The Raether limit is the physical limiting value of the multiplication factor (M) or in an ionization avalanche process (Townsend avalanche). Even though, theoretically, it seems as if M can increase without limit (exponentially), physically, it is limited to about M < 108 or αx < 20 (where α is the first Townsend coefficient and x is the length of the path of ionization, starting from the point of the primary ionization. Heinz Raether postulated that this was due to the effect of the space charge on the electric field.