  We investigate enhanced field emission due to a continuous or pulsed
oscillating field added to a constant electric field $E$ at the emitter
surface. When the frequency of oscillation, field strength, and property of the
emitter material satisfy the Keldysh condition $\gamma<1/2$ one can use the
adiabatic approximation for treating the oscillating field, i.e. consider the
tunneling through the instantaneous Fowler-Nordheim barrier created by both
fields. Due to the great sensitivity of the emission to the field strength the
average tunneling current can be much larger than the current produced by only
the constant field.
  We carry out the computations for arbitrary strong constant electric fields,
beyond the commonly used Fowler-Nordheim approximation which exhibit in
particular an important property of the wave function inside the potential
barrier where it is found to be monotonically decreasing without oscillations.
