Bohm Sheath Criterion
A fundamental boundary condition stating that positive ions entering a non-neutral plasma sheath must enter with a velocity at or exceeding the ion acoustic sound speed c_s.
Governing formula
v_ion ≥ c_s = √(k_B T_e / m_i) (at Sheath Entrance)
SI unit
m/s (Ion Acoustic Sound Speed c_s)
In depth
Formulated by David Bohm. Requires a quasi-neutral 'pre-sheath' region to accelerate ions to the Bohm speed v = c_s before they enter the non-neutral sheath boundary layer.
Examples in the real world
Calculating ion flux and energy delivered to substrate walls in industrial plasma processing.