Mapping of the HIDRA stellarator magnetic flux surfaces

Abstract

The Hybrid Illinois Device for Research and Applications (HIDRA) is a classical stellarator designed for conducting plasma material interaction experiments and developing novel Plasma Facing Components (PFCs). Notably, the testing of two open-channel liquid lithium PFCs is imminent. Determining the shape of the plasma and its magnetic structure inside HIDRA is essential to carry out these tests. For this, electron traces were captured to build up the images of the HIDRA magnetic flux surfaces for several magnetic configurations, following the same procedure previously employed on the WEGA stellarator coupling an electron gun with a fluorescent detector. The FIELDLINES code has then been used to generate computational surfaces matching the experimental results. The obtained surfaces were found to be subject to a similar n = 1 error field as the one observed on WEGA, suggesting that the origin of this error field is inherent to the HIDRA vacuum vessel. Also, the effect of adding a vertical field was investigated, demonstrating the ability to radially shift the magnetic axis and move to a regime free of low-order rational resonances. This additional control over the HIDRA plasma and magnetic structure allows more freedom in setting up the PFC tests in the limiter and divertor regions.

First lithium experiments in HIDRA and evidence of helium retention during quasi-steady-state stellarator plasma operations

“…The pressure of the vacuum vessel is monitored via a Pfeiffer Vacuum Full Range gauge. The spectrometer is looking directly into the center of the vacuum chamber, however, FIELDLINES tracing and electron beam measurements, as well as past measurements in Greifswald, show that the plasma center is about 15 mm towards the high field side and has a hollow profile [33,37]. Thus, until further measurements of the electron density and temperature profile across the plasma using a Langmuir probe can be taken, the values of electron temperature and density are used in discussion with the understanding that more diagnostic data has to be collected.…”

“…The magnetic flux surfaces have been measured using an electron beam and fluorescing rod technique. A long exposure camera allows images of the flux lines to be superimposed to show the shape of the field and compare it to a ray-tracing program [33]. These show that the ray tracing program (FIELDLINES) is able to predict the shape of the magnetic flux surfaces very well.…”

“…HIDRA and its various systems are described by Johnson et al, in the following [31] and the on-axis magnetic field has been verified using a direct current (DC) hall probe [32]. The central iota profile was determined in Germany and has also been verified in Illinois [33]. The magnetic flux surfaces have been measured using an electron beam and fluorescing rod technique.…”

“…Figure 2(a) shows the current going into the coils and figure 2(b) is the on axis magnetic field. Figure 2(c) shows the calculated central iota that is calculated from [33]. Figure 2(d) shows the total amount of power going into the plasma and figure 2(e) shows the neutral pressure inside the vacuum chamber.…”

“…The poor particle confinement time in HIDRA, on the order of 1-5 ms, means that there will be a significant particle flux at the edge, with modeling suggesting a particle flux on the order of 10 20 -10 22 m −2 s −1 . This makes and helical (blue) coils, (b) is the on axis toroidal magnetic field, (c) is the calculated on-axis rotational transform from [33], (d) is the neutral gas pressure inside the vacuum vessel, (e) is the net power going into the vessel and thus available to heat the plasma, and (f) and (g) are the plasma parameters plasma density and plasma temperature respectively. HIDRA and ideal device to study PMI and to design PFCs and related materials.…”