Bohm velocity: Why? Wall charged negatively, or facing extractor collisional"drag nmi=t =nE坛 small mall not near wall Add constant . my, dv d(+P)=-F ne vi =const =T amv+P。+P)=-Fn Pe+P=nk(Te+T)=nkTe dx/"、,/- d min at v.= 16.522, Space P pessan Lecture Prof. Manuel martinez Page 2 of16.522, Space Propulsion Lecture 15 Prof. Manuel Martinez-Sanchez Page 2 of 11 Bohm velocity: Why? collisional “drag” i i i i i i x in dv dP nm v + = enE - F dx dx e e e e e e x en dv dP n m v + = -en E - F dx dx small small not near wall, n =n. e i Add: constant ( ) e i i e i i in dv d P +P n m v + - F dx dx  and n v = const = e i Γi ( ) 2 e i i e in i d n m v + P + P - F dx  P + P = n k T + T n kT e i e e i ee ( )  i i i i e in i d m v + kT = -F dx v ⎡ ⎤ Γ ⎢ ⎥ Γ ⎣ ⎦ min. at e i i kT v = m