Shuangwei Xie

The HelCat dual-source plasma device

The HelCat (Helicon-Cathode) device has been constructed to support a broad range of basic plasma science experiments relevant to the areas of solar physics, laboratory astrophysics, plasma nonlinear dynamics, and turbulence. These research topics require a relatively large plasma source capable of operating over a broad region of parameter space with a plasma duration up to at least several milliseconds. To achieve these parameters a novel dual-source system was developed utilizing both helicon and thermionic cathode sources. Plasma parameters of n(e) approximately 0.5-50 x 10(18) m(-3) and T(e) approximately 3-12 eV allow access to a wide range of collisionalities important to the research. The HelCat device and initial characterization of plasma behavior during dual-source operation are described.

Extremum-Seeking-Based Fluctuation Mitigation and Azimuthal Velocity Profile Regulation by

Turbulence and turbulence-driven transport are ubiquitous in magnetically confined plasmas, where there is an intimate relationship between turbulence, transport, destabilizing mechanisms, such as gradients and currents, and stabilizing mechanisms like shear. Active control of fluctuations is investigated in this paper via manipulation of flow profiles in a magnetized laboratory plasma device helicon-cathode (HELCAT). Fluctuations are monitored by electrostatic probes, and E×B flow profiles are controlled via bias ring electrodes. First, a nonmodel-based extremum-seeking optimal control algorithm is implemented in HELCAT to seek the bias ring voltages that minimize a cost function related to the fluctuation amplitude. The experimental results in HELCAT show that the proposed controller is able to not only suppress the fluctuations but also to regulate their average amplitude around a predefined desired level. It is anticipated that this controller can become a valuable tool for physics-oriented studies designed to elucidate the relationship between the shape of the azimuthal flow profile and the amplitude of the fluctuations once the capability of measuring the flow profile in real time becomes available in HELCAT. Second, with the assistance of a HELCAT-tailored transport code capable of predicting the evolution of the azimuthal flow at several radial points within the plasma, the potential of an extremum-seeking controller for directly regulating the azimuthal flow profile around a prescribed target profile is illustrated numerically.

E\times B

We investigate active control of fluctuations via manipulation of flow profiles in a magnetized laboratory plasma device (HELCAT). Fluctuations and particle transport are monitored by electrostatic probes, and E×B flow profiles controlled via biased ring electrodes. A non-model-based optimization algorithm is implemented to seek control inputs that minimize a cost function related to the fluctuation amplitude. The algorithm is also able to identify radial poloidal flow profiles associated with specific levels of RMS fluctuations.

Actuation in HELCAT

Summary form only given. The HELCAT (helicon-cathode) device is a dual-source linear plasma device that has recently begun full operation at the University of New Mexico. HELCAT is 4 m long, 50 cm diameter, with axial magnetic field < 2.2 kG. An RF helicon source of tunable frequency 10 -30 MHz and P < 5 kW, resides at one end of the device, while a thermionic BaO-Ni cathode capable of discharge currents up to 2.5 kA is located at the other end. Nominal parameters are: Te ~ 5 -10 eV, ne ~ 1012 /cc (cathode), 1013 - 1014 /cc (helicon), plasma diameter 15-20 cm. An overview of the device characteristics and initial experimental results are presented.

Extremum-seeking-based fluctuation mitigation by E×B actuation in HELCAT

Summary form only given. Many toroidal fusion devices now routinely generate edge and/or core transport barriers, where heat and particle transport are reduced far below Bohm diffusion levels. However, minimal particle transport is not necessarily desirable, since it can lead to core impurity accumulation, or alpha particle buildup. Ideally, active, stable control over the transport, rather than simple minimization, could be obtained. To this effect, research is now underway to investigate active control of particle transport. Turbulence and transport dynamics are, of course, strongly nonlinear, and apparently not deterministic. However, modern nonlinear control methods now exist, such as chaotic control and fuzzy control, which do not rely on a model of the system dynamics to affect stable control. Experiments are being conducted in the HELCAT (HELicon-CAThode) linear device at UNM. HELCAT is a 4 m long device, with B < 0.22 T, and cathode-produced densities, n ~ 1-5x10nland12 cmnland-3. Sheared ExB flows, generated via biased concentric rings, are utilized to modify the transport. Fluctuations and flux are monitored with probe arrays. Parameters, such as RF power, gas pressure and magnetic field, are investigated for their effects on plasma behavior (turbulence, blobs, profile, shear layer, fluctuations, etc.). Open loop experiments have demonstrated that drift fluctuations can be fully suppressed by simple biasing. Additionally, a bias regime between drift instability and full suppression exits where fluctuations are chaotic. Experimental results and analysis will be presented.

Characterization and Overview of the Helcat (Helicon-Cathode) Dual Source Linear Plasma Device

Summary form only given. Many toroidal fusion devices now routinely generate edge and/or core transport barriers, where heat and particle transport are reduced far below Bohm diffusion levels. However, minimal particle transport is not necessarily desirable, since it can lead to core impurity accumulation, or alpha particle buildup. Ideally, active, stable control over the transport, rather than simple minimization, could be obtained. To this effect, research is now underway to investigate active control of particle transport. Turbulence and transport dynamics are, of course, strongly nonlinear, and apparently not deterministic. However, modern nonlinear control methods now exist, such as chaotic control and fuzzy control, which do not rely on a model of the system dynamics to affect stable control. Experiments are being conducted in the HELCAT (HELicon-CAThode) linear device at UNM. HELCAT is a 4 m long device, with B < 0.22 T, and cathode-produced densities, n ~ 1-5x10nland12 cmnland-3. Sheared ExB flows, generated via biased concentric rings, are utilized to modify the transport. Fluctuations and flux are monitored with probe arrays. Parameters, such as RF power, gas pressure and magnetic field, are investigated for their effects on plasma behavior (turbulence, blobs, profile, shear layer, fluctuations, etc.). Open loop experiments have demonstrated that drift fluctuations can be fully suppressed by simple biasing. Additionally, a bias regime between drift instability and full suppression exits where fluctuations are chaotic. Experimental results and analysis will be presented.