivity.

Many people believe that the only way to meet the world's increasing energy needs is through nuclear fusion, which is the release of energy through the combination of atomic nuclei. Nuclear fusion has the ability to provide almost endless amounts of clean energy with minimum waste, in contrast to nuclear fission, which breaks atoms and creates dangerous waste. the most up-to-date developments in nuclear fusion technology, with an emphasis on the strides taken towards net-positive energy generation, the major obstacles now standing in the way, and the bright prospects for fusion going forward as a sustainable energy source. Progress in experimental fusion reactors, including stellarators and tokamaks, is reviewed, with special emphasis on advances in laser-based inertial confinement methods, plasma confinement, and superconducting magnets. The International Thermonuclear Experimental Reactor (ITER) is one of several global cooperative initiatives that are working to prove that commercial-scale fusion electricity is possible. In spite of a lot of progress, there are still certain technical hurdles to overcome, such keeping the plasma stable, getting the ignition conditions right, and scaling up fusion reactors. Nevertheless, nuclear fusion has the potential to completely transform the way the world produces energy if current trends in materials science, superconductivity, and sophisticated computer simulations continue. the fusion energy's part in creating a sustainable and carbon-neutral future, the possible economic and environmental implications, and the commercialization timescale.

Abstract Understanding and accounting for uncertainty is one aspect of ensuring next-step tokamaks such as SPARC will robustly achieve their goals. While traditional Plasma OPerating CONtour (POPCON) analyses guide design, they often overlook the significant impact of uncertainties in scaling laws, plasma profiles, and impurity concentrations on performance predictions. This work confronts these challenges by introducing statistical POPCONs, which leverage Monte Carlo analysis to quantify the sensitivity of SPARC’s operating points (Creely et al 2020 J. Plasma Phys. 86 5) to these crucial variables. For profiles, a physically motivated gradient-based functional form is introduced. We further develop a multi-fidelity Bayesian optimization workflow that effectively identifies operating points maximizing the probability of meeting performance goals, which gives a significant speed-up over brute force search methods. Our findings reveal that accounting for these uncertainties leads to an optimal operating point different from deterministic predictions, which balances H-mode access, confinement, impurity dilution, and auxiliary power.

Cd3As2 is a Dirac semimetal that hosts a chiral anomaly, functioning as a platform to realize energy applications. We use fast-ion implantation to enhance the negative longitudinal magnetoresistance (NLMR)─signature of a chiral anomaly─in Nb-doped Cd3As2 thin films. High-energy ion implantation is used to investigate semiconductors and nuclear materials but is rarely employed to study topological materials. We use electrical transport and transmission electron microscopy to characterize the NLMR and crystallinity of Nb-doped Cd3As2. We find surface-doped thin films display a maximum NLMR around B = 7 T and bulk-doped thin films display a maximum over B = 9 T─all while maintaining crystallinity. This is more than a 100% relative enhancement of the maximum NLMR compared to pristine Cd3As2. Our work demonstrates the potential of high-energy ion implantation as a practical route to explore chiralitronic properties in topological semimetals.

We present an improved diffraction model for phase contrast imaging (PCI), relevant to the measurement of electron density fluctuations in magnetically confined fusion plasmas. The model applies to optical systems containing multiple special components, such as phase plates, localization masks, and apertures, connected by segments describable in terms of ray transfer matrices. Fresnel diffraction in the connecting segments may be described using linear canonical transforms, while the special components are characterized by their respective transfer functions. For a PCI system with ideal placement of the phase plate and localization mask, deviations between the Fresnel and Fraunhofer diffraction models are entirely determined by the distance from the diffraction plane to the object plane of the optical system. Application of the developed model to the synthetic PCI diagnostic at the Wendelstein 7-X (W7-X) stellarator shows minimal impact of diffraction on the DC detector power. However, significant amplitude modifications due to phase scintillation from diffraction are expected for high-wavenumber PCI sound wave calibration signals at W7-X, in qualitative agreement with the experimental observations. In contrast to previous expectations, diffraction is found to have only a small impact on the localization mask response characteristics at W7-X. Based on this observation, we show that the synthetic PCI mask response at W7-X can be reproduced by a model which only considers the signal at the beam center in the image plane without diffraction. We further use a similar model to reproduce the experimentally observed mask response from the DIII-D tokamak, indicating the general applicability of such models.