Observation of three-dimensional motion of the pellet ablatant in the Large Helical Device

Abstract

Application of two-point stereoscopic diagnostics using a fast camera and bundled fibre has enabled the observation of the three-dimensional nature of the pellet trajectory in the Large Helical Device. It has been observed that the pellet trajectory deviates from its injection direction, toroidally and vertically depending on the direction of the tangentially applied neutral beams. The magnitude of the toroidal deviation is similar in the clockwise as well as counter-clockwise neutral beam directions and is of 15–20 cm with a deflection speed of up to 400 m s−1. In contrast, an asymmetry in trajectory deflection has been observed in the vertical direction. Experimental results also indicate that the starting radius of the pellet trajectory bending in the counter-clockwise neutral beam case is more inward to that of the plasma. Collectively this leads to less penetration of the pellet inside the plasma and is prominent in the case of the clockwise neutral beam. Additionally, this fact supports evidence that the fast ion plays an important role in the pellet ablation process in the Large Helical Device. The pellet deflection is explained by the rocket effect due to unilateral ablation by the fast ions. The possible cause of the difference in the vertical deflection is explained by considering the geometrical aspects of the magnetic field structure.

A comparison of the influence of plasmoid-drift mechanisms on plasma fuelling by cryogenic pellets in ITER and Wendelstein 7-X

“…Outward drifts are indeed observed for LFS injection [23,24]. However, experimental results obtained in the Large Helical Device, where HFS injections were carried out for the first time in a helical device, show that material drift is different from that observed for tokamaks [25,26]. More recently, first experiments in Wendelstein 7-X (W7-X), carried out with a blower-gun injector [27] during its OP 1.2 experimental campaign, appear to reaffirm this.…”

Three-dimensional dynamics of fluctuations appearing during pellet ablation process around a pellet in a fusion plasma experiment

“…The recent development of fast camera technology with higher time resolution has been expected to open a new horizon for understanding the dynamics of plasma 16 – 18 including pellet physics. At present, a fast camera is an optimum tool to visualize the pellet dynamics and ablation process with high spatial and temporal resolutions 19 – 25 , although quantitative measurements of physical quantities are difficult.…”

Experimental investigation of stereocamera's error to optimize dust observation on HL-2A tokamak

“…For example, on the stellarator of the Large Helical Device (Sakamoto & Yamada 2005; Mishra et al. 2011), the angle between the orientation directions of the two cameras is , which is able to achieve a measurement uncertainty of (Sakamoto & Yamada 2005). During an extremely busy tokamak operation campaign with multiple experiments performed in parallel, it is too difficult to reserve two viewing ports for a long time to observe dust dynamics from two perpendicular directions, as was performed by Roquemore et al.…”

“…From the principle of stereocameras (Brown 1971;Ivanov et al 1995;Prasad & Jensen 1995;Zhang 2000;Maddonni et al 2001;Luhmann et al 2011), for any orientation directions of two cameras, the 3-D coordinates of the observed objects are always able to be determined by using the triangulation method; however, the accuracy is related to the orientation directions of two cameras. For example, on the stellarator of the Large Helical Device (Sakamoto & Yamada 2005;Mishra et al 2011), the angle between the orientation directions of the two cameras is 18 • , which is able to achieve a measurement uncertainty of ≈30 mm (Sakamoto & Yamada 2005). During an extremely busy tokamak operation campaign with multiple experiments performed in parallel, it is too difficult to reserve two viewing ports for a long time to observe dust dynamics from two perpendicular directions, as was performed by Roquemore et al (2007), De Temmerman et al (2010, Yang et al (2013) and Shalpegin et al (2015).…”