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The group "Experimental Physics II - Reactive Plasmas" at the faculty of physics and astronomy at Ruhr University Bochum.

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Synchronising optical emission spectroscopy to spokes in magnetron sputtering discharges

Spokes are patterns of increased light emission, observed to rotate in front of the targets of magnetron sputtering discharges. They move through the plasma at velocities of several km/s in or against the E⃗ × B⃗ direction of the discharge. The high velocity and their initial creation at arbitrary positions render measurements of spokes challenging. For more demanding plasma diagnostic techniques that require data acquisition over multiple discharge pulses, synchronisation to the spoke movement is necessary.
In this publication, we present optical emission spectroscopy of spokes in both high power impulse magnetron sputtering (HiPIMS) as well as direct current magnetron sputtering (DCMS) discharges, performed by triggering an iCCD camera on the spoke movement. Optical filters between plasma and camera allow us to isolate emission lines of metal and working gas neutrals and ions.
Based on these optical measurements and previous probe studies, the dynamics of electrons drifting through spokes in both DCMS and HiPIMS is discussed. In HiPIMS, the much shorter mean free path for inelastic electron collisions enables strong ionisation inside the spoke, causing a sudden variation in electron density which leads to the distinct spoke shape. In contrast, the spoke shape for DCMS discharges seems to rather be indicative of electron energy variations.

Release Date
Permanent Identifier (URI)
Is supplementing
Plasma Source Name
Plasma Source Application
Plasma Source Specification
Plasma Source Properties
magnetron: Thin Film Consulting IX2U, 2 inch Al target 3mm thickness HiPIMS power supply: TRUMPF Hüttinger TruPlasma Highpulse 4002, voltage: 670 V applied, 570 V after breakdown, peak current: 40 A or 2 A/cm², pulse length: 100 µs, repetition frequency: 30 Hz DCMS power supply: TRUMPF Hüttinger TruPlasma Highpulse 4001 G2, voltage: 305 V, current: 60 mA or 3 mA/cm²
English (United Kingdom)
Plasma Medium Name
Plasma Medium Properties
41 sccm, 0.5 Pa
Plasma Medium Procedure
Chamber base pressure of 10^-6 Pa, argon is continuously flowing and pumped during discharge operation. Gas tubes are pumped and flushed prior to the experiment.
Plasma Target Name
Contact Name
Maaß, Philipp A.
Plasma Target Properties
50 mm diameter, 3 mm thickness
Contact Email
Plasma Diagnostic Properties
Used Andor iStar sCMOS 18U-E4 with optical filters observing plasma from the front and from the side. PMT or probe was used to synchronise the camera on the movement of spokes.
Public Access Level
Plasma Diagnostic Name
Funding Agency

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