String field theory solution corresponding to constant background   magnetic field
  Following the method recently proposed by Erler and Maccaferri, we construct solutions to the equation of motion of Witten's cubic string field theory, which describe constant magnetic field background. We study the boundary condition changing operators relevant to such background and calculate the operator product expansions of them. We obtain solutions whose classical action coincide with the Born-Infeld action. 
Nobuyuki Ishibashi, Isao Kishimoto, Tomohiko Takahashi
hep-th