Investigation of helium plasmas near material surfaces

Experiments have been performed on a linear, lower-hybrid-wave-heated plasma device in which a low-temperature, magnetized, helium plasma of collisionality ∼1-10 is produced. The plasmas are directed onto a biasable graphite target. Axial and radial Langmuir probe measurements were taken with 1 mm spatial resolution and 10 ms temporal resolution as a function of target bias. The achieved peak pressure is nT≈1.5×10¹⁴ cm^-3 eV. In the low recycling mode of operation, the plasma density is found to decrease about a factor of 2 with decreasing distance from the target, while the electron temperature remains constant. These results have been compared with plasma simulations made with the B2 2D fluid code, modified to include radial transport and atomic physics of helium.