Schematic of feedback system for controlling plasma turbulence and transport
In most all plasmas turbulence causes the plasma to slowly (or quickly!) leak out of the trapping magnetic field. This is called transport. Transport barriers (internal and H-mode edge barriers) are now routinely created in toroidal fusion devices, like tokamaks and stellarators. Radial diffusion levels far below Bohm diffusion are achieved. However, while minimal heat transport is desirable, very low particle transport is not, since it can lead to such undesirable effects as core impurity accumulation, or buildup of alpha particle ash in a reactor. Ideally, the turbulence-driven particle transport could be actively controlled to a desired stable operating point. It is well known that changes in plasma flow profiles (e.g. flow shear) can result in significant reductions or enhancements in turbulence and turbulence-driven transport, e.g. H-mode. Although turbulence-transport-flow shear physics remain poorly understood, manipulation of flow profiles clearly provides a “control knob” for manipulating the transport.
In HelCat we study plasma turbulence and how to control it and the associated transport. As shown in the schematic below, our idea is fairly simple. We measure the turbulence with a Langmuir probe, in this case measuring the ion saturation current. This single is then analyzed and used to modify the radial electric field via a set of bias rings. These six bias rings can create an electric field profile that induces a sheared ExB flow in the plasma, and thus modifies the turbulence. Ultimately, the goal is to be able to suppress (or enhance) the turbulence in real time.
At this point (November 2006), we've been able to "manually" modify the turbulence using the bias rings. In the "standard" helicon discharge we see drift wave type fluctuations in the gradient region of the plasma. The frequency is proportional to the density gradient, i.e.
For argon at B~350 gauss the frequency is f ~700 Hz. The plots below show what happens when we put a constant potential on all bias rings. In the plot on the left the potential is 0V, and the drift wave oscillations are large. With a 14V bias on the rings, the oscillations disappear. With increasing bias voltage other modes of oscillation are observed. For more details, see the recent presentations.
In fact, the effect is more interesting than just the fact that the oscillations are suppressed. We observe a classic period doubling route to chaos as the bias voltage is increased. In the plots below are depicted a "phase space" plot, plotting the ion saturation current at time t versus the current at time t-dt. What you see is a "chaotic attractor". As the voltage is increased the attractor first adds one additional "loop", then these loops double, and ultimately the attractor becomes random-like, or chaotic.
Phase delay plot showing period doubling route to chaos
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