Transient effects in pulsed electron cyclotron resonance (ECR) plasmas are significant for applications such as; plasma processing industry, particle accelerators, etc. ESS Bilbao, based in Spain, are conducting a research project on this topic as part of a larger program associated with spallation neutron generation and accelerator technology. Test bench for Ion-sources Plasma Studies (T.I.P.S) is an ECR plasma generator, powered by a 2.45 GHz and 3k W modifiable power magnetron. This power magnetron has been developed to perform study programs, dedicated to the plasma physics associated with the performance of ECR ion sources.
The diagnostic setup uses a number of complimentary methods such as; directional couplers for incident and reflected power measurement, a Langmuir probe, and a vacuum-ultraviolet (VUV) spectrometer (Figure 1). The study aims to gain a better insight into the plasma breakdown dynamics of ECR plasmas. Specifically it uncovers the similarity of the transient effects of the plasma breakdown dynamics that were previously seen, with high frequency ECR ion sources intended to produce 2.45 GHz microwave discharges and multi-charged ions, which are frequently used to generate mono-charged, high current ion beams.
Figure 1. A view of the experiment with time resolved Langmuir probe and VUV spectroscopy diagnostics
Langmuir Probe System
With the aid of the Langmuir probe system, I-V curves can be obtained to estimate plasma electron density and temperature. A tungsten wire measuring 0.5 mm in diameter and 6 mm in length is what makes up the tip of the probe. To study the transient effects the probe is synchronized with the VUV-emission measurement through a delay generator, and the I-V curve is built from the data acquired over successive pulses. Hiden Analytical have developed the Langmuir probe driver circuit (ESPion), which is capable of acquiring a single I-V point in just 62.5 ns and also rearming itself for another 14.6 µs needed for data handling. The jitter between the timing signals is maintained at less than 200 ns.
Each I-V curve point represents the average probe current received at a fixed probe voltage of more than 100 consecutive pulses. Acquiring a complete I-V curve corresponding to a temporal data point takes several minutes. Using the Maxwellian electron energy distribution function (EEDF), the electron temperature Te is calculated from the I-V curve (Figure 2).
Figure 2. ESPion Langmuir Probe I-V curves. Curve (a) typical data corresponding to plasma breakdown, Curve (b) typical data taken during steady-state plasma conditions (15 and 60 μs after an incoming microwave pulse)
During the microwave coupling process a 20 eV temperature peak is seen with a considerable reduction of the reflected power. This peak decreases gradually reaching a final steady state temperature of approximately 5 eV, which almost remains constant during flat top microwave pulse. Electron density reaches a stable value of about 1.5 × 1016 m-3 during the generation of the temperature peak, suggesting that this process is related to plasma evolution during breakdown. This factor is proven by time resolved VUV spectroscopy measurements, where identical peaking behaviors were seen and documented for Lyman band and Lyman-alpha emission.
This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.
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