def get_kappa_ex(E, B, D):
    """
    Find kappa_ex for a given B field.
    Based on the deflection approximation used in 
    Achterberg et al. 1999.
    """
    l = 1
    kappa_ex = []
    for i, e in enumerate(E):
        theta_p = 0.0401 * (e / 50) ** -1 * B * (D[i] / 10) ** 0.5 * l ** 0.5
        kappa_ex.append(2.3 / theta_p ** 2)
    return kappa_ex
