Helically Symmetric Experiment

HSX
Helically Symmetric eXperiment
Device typeStellarator
LocationMadison, Wisconsin, US
AffiliationUniversity of Wisconsin–Madison
Technical specifications
Major radius1.2 m (3 ft 11 in)
Minor radius0.15 m (5.9 in)
Plasma volume0.44 m3
Magnetic field1.25 T (12,500 G)
Heating power100 kW (ECH)
Discharge duration0.2 s (pulse)
Plasma current13.4 kA
Plasma temperature2000–2500 eV (electron temp.)
History
Year(s) of operation1999–present
Links
Other linksHSX Device Parameters

The Helically Symmetric Experiment (HSX, stylized as Helically Symmetric eXperiment), is an experimental plasma confinement device at the University of Wisconsin–Madison, with design principles that are intended to be incorporated into a fusion reactor. The HSX is a modular coil stellarator which is a toroid-shaped pressure vessel with external electromagnets which generate a magnetic field for the purpose of containing a plasma. It began operation in 1999.

Background

A stellarator is a magnetic confinement fusion device that uses external magnetic coils to generate the magnetic fields needed to confine the high temperature plasma. In tokamaks and reversed field pinches, part of the necessary magnetic field is created by the interaction of external magnets and an electrical current flowing through the plasma. Stellarators may or may not have a large plasma current, and do not require one. Lack of a large plasma current makes a device suitable for steady-state fusion power plants.

However, due to non-axisymmetric nature of the fields, old stellarators have a combination of toroidal and helical modulation of the magnetic field lines, which leads to high transport of plasma out of the confinement volume at fusion-relevant conditions. This large transport in old stellarators can limit their performance as fusion reactors.

This loss of plasma can be reduced by tailoring the magnetic field geometry, for example by approximating a "quasi-symmetry". Quasi-helical symmetry used by HSX is one form of quasi-symmetry. The concept was proposed theoretically in 1983 by Allen Boozer. A usable approximation was first demonstrated numerically in 1988 by Jürgen Nührenberg.

Improvements in computer modeling capability have since made such configurations easier to calculate. Numerically calculated electromagnets shapes can directly produce a magnetic field configuration that combines the good confinement properties of tokamaks and the steady-state nature of conventional stellarators. Proving that such a machine can be built, and that it actually reduces transport as predicted, were the design goals of HSX.

Another successful result was obtained with the large optimised stellarator Wendelstein 7-X, which has a better particle confinement than that expected in ITER and achieves plasma duration of 30 minutes. Wendelstein 7-X does not use quasi-symmetry as the basis of its optimization.

Device

The magnetic field in HSX is generated by a set of 48 twisted coils arranged in four field periods. HSX typically operates at a magnetic field of 1 tesla at the center of the plasma column. A set of auxiliary coils is used to deliberately break the symmetry to mimic conventional stellarator properties for comparison.

The HSX vacuum vessel is made of stainless steel, and is helically shaped to follow the magnetic geometry.

Plasma formation and heating is achieved using 28 GHz, 100 kW electron cyclotron resonance heating (ECRH). A second 100 kW gyrotron has recently been installed on HSX to perform heat pulse modulation studies.

Operations

Plasmas as high as 3 kiloelectronvolts in temperature and about 8×1012/cc in density are routinely formed for various experiments.[citation needed]

Experiments have shown that edge magnetic islands affect particle fueling and exhaust. In HSX, the presence of a magnetic island chain at the plasma edge increases the plasma sourcing to exhaust ratio but reduces fueling efficiency by 25%. Moving the island radially inward decreases both the effective and global particle confinement times. This process is effective for controlling plasma fueling and helium exhaust times.

Subsystems, diagnostics

HSX has a large set of diagnostics to measure properties of plasma and magnetic fields. The following gives a list of major diagnostics and subsystems.

Goals and major achievements

HSX has made and continues to make fundamental contributions to the physics of quasisymmetric stellarators that show significant improvement over the conventional stellarator concept.[citation needed] These include:

Ongoing experiments

A large number of experimental and computational research works are being done in HSX by students, staff and faculties. Some of them are in collaboration with other universities and national laboratories, both in the US and abroad. Major research projects at present are listed below:

References

  1. ↑ Lobner, Pete (30 August 2017). "Helically Symmetric Experiment | The Lyncean Group of San Diego". Retrieved 2020-06-20.
  2. ↑ Nührenberg, J. (2010). "Development of quasi-isodynamic stellarators". Plasma Phys. Control. Fusion. 52 (124003). doi:10.1088/0741-3335/52/12/124003.
  3. ↑ Boozer, Allen (1983). "Transport and Isomorphic Equilibria". Phys. Fluids. 26 (496). doi:10.1063/1.864166.
  4. ↑ Nührenberg, J.; Zille, R. (1988). "Quasi-helically symmetric toroidal stellarators". Physics Letters A. 129 (2). doi:10.1016/0375-9601(88)90080-1.
  5. ↑ "HSX Device Parameters". HSX - Helically Symmetric eXperiment. Retrieved 2020-06-20.
  6. ↑ Stephey, L.; Bader, A.; Effenberg, F.; Schmitz, O.; Wurden, G.A.; et al. (2018). "Impact of magnetic islands in the plasma edge on particle fueling and exhaust in the HSX and W7-X stellarators". Physics of Plasmas. 25 (6). Bibcode:2018PhPl...25f2501S. doi:10.1063/1.5026324. hdl:21.11116/0000-0001-6AE2-9. S2CID 125652747.

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