An article of manufacture includes a support structure including a cladding material and defining therein a plurality of substantially parallel cores. The article also includes a plurality of conically-shaped spikes protruding from a first side of the support structure. Each respective conically-shaped spike of the plurality of conically-shaped spikes includes a core material (i) extending through a corresponding core of the plurality of substantially parallel cores and (ii) comprising an axial protrusion that protrudes axially from the cladding material at the first side of the support structure. The axial protrusion of the core material is tapered to form the respective conically-shaped spike. The article also includes a refractory metal layer coating at least a portion of each respective conically-shaped spike and one or more electrodes connected to the refractory metal layer and configured to apply a voltage to the refractory metal layer.

An article of manufacture includes a support structure including a cladding material and defining therein a plurality of substantially parallel cores. The article also includes a plurality of conically-shaped spikes protruding from a first side of the support structure. Each respective conically-shaped spike of the plurality of conically-shaped spikes includes a core material (i) extending through a corresponding core of the plurality of substantially parallel cores and (ii) comprising an axial protrusion that protrudes axially from the cladding material at the first side of the support structure. The axial protrusion of the core material is tapered to form the respective conically-shaped spike. The article also includes a refractory metal layer coating at least a portion of each respective conically-shaped spike and one or more electrodes connected to the refractory metal layer and configured to apply a voltage to the refractory metal layer.