Inverse Bremsstrahlung current drive

The generation of the plasma current resulting from Bremsstrahlung absorption is considered. It is shown that the electric current is higher than the naive estimates assuming that electrons absorb only the photon momentum and using the Spitzer conductivity would suggest. The current enhancement is in part because electrons get the recoil momentum from the Coulomb field of ions during the absorption and in part because the electromagnetic power is absorbed asymmetrically within the electron velocity distribution space.

Figure 1

Schematic diagram of Bremsstrahlung absorption (a) and emission (b).

Figure 2

The probability of Bremsstrahlung absorption in arbitrary units versus the angle between the electron velocity and the incoming photon direction $cos\theta =v_z/v$ for $\beta =0.08$.

Figure 3

The momentum absorption rate per electron as a function of $cos\theta =v_z/v$ for $\beta =0.08$: along the $z$ axis taking into account the recoil (solid blue line), along the $z$-axis taking into account only the photon momentum (dashed red line).

Figure 4

The generated current density versus the ion charge $Z$: fluid approximation with the Spitzer conductivity given by Eq. (30) (solid blue line), current drive approximation keeping only the first term in Eq. (39) (dotted red line), current drive approximation keeping both terms in Eq. (39) (dashed orange line), hybrid current given by Eq. (41) (dash-dotted green line).