def METHOD_NAME( z, r, epsilon, k0, w0, wp, t) :
    """
    Return the longitudinal electric field as an array
    of the same length as z and r, in the half-plane theta=0
    """
    Ez_array = \
        - epsilon * m_e*c**2/e * k0 * \
            np.exp( -r**2/w0**2 ) * np.cos( k0*z ) * np.sin( wp*t )
    return( Ez_array )