Density Gradient and Absorption Effects in Gas-Filled Magnetic Axion   Helioscopes
  The effects of absorption in the gas, and of density variations on the sensitivity of gas-filled solar-axion helioscopes are theoretically investigated. It is concluded that the 10-meter long CAST helioscope, the most sensitive experiment to date is near the limit of sensitivity in axion mass. Increasing the length, gas density, or tilt angle all have negative influences, and will not improve the sensitivity. 
R.J. Creswick, S. Nussinov, and F.T. Avignone III