Collisionless alpha -particle confinement in stellarators

One of the main goals of Wendelstein 7-X (W7-X) is to demonstrate the fast particle confinement properties of the quasi-isodynamic stellarator concept. Fast particle populations will be produced either by Neutral Beam Injection (NBI) or by minority Ion Cyclotron Resonant Heating (ICRH). A fraction of these particles are expected to be lost (even without collisions), despite the optimisation procedure used for the W7-X design. Confinement properties of NBI particles in W7-X were presented in the paper of M. Drevlak et al Nucl. Fusion 2014. A detailed study is presented here where the loss patterns of an NBI population are described. In particular, focusing on a high-mirror equilibrium, the confinement of fast ions with varying energy injection is studied under collisional conditions. It is found that collisions are not only responsible for classical transport losses but also enhance drift induced losses caused by the finite orbit width of trapped particles. Moreover, an asymmetry is found in the toroidal position of particle losses which can be explained by local variation in the equilibrium field. The effects of a neoclassically resolved radial electric field are also investigated. Fast particle confinement is significantly improved by the associated E × B drift. In particular, an increasing radial electric field helps to reduce and even stop the losses due the 3D equilibrium structure for times comparable to slowing down time.

show abstract

“…Losses induced by collisionless stochastic diffusion of fast ions was introduced in Ref. [1] and further developed in Ref. [2].…”

Section: Introductionmentioning

confidence: 99%

Fast particle loss channels in Wendelstein 7-X

et al. 2016

View full text Add to dashboard Cite

show abstract

“…Hence, closure of separatrix shapes within the plasma could be added as an additional criterion during the optimization of stellarator configurations. Minimization of fast particle loss calculated with a Monte Carlo code is often included in the set of criteria in the process of the stellarator optimization (see, e.g., [13]). However, closure of separatrix shapes within the plasma is a criterion evaluated much faster than the particle loss with a Monte Carlo code (although, admittedly, the latter criterion takes into account loss channels other than stochastic diffusion of transitioning particles).…”

Section: Discussionmentioning

confidence: 99%

“…Numerical simulations of the alpha particle behaviour [13] have shown that the alpha particle confinement in optimized configurations is indeed much better than that in traditional stellarators. Most particles in the optimized configurations, including a majority of trapped particles, are confined for times much longer than the characteristic times of the orbital motion.…”

Section: Introductionmentioning

confidence: 99%

Mitigation of stochastic diffusion losses in optimized stellarators

Tykhyy

Kolesnichenko

Yakovenko

et al. 2007

Plasma Phys. Control. Fusion

View full text Add to dashboard Cite

Abstract. The stochastic diffusion (SD) resulting from collisionless transformations between locally trapped and locally passing orbits may lead to a significant loss of energetic particles from stellarator plasmas (Beidler C D et al 2001 Phys. Plasmas 8 2731. In this work, a method to mitigate the consequences of SD is suggested. The method consists in changing the shapes of the separatrices between the locally trapped and locally passing states so that most separatrices do not cross the plasma boundary, which is achieved with a suitable modification of the magnetic configuration. It is demonstrated analytically that in this way the particle loss because of the SD may be reduced significantly, even though the SD still occurs. In particular, it is possible to protect fast particles inside a certain radius from SD loss. It is shown that a variation of the electric field can also affect the SD loss. The modifications of the magnetic configuration and the electric field required for the mitigation are quantified for a configuration of the stellarator Wendelstein 7-X.

show abstract

“…For thermal particles, the problem is mitigated by collisionless detrapping associated with poloidal ExB drift, but for fast particles this helpful process is weak due to the smaller ratio of ExB to parallel speed. The scale of the fast-particle confinement problem is clearly shown in [29], which found all trapped alpha particles to be lost in ∼ 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 reactor parameters can be estimated as ≥ 0.1s. Fast particle confinement in modern optimized designs such as W7-X and NCSX is much improved compared to conventional stellarators [30], but remains one of the main challenges for the concept.…”

Section: Energetic Ion Confinementmentioning

confidence: 99%

Recent advances in stellarator optimization

et al. 2017

View full text Add to dashboard Cite

Computational optimization has revolutionized the field of stellarator design. To date, optimizations have focused primarily on optimization of neoclassical confinement and ideal MHD stability, although limited optimization of other parameters has also been performed.The purpose of this paper is to outline a select set of new concepts for stellarator optimization that, when taken as a group, present a significant step forward in the stellarator concept.One of the criticisms that has been leveled at existing methods of design is the complexity of the resultant field coils. Recently, a new coil optimization code -COILOPT++, which uses a spline instead of a Fourier representation of the coils, -was written and included in the STELLOPT suite of codes. The advantage of this method is that it allows the addition of real space constraints on the locations of the coils. The code has been tested by generating coil designs for optimized quasi-axisymmetric stellarator plasma configurations of different aspect ratios. As an initial exercise, a constraint that the windings be vertical 1 was placed on large major radius half of the non-planar coils. Further constraints were also imposed that guaranteed that sector blanket modules could be removed from between the coils, enabling a sector maintenance scheme. Results of this exercise will be presented.New ideas on methods for the optimization of turbulent transport have garnered much attention since these methods have led to design concepts that are calculated to have reduced turbulent heat loss. We have explored possibilities for generating an experimental database to test whether the reduction in transport that is predicted is consistent with experimental observations. To this end, a series of equilibria that can be made in the now latent QUASAR experiment have been identified that will test the predicted transport scalings. Fast particle confinement studies aimed at developing a generalized optimization algorithm are also discussed. A new algorithm developed for the design of the scraper element on W7-X is presented along with ideas for automating the optimization approach.

show abstract

Collisionless alpha -particle confinement in stellarators

Cited by 82 publications

References 7 publications

Fast particle loss channels in Wendelstein 7-X

Fast particle loss channels in Wendelstein 7-X

Mitigation of stochastic diffusion losses in optimized stellarators

Recent advances in stellarator optimization

Contact Info

Product

Resources

About