The addition of a gas to an FEL wiggler changes the phase velocity of the electromagnetic wave, and so provides a fairly simple way of wavelength tuning. This effect has been demonstrated by reducing the wavelength of an infrared FEL by 0.73 μm with the introduction of 200 Torr of hydrogen gas. In this paper, a helium-loaded FEL experiment, which is to operate at λ = 600 Å with λ = 220, is proposed, and detailed design calculations are presented. Near the electronic resonance of helium, which occurs at λ = 584 Å, the index of refraction becomes large, so it is possible to use low gas pressures. This is advantageous because at lower pressures there is less degradation of the beam quality due to multiple scattering. Using typical electron beam parameters, in 115 cm of wiggler length, the power in the wave is increased by a factor of ∼ 9 at λ = 600 Å.