Fast scanning probe for the NSTX spherical tokamak

Area of Science:

• Plasma physics
• Fusion energy research
• Diagnostic instrumentation

Background:

• Langmuir probes are essential for in-situ plasma characterization.
• Previous probe designs faced limitations in speed, data acquisition, and maintenance.
• Characterizing plasma in fusion devices like the National Spherical Torus Experiment (NSTX) requires robust diagnostic tools.

Purpose of the Study:

• To introduce a novel fast reciprocating Langmuir probe and drive system.
• To enhance diagnostic capabilities for scrape-off layer (SOL) and pedestal regions in fusion plasmas.
• To enable accurate measurement of a wide range of plasma parameters and their fluctuations.

Main Methods:

• Development of a probe system with high-temperature circuit boards, a rotatable/removable shaft, and a 10-tip configuration.
• Integration with a fast pneumatic drive system achieving ~7 m/s speeds and ~20g acceleration.
• Deployment in the National Spherical Torus Experiment (NSTX) to measure plasma parameters in high-power density environments (1-10 MW/m²).

Main Results:

• The probe system successfully measures electron temperature (T(e)), electron density (n(e)), Mach number (M(r)), floating potential (V(f)(r)), electric fields (E(theta)(r), E(rho)(r)), and saturation current (I(sat)(r)).
• Measurements capture profiles and fluctuations up to 3 MHz.
• The system operates effectively in NSTX's SOL (T(e) ~10-30 eV, n(e) ~0.1-5x10¹² cm⁻³) and pedestal (n(e) ~0.5-1.5x10¹³ cm⁻³, T(e) ~30-150 eV) regions.

Conclusions:

• The described fast reciprocating Langmuir probe represents a significant advancement in plasma diagnostic technology.
• Its design features facilitate efficient data collection and maintenance in demanding fusion research environments.
• The system provides valuable data for understanding plasma behavior in critical regions of fusion devices.