The Helically Symmetric Experiment (HSX, stylized as Helically Symmetric eXperiment), is an experimental
plasma confinement device at the
University of Wisconsin–Madison
A university () is an educational institution, institution of higher education, higher (or Tertiary education, tertiary) education and research which awards academic degrees in several Discipline (academia), academic disciplines. Universities ty ...
, with design principles that are intended to be incorporated into a
fusion reactor
Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion reactions. In a fusion process, two lighter atomic nuclei combine to form a heavier nucleus, while releasing energy. Devices ...
. The HSX is a modular coil
stellarator
A stellarator is a plasma device that relies primarily on external magnets to confine a plasma. Scientists researching magnetic confinement fusion aim to use stellarator devices as a vessel for nuclear fusion reactions. The name refers to the ...
which is a
toroid
In mathematics, a toroid is a surface of revolution with a hole in the middle. The axis of revolution passes through the hole and so does not intersect the surface. For example, when a rectangle is rotated around an axis parallel to one of its ...
-shaped
pressure vessel with external
electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in ...
s which generate a
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
for the purpose of containing a plasma. It began operation in 1999.
Background
A
stellarator
A stellarator is a plasma device that relies primarily on external magnets to confine a plasma. Scientists researching magnetic confinement fusion aim to use stellarator devices as a vessel for nuclear fusion reactions. The name refers to the ...
is a
magnetic confinement fusion
Magnetic confinement fusion is an approach to generate thermonuclear fusion power that uses magnetic fields to confine fusion fuel in the form of a plasma. Magnetic confinement is one of two major branches of fusion energy research, along with ...
device which generates all required magnetic fields to confine high temperature plasma by external magnetic coils. In contrast, in
tokamaks
A tokamak (; russian: токамáк; otk, 𐱃𐰸𐰢𐰴, Toḳamaḳ) is a device which uses a powerful magnetic field to confine plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being ...
and
reversed field pinch
A reversed-field pinch (RFP) is a device used to produce and contain near-thermonuclear Plasma (physics), plasmas. It is a Pinch (magnetic fusion), toroidal pinch which uses a unique magnetic field configuration as a scheme to magnetically con ...
es, the magnetic field is created by the interaction of external magnets and an
electrical current flowing through the plasma. The lack of this large externally driven plasma current makes stellarators 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, solved in the
Wendelstein 7-X
The Wendelstein 7-X (abbreviated W7-X) reactor is an experimental stellarator built in Greifswald, Germany, by the Max Planck Institute for Plasma Physics (IPP), and completed in October 2015.[fusion reactor
Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion reactions. In a fusion process, two lighter atomic nuclei combine to form a heavier nucleus, while releasing energy. Devices ...]
s.
This problem can be largely reduced by tailoring the magnetic field geometry. The dramatic improvements in computer modeling capability in the last two decades has helped to "optimize" the magnetic geometry to reduce this transport, resulting in a new class of stellarators called "
quasi-symmetric stellarators". Computer-modeled odd-looking
electromagnets
An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in t ...
will directly produce the needed magnetic field configuration. These devices combine the good confinement properties of tokamaks and the steady-state nature of conventional stellarators. The Helically Symmetric Experiment (HSX) at the University of Wisconsin-Madison is such a quasi-helically symmetric stellarator (
helical axis of symmetry).
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
Electron cyclotron resonance (ECR) is a phenomenon observed in plasma physics, condensed matter physics, and accelerator physics. It happens when the frequency of incident radiation coincides with the natural frequency of rotation of electrons in m ...
heating (ECRH). A second 100 kW
gyrotron
High-power 140 GHz gyrotron for plasma heating in the Wendelstein 7-X fusion experiment, Germany.
A gyrotron is a class of high-power linear-beam vacuum tubes that generates millimeter-wave electromagnetic waves by the cyclotron resonance of e ...
has recently been installed on HSX to perform heat pulse modulation studies.
Operations
Plasmas as high as 3
kiloelectronvolt
In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacuum ...
s in temperature and about 8/cc in density are routinely formed for various experiments.
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.
*
Thomson scattering
Thomson scattering is the elastic scattering of electromagnetic radiation by a free charged particle, as described by classical electromagnetism. It is the low-energy limit of Compton scattering: the particle's kinetic energy and photon frequen ...
* Diagnostic neutral beam
*
Electron cyclotron resonance
Electron cyclotron resonance (ECR) is a phenomenon observed in plasma physics, condensed matter physics, and accelerator physics. It happens when the frequency of incident radiation coincides with the natural frequency of rotation of electrons in m ...
heating system
* Electron cyclotron emission
radiometers
* Charge exchange recombination spectroscopy
*
Interferometer
* Motional
Stark effect
* Heavy ion beam probe (coming soon)
* Laser blow-off
* Hard and soft-
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
detectors
*
Mirnov coil
Mirnov oscillations (a.k.a. magnetic oscillations) are amplitude perturbations of the magnetic field in a plasma. It is named after Sergei V. Mirnov who designed a probe to measure these oscillations in 1965. The probe name is Mirnov coil.
Mir ...
s
*
Rogowski coil
A Rogowski coil, named after Walter Rogowski, is an electrical device for measuring alternating current (AC) or high-speed current pulses. It sometimes consists of a helical coil of wire with the lead from one end returning through the centre o ...
s
* Passive spectroscopy
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. These include:
* Measuring large ion flows in the direction of quasisymmetry
* Reduced flow damping in the direction of quasisymmetry
* Reduced passing particle deviation from a
flux surface
In magnetic confinement fusion, a flux surface is a surface on which magnetic field lines lie. Since the magnetic field is divergence-free (and magnetic nulls are undesirable), the Poincare-Hopf theorem implies that such a surface must be either ...
* Reduced direct loss orbits
* Reduced neoclassical transport
* Reduced equilibrium parallel currents because of the high effective transform
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 USA and abroad. Major research projects at present are listed below:
* Effect of quasi-symmetry on plasma flows
* Impurity transport
* Radio frequency heating
*
Supersonic plasma fueling and the neutral population
* Heat pulse propagation experiments to study thermal transport
* Interaction of turbulence and flows in HSX and the effects of quasi-symmetry on the determination of the radial electric field
* Equilibrium reconstruction of the plasma density, pressure and current profiles
* Effects of viscosity and symmetry on the determination of the flows and the radial electric field
* Divertor flows, particle edge fluxes
* Effect of radial electric field on the bootstrap current
* Effect of quasi-symmetry on fast ion confinement
References
Additional resources
*
External links
*
Experimental Tests of Quasisymmetry in HSX. TalmadgeSlide 4 compares with tokamak
{{Authority control
Stellarators
Plasma physics
University of Wisconsin–Madison