Deposition of Beam Neutrals

• Each beam source is specified individually by power, energy, tangency radius, component mix (full, half, third, by particle fraction), and by the shape of its cross-sectional power profile (``footprint'') in vacuum (see Section 6.2.2).
• SNAP tracks the linear beam trajectory for each component of each source, locally depositing beam neutrals as fast ions through the following processes:
  • Charge-exchange on thermal ions.
  • Ionization by thermal ions.
  • Ionization by beam ion population.
  • Ionization by thermal electrons.
  • Charge-exchange with impurities.
  If SNAP's rotation switch is turned on, then the cross-sections for all of these interactions are performed in the rotating plasma frame.

  SNAP does not consider ``multi-step'' ionization processes, by which a beam neutral is kicked into an excited state (but not ionized) by an initial collision, and subsequently ionized in a second collision. These processes have the net effect of increasing beam attenuation, but they are thought to be relatively unimportant for TFTR parameters.

• At each point along the linear beam trajectory, the collisionless first-orbit of the deposited fast ions is calculated. If the orbit is found to lie outside the plasma, it is considered lost, and the calculated first-orbit beam power loss term is incremented accordingly.
• Otherwise, the fast ions are considered born in the plasma; their subsequent behavior is calculated by a Fokker-Planck routine.
• The outputs of the beam neutral deposition code are:
  • The total shine-thru power.
  • The total first-orbit power loss.
  • The deposition profile of fast ions as a function of energy, pitch angle, and radius.

 
• Solution of Fokker-Planck Equation
• Details of Beam Power Profiles