Pulsed Cathodic Arc Plasma Diagnostics Optical Emission Spectroscopy Results Aluminium

Time-Resolved Spectroscopy Single aluminium cathode (  = 50 mm). Different cathode currents (Pulse length 600 us, except for Ic = 2.4 kA, being 450 us). Transport parameters set to reduce the coating of the window. Spec ICCD PC OF L Al I, Al II and Al III transitions Slit aperture: 50  m Gate width: 2  s Gate interval: 25  s Software accumulations to improve S/N ratio Ic = kA: 20 accum. Ic = 1.6 kA: 10 accum. Ic = 2.0 kA: 8 accum. Ic = 2.4 kA: 5 accum.

Analysis Parameters Experimental intensity (Counts) was divided by the number of the respecting accumulations. As a result, the resulting values for all cathode currents are comparable. Area under the peaks obtained by numerical integration. From the fitted curve (interpolation of experimental points), FWHM and Centroid were obtained. These calculations were done automatically by Origin. For some transitions: nm (Al II), and nm (Al III); it was done a double peak Gaussian fitting. Information such as blue shift peak was obtained.

TOTAL AREA -Neutral Al- Very low intensity and small signal-to noise ratio. Intensity increases towards the end of the pulse.

Total Area (Al 1+ ) Same behaviour for all transitions at their corresponding cathode current. Similar production rate for up to ~100 us Saturation for the lower cathode currents

Total Area (Al 2+ ) High peak intensity at short times (~100 us), except for Ic = 2.4 kA. Perhaps, production rate is a function of the local spots clusters density.

Gaussian fitting Realized for one Al II ( nm) and two Al III transitions (569.66, nm). Double gaussian peak fitting for each transition, which are made by ~ 15 experimental points. For all cathode currents and times, a “blue hump” appeared. This hump is modelled (R^2 > 0.98) by two gaussian peaks. Plots for the two peaks relative separation (blue shift) and the central peak location are provided. Information about the width can also be obtained; however the Fizeau interferometer may provide a much better reliable measurements.

Central peak location Transitions have a blue shift with decreasing cathode current. Error bars provided by the fitting process.

Blue shift within the same transition Blue shift is calculated by the difference between the central and the blue peak within the same transition. Al II shows the same shift for all cathode currents (~0.55 Å). Al III has more variations, specially for early times.

Results by cathode current Comparison among the two different species employed for the Gaussian fitting analysis. Normalised total area. Al ll total area (obtained by numerical integration) is normalised against its own maximum and compared with the corresponding one for Al III. Particle velocity. Obtained by Non-relativistic Doppler effect for motion along the line of sight: v =  c v = particle velocity  = blue shift:= (Central peak) - (Blue peak centre)  = central peak wavelength c = speed of light

Normalised area Production rate of Al II is higher and saturates at early times as the current is diminished. Al III has the same production rate for the highest current; while for the lower currents peaks at early times (~100 us) and then keeps constant but at much lower values.

Results by cathode current Al II keeps ~ constant velocity during the pulse and has ~ the same value for all currents. Al III has more electromagnetic interaction with the (charged) cathode (as expected) showing dependency with the Cathode voltage (?) as the current (and the voltage) increases (decreases). Unfortunately I couldn’t open the Database files and obtain this information.

Summary From the intensity (total area) results, the higher the species’ charge state, the more interaction with the fields on the cathode. The effects are more clear when the cathode current is increased. Higher charge states species production rate (Al 2+ and higher) are definitely linked with the local density of cathode spots; while Al 1+ and neutrals are more related with charge exchange produced by collisions (and the absolute number of cathode spots or plasma sources). It is evident a change of behaviour for Ic = 2.4 kA. These changes: higher charge states production rate, but slow down of these species (meaning more interaction with the cathode-anode); might be the responsible on the change of transport conditions observed at this cathode current.