Diamond-based microplasmas

A microplasma is an electrical discharge where one of the critical dimensions is < 1 mm. They are often smaller-scale versions of well-known hollow-cathode discharges.

Hollow-cathode discharge

The products pd and pD determine the discharge properties, e.g. the breakdown voltage for a given pressure, and this is known as the Paschen scaling law (1916). The hollow-cathode design makes them very stable and efficient. As D decreases, the operating pressure increases, and for D < 10-50 µm, it is possible to sustain plasmas at pressures equal to or above 1 atmosphere. It is possible to fabricate thousands of microplasmas on a large area plate to make microplasma arrays (MPAs).

A microplasma array


Why use diamond for MPAs?

Diamond microplasmas

The all-diamond microplasmas are made by depositing a layer of conducting B-doped diamond on either side of a thick insulating undoped diamond layer, and then laser drilling a hole through the sandwich (see photo, below, right). Acid cleaning removes any graphitic residues in the cavity. The device is connected to a DC power supply and a discharge is struck in a suitable gas (e.g. helium) at pressures of a few 100 Torr, depending on the dimensions of the cavity.

All-diamond microplasma cavity Laser hole

A microplasma struck in a diamond cavityTo the right you can see a photograph of a 200 μm-diameter microplasma operating in helium gas at ~700 Torr. A movie of a 800 Torr microplasma operating at different voltages can be seen here. Our highest pressure microplasma to date operates at 9.5 atmospheres!

Refs: S. Mitea, M. Zeleznik, M.D. Bowden, P.W. May, N.A. Fox, J.N. Hart, C. Fowler, R. Stevens and N.St.J. Braithwaite, "Generation of microdischarges in diamond substrates", Plasma Sources Sci. Technol. 21 (2012) 022001.