  We demonstrate experimentally that the trident process $e^-\rightarrow
e^-e^+e^-$ in a strong external field, with a spatial extension comparable to
the effective radiation length, is well understood theoretically. The
experiment, conducted at CERN, probes values for the strong field parameter
$\chi$ up to 2.4. Experimental data and theoretical expectations using the
Local Constant Field Approximation show remarkable agreement over almost 3
orders of magnitude in yield.
