Alcator C-Mod

Noah Smick

Updated: June 27, 2006

I am a graduate student in the department of Nuclear Science and Engineering starting my 5 th year working on the Alcator C-Mod tokamak. I work in the area of edge plasma physics in which we study the behavior of the cooler plasma surrounding the hot confined plasma in a magnetic fusion device. Studying this region provides insight into how particles and energy are lost from the confined plasma. In addition, the edge region can impose boundary conditions on the confined plasma that affect its behavior. The edge also interacts with the walls of the plasma chamber, introducing impurity wall material into the plasma, and transferring plasma energy and particles to the walls.

One unique property of the plasma edge is that the density and temperature are sufficiently low to permit the use of physical probes to measure properties of the plasma. A probe that is inserted briefly and then removed can survive the edge temperatures of hundreds of thousands of degrees, but a probe used in the core would quickly succumb to temperatures of tens of millions of degrees. The dramatic death of such a probe would also extinguish the plasma. For this reason the insertion depth and dwell time of a probe in the plasma must be carefully controlled.

The simplest and most commonly used physical plasma probe is called the Langmuir Probe. It consists of an electrode placed in the plasma and biased to a particular voltage. The electrode then draws current from the plasma, the value of which depends on many plasma properties, including density, temperature, potential, and flow velocity vector. It is therefore possible to reconstruct all of these plasma properties by examining the current-voltage profiles from an array of electrodes.

The standard Langmuir probe configuration for edge measurements on Alcator C-Mod is four-electrode probe with one electrode embedded in each face of a four-sided pyramid. The probe is rapidly plunged into the edge plasma using compressed air and then bounces back using a limit spring. Access is gained by using one of the ten ports around the outside of the doughnut-shaped tokamak.

Recent studies have shown that a number of interesting edge phenomena take place on the inside of the donut where traditional probes cannot access. The inside of the donut seems to have reduced plasma leakage out of the confined core. There are also near-sonic plasma flows present in the interior edge region, which could be linked to this asymmetry. There is speculation that these flows could couple back to the confined plasma and affect global flow shear, an important parameter controlling confinement. Because of these observations, there is reason to undertake a careful study of plasma conditions in the inner edge region.

My project is called the WASP (Wall-Acutated Scanning Probe). It is a linear plunge four electrode scanning probe that will be placed on the inner wall to measure the inner plasma edge. Since there is no port access in this region, a pneumatic drive is not possible and the probe will be driven electro-mechanically. A coil is embedded in the probe in which we can drive current. It will then feel a torque from the large magnetic field in the tokamak. This is similar to how an electric motor uses permanent magnets to produce motion from electric current. The WASP instead uses the plasma confinement magnetic field already present in the tokamak. This design is compact and versatile, allowing a higher degree of control over plunge dynamics than the traditional pneumatic drive. It can also fit in the limited space available on the inner wall.

The WASP (Wall-Actuated Scanning Probe)

The WASP project is currently ready for installation. We have built three probes and plan to install two of them during our next up-to-air break which should begin late in the summer of ‘06. Only one of these probes will be operated, with the other reserved as a spare to be used in the event of a failure of the first probe. If all goes well, we should begin collecting data from the WASPs during our fall campaign.