Results 1 - 10 of 502
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[en] Experiments on the Pegasus ST are advancing the physics and technology basis of local helicity injection (LHI). LHI injects helicity with relatively intense electron current sources in the plasma edge. It creates high toroidal current, toroidally-averaged tokamak-like plasmas that have been efficiently transitioned to Ohmically driven tokamak plasmas. Tradeoffs between physics and engineering goals are tested with LHI systems on the low-field-side and the high-field-side of Pegasus, producing plasmas predominantly driven by non-solenoidal induction and DC helicity drive, respectively. An extensive LHI source development campaign comparing active arc sources, passive and gas-effused electrode sources lead to the selection of active arc sources for present and next-step LHI deployments. LHI plasmas with net toroidal current MA, eV, and m−3 are attained to date. A predictive 0D power-balance model describes experimental and partitions the active current drive sources. High-frequency MHD activity is found to be present during LHI current drive, in addition to modes previously found in NIMROD simulation and experiment. A new regime of reduced MHD activity was discovered where activity is suppressed, LHI CD efficiency improves, and long-pulse plasmas are sustained with . (paper)
[en] Understanding the many aspects of tokamak physics requires the development of quite sophisticated models. Moreover, in the operation of the devices, prediction of the future evolution of discharges can be of crucial importance, particularly in the case of the prediction of disruptions, which can cause serious damage to various parts of the machine. The determination of the limits of predictability is therefore an important issue for modelling, classifying and forecasting. In all these cases, once a certain level of performance has been reached, the question typically arises as to whether all the information available in the data has been exploited, or whether there are still margins for improvement of the tools being developed. In this paper, a theoretical information approach is proposed to address this issue. The excellent properties of the developed indicator, called the prediction factor (PF), have been proved with the help of a series of numerical tests. Its application to some typical behaviour relating to macroscopic instabilities in tokamaks has shown very positive results. The prediction factor has also been used to assess the performance of disruption predictors running in real time in the JET system, including the one systematically deployed in the feedback loop for mitigation purposes. The main conclusion is that the most advanced predictors basically exploit all the information contained in the locked mode signal on which they are based. Therefore, qualitative improvements in disruption prediction performance in JET would need the processing of additional signals, probably profiles. (paper)
[en] A computational study of long-legged tokamak divertor configurations is performed with the edge transport code UEDGE. Several divertor configurations are considered, with radially or vertically extended, tightly baffled, outer divertor legs and with or without a secondary X-point in the divertor leg volume. For otherwise identical conditions, a scan of the input power from the core plasma is performed. As the power is reduced to a threshold value, the plasma in the outer leg transitions to a fully detached state, which defines the upper limit on the power for detached divertor operation. Reducing the power further results in the detachment front shifting upstream but remains stable. At low power, the detachment front eventually moves all the way to the primary X-point, which is usually associated with degradation of the core plasma, and this defines the lower limit on the power for the detached divertor operation. For the studied parameters, for long-legged divertors, the detached operation window is quite large, in particular, for the X-point target configuration using a secondary X-point in the divertor leg volume, allowing a factor of 5–10 variations in the input power. For the same parameters, for the standard divertor configuration, the detached operation window is very small or even non-existent. In conclusion, the present modeling results suggest the possibility of stable fully detached divertor operation for a tokamak with tightly baffled extended divertor legs.
[en] Global gyrokinetic simulations find that realistic density gradients of energetic particles can simultaneously excite low frequency Alfven eigenmodes in toroidal geometry, beta-induced Alfven-acoustic eigenmode (BAAE) and beta-induced Alfven eigenmode (BAE), with similar radial mode widths and comparable linear growth rates even though damping rate of BAAE is much larger than BAE in the absence of energetic particles. This surprising result is attributed to non-perturbative effects of energetic particles that modify ideal BAAE mode polarizations and nonlocal geometry effects that invalidate radially local dispersion relation. Dominant mode changes from BAAE in a larger tokamak to BAE in a smaller tokamak due to the dependence of wave-particle resonance condition on the tokamak size. (letter)
[en] In reply to the Comment by Biel et al (2016 Nucl. Fusion 57 038001) on our recent papers Costley et al (2015 Nucl Fusion 55 033001) and Costley (2016 Nucl. Fusion 56 066003), we point out that the fusion triple product, nTτ E, and fusion power gain, Q fus, cannot be expressed solely in terms of independent engineering design variables such as major radius, R, and toroidal field, B; output performance variables such as normalised beta, β N, safety factor, q, and fusion power P fus, have to be invoked. Further, we show that the density limit has the effect of largely cancelling the size dependence in nTτ E and Q fus, which would otherwise be present, when these parameters are expressed in terms of P fus. Considerations of engineering aspects are also briefly discussed. (reply)
[en] For many years, machine learning tools have proved to be very powerful disruption predictors in tokamaks. On the other hand, the vast majority of the techniques deployed assume that the input data is independent and is sampled from exactly the same probability distribution for the training set, the test set and the final real time deployment. This hypothesis is certainly not verified in practice, since the experimental programmes evolve quite rapidly, resulting typically in ageing of the predictors and consequent suboptimal performance. This paper describes various adaptive training strategies that have been tested to maintain the performance of disruption predictors in non-stationary conditions. The proposed approaches have been implemented using new ensembles of classifiers, explicitly developed for the present application. The improvements in performance are unquestionable and, given the difficulties encountered so far in translating predictors from one device to another, the proposed adaptive methods from scratch can therefore be considered a useful option in the arsenal of alternatives envisaged for the next generation of devices, particularly at the very beginning of their operation. (paper)
[en] Radial fluxes of parallel momentum due to and magnetic drifts are shown to be correlated in tokamak plasmas. This correlation comes from the onset of poloidal convective cells generated by turbulence. The entire process requires a symmetry breaking mechanism, e.g. a mean shear flow. An analytical calculation shows that anti-correlation between the poloidal and parallel components of the turbulent Reynolds stress results in anti-correlation of the fluxes of parallel momentum generated by and curvature drifts. (paper)
[en] The achievement of new operational conditions in the RFX-mod-shaped tokamak allowed for the investigation of n = 0 stability and its relationship with a plasma shape cross-section. For this purpose, plasma response models have been produced from CREATE-L code equilibrium reconstructions based on experimental data, and their properties related to n = 0 stability have been studied in detail. In particular, we find that a pure proportional controller is unable to stabilize the n = 0 mode in all models related to experimental shots with the new operational conditions due to the peculiar property that the plasma response models are non-minimum phase systems. This phenomenon has been correlated with increased poloidal beta and enhanced plasma shape conditions. A physical interpretation is given as the rise of a combination of n = 0 vertical instability and n = 0 horizontal instability leading to actuators being ineffective at achieving complete stabilization. This conjecture has been confirmed by exploring the relationship between n = 0 stability of plasma equilibrium and the shape of a plasma cross-section with both vacuum equilibrium field harmonic analysis and field decay index investigation. (paper)
[en] A novel, rapid time-response, disruption mitigation system referred to as the electromagnetic particle injector (EPI) is described. This method can accurately deliver the radiative payload to the plasma center on a <10 ms time scale, much faster, and deeper, than what can be achieved using conventional methods. The EPI system accelerates a sabot electromagnetically. The sabot is a metallic capsule that can be accelerated to desired velocities by an electromagnetic impeller. At the end of its acceleration, within 2 ms, the sabot will release a radiative payload, which is composed of low-z granules, or a shell pellet containing smaller pellets. The primary advantage of the EPI concept over gas propelled systems is its potential to meet short warning time scales, while accurately delivering the required particle size and materials at the velocities needed for achieving the required penetration depth in high power ITER-scale discharges for thermal and runaway current disruption mitigation. The present experimental tests from a prototype system have demonstrated the acceleration of a 3.2 g sabot to over 150 m s−1 within 1.5 ms, consistent with the calculations, giving some degree of confidence that larger ITER-scale injector can be developed. (paper)
[en] At a typical fusion research site, experimental data is stored using archive technologies that deal with each discharge as an independent set of data. These technologies (e.g. MDSplus or HDF5) are typically supplemented with a database that aggregates metadata for multiple shots to allow for efficient querying of certain predefined quantities. Often, however, a researcher will need to extract information from the archives, possibly for many shots, that is not available in the metadata store or otherwise indexed for quick retrieval. To address this need, a new search tool called TokSearch has been added to the General Atomics TokSys control design and analysis suite. This tool provides the ability to rapidly perform arbitrary, parallelized queries of archived tokamak shot data (both raw and analyzed) over large numbers of shots. The TokSearch query API borrows concepts from SQL, and users can choose to implement queries in either Matlab™ or Python.