In first approximation, charged particles spiral along magnetic field-lines. To avoid particle losses along the field-lines, field-lines have to be closed. To contrast the tendency of the plasma to expand like a gas of finite pressure, field-lines have to be helically twisted. Closed, helical field-lines are the basic principle by which we confine plasmas.
Closed, helical field-lines can be entirely generated by currents external to the plasma. No current is needed in the plasma, and yet an equilibrium can be established, in three dimensions (3D), between the kinetic pressure-gradient that tends to expand the plasma like a gas and the magnetic forces that tend to compress it. This type of plasma, called a stellarator, is very stable and inherently steady-state, but at the cost of complicated 3D shapes for the external coils and the plasma itself.
We started a program to study these equilibria under extreme conditions of high pressure and low “aspect ratio” (the ratio of the average major radius and average minor radius of the toroidal plasma) at the CNT stellarator. We also investigate whether the elegant physics of 3D equilibria can be satisfactorily reproduced by simplified coils in the CNT and CIRCUS devices, as well as in new concepts. Finally, we conceived, modeled and hope to build soon a non-fusion stellarator, TARALLO, that will serve as source of highly charged ions for accelerators.
The Columbia Non-neutral Torus (CNT) was built in the early 2000's to study non-fusion plasmas that were mostly or exclusively made of electrons. We now operate it as an electron-cyclotron-heated neutral plasma for fusion studies relevant to bigger stellarators. Initial measurements of density, temperature and confinement were encouraging. We also reconstructed stellarator equilibria at very low aspect ratio –in fact, the lowest in the world- by means of the VMEC code.
At present we are undergoing an upgrade to higher heating power. Achieving higher plasma temperatures will serve two goals:
- lowering the collisionality to mimic the plasma edge of stellarator reactors and study its interaction with a plasma-facing surface. Infrared cameras will confirm whether the exhausted power spreads on a broader area than in tokamaks.
- studying stellarator equilibrium and stability at high, possibly unprecedented values of plasma pressure normalized to the confining magnetic pressure, β, also thanks to the low CNT magnetic field, and high density achievable if special electrostatic “electron Bernstein” waves are used for heating.
Another goal is to perturbatively measure the effects of "error fields" (imperfections in the magnetic field that confines the plasma) on stellarator confinement, transport and stability with the intent of perhaps relaxing the tolerances in stellarator construction.
Some stellarator plasmas are highly three-dimensional, perhaps detrimentally so from some physical and/or constructive points of view. For instance, many particles are “trapped” in some sectors of the torus, leading to undesired effects such as turbulence and enhanced particle losses. For these reasons, and in collaboration with external theorists, we constructed the table-top, remarkably simple CIRCUlar coil Stellarator (CIRCUS). To be precise it is actually a tokamak-torsatron hybrid, but yes, the coils are circular, just vertically tilted compared with a tokamak. Like a stellarator or torsatron, CIRCUS is expected to generate helical field-lines and confine a current-free or nearly current-free plasma. At the same time, the plasma is expected to be more uniform in the toroidal direction (in some sense, less three-dimensional), similar to a tokamak. Currently we are installing an electron gun to “visualize” the field-lines, and confirm helical-field generation in the absence of plasma current.
Last but not least, we conceived and modeled TARALLO: the first toroidal Electron Cyclotron Resonance Ion Source (ECRIS) for accelerators. High-energy collisions of heavy ions re-create a state to be found in the first 10 μs after the Big Bang, called the quark-gluon plasma. They can answer fundamental questions on the stability of nuclei in neutron stars and supernovae, and explain how elements from iron to uranium formed in the universe. They can “test” the Standard Model and our understanding of Quantum Chromodynamics and are essential vehicles in the exploration of transuranic elements.
TARALLO will also be a stellarator, but will confine non-fusion plasmas of gold, lead and other elements. It is interesting that the conditions of density, temperature and confinement needed for ion production from these plasmas are similar to those needed to fuse nuclei of hydrogen isotopes. We used our understanding of fusion plasmas to conceive a better source, with the potential of producing more ions and of higher charge, and so help to push the so-called intensity and energy frontiers in particle physics. TARALLO has the peculiarity of being toroidal. Ion extraction is more complicated than from conventional ECRIS sources, but feasible, as our numerical modelling of particle trajectories suggests.