A heave compensator with adjustable dampening properties includes a length extension device having an inner space divided by a slide-able piston into a vacuum chamber and a liquid filled chamber, a gas accumulator divided by a slide-able piston into a gas filled chamber and a liquid filled chamber, and a gas tank having an expansion chamber. A compositionally graded inorganic crystalline material can be formed by preparing a crystalline substrate by forming crystallographically oriented pits across an exposed surface of the substrate. A first vacuum chamber 16 containing the electron source 1 is connected to a second vacuum chamber 9 via an aperture 8 provided in the extraction electrode 6. Each finger element is secured at one end to the stationary press crown.