1. A device comprising:a cylindrically-symmetric element rotatable about an axis and having a surface coated with a band of plasma-forming target material for irradiation by a drive laser to produce plasma;
a subsystem for replenishing plasma-forming target material on the cylindrically-symmetric element; and
a wiper positioned to scrape plasma-forming target material on the cylindrically-symmetric element to establish a uniform thickness of plasma-forming target material; and
a measurement system outputting a signal indicative of a radial distance between the wiper edge and the axis, wherein the measurement system comprises a light emitter and a light sensor.