The fitted curve of the relationship between the equivalent temperature and the discharge current is also shown in Figure 1:0 The fitted result show that \(a=4.003\times 10^{16}\) /(mA\(\cdot\)m\({}^{3}\)), and \(b=0.047\) /mA. The curve of Equation (2) is shown in Figure 1:1 With the discharge current increasing, the atomic density increases but the increasing rate becomes slow. Figure 1:2 All the transmittance spectra for various magnetic fields and the discharge current of 18 mA. Figure 1:3 The measured transmittance spectra (blue, solid) and the fitted transmittance spectra (orange, dashed) at the discharge current of 18 mA and the magnetic field from 225 G to 337 G. The fitted equivalent temperatures (T) and magnetic fields (B) are listed on each subfigure. (**a**) The measured and the fitted transmittance spectra at the magnetic field at 225 G, and the fitted equivalent temperature is 61.54 \({}^{\circ}\)C and the fitted magnetic field is 251.4 G; (**b**) The measured and the fitted transmittance spectra at the magnetic field at 260 G, and the fitted equivalent temperature is 61.14 \({}^{\circ}\)C and the fitted magnetic field is 270.6 G; (**c**) The measured and the fitted transmittance spectra at the magnetic field at 280 G, and the fitted equivalent temperature is 60.29 \({}^{\circ}\)C and the fitted magnetic field is 291.4 G; (**d**) The measured and the fitted transmittance spectra at the magnetic field at 337 G, and the fitted equivalent temperature is 61.86 \({}^{\circ}\)C and the fitted magnetic field is 373.5 G.  
Figure 1:4 The fitted equivalent temperatures of the HCL at different magnetic fields versus the discharge current (color dots) and the fitted curve (black line) of the equivalent temperature of the HCL versus the discharge current. This experiment shows that the HCL can really improve the atomic density at room temperature. This can help us to expand the wavelengths of atomic filters and decrease the requirement of ambient temperature for using FADOF. However, there are also slight differences between the HCL-FADOF and the VC-FADOF. The broadening and the overlap of the HCL-FADOF, which broaden the bandwidth of the HCL-FADOF, are more serious than those of the VC-FADOF. And the peak transmittances of the HCL-FADOF are lower 1-2% than those of the VC-FADOF. These are contrary to the design goal (high transmittance and narrow bandwidth) of the atomic filters. The first reason may be the collision between the rubidium atoms and the ions. As mentioned before, the positively charged ions strike the surface of the cathode under the acceleration of the electric field between the anode and the cathode. Simultaneously, the atoms or ions sputtered from the cathode are also struck by the positively charged ions. The second reason may be the Stark effect. Unlike the saturated vapor cell, there is a large electric field, which could cause the Stark effect, between the anode and the cathode. Hence there is not only the Zeeman effect but also the Stark effect in the HCL-FADOF. The impact of the Stark effect in an HCL-FADOF deserves further study. In conclusion, a rubidium HCL-FADOF working at 780 nm was established and carefully tested. The transmittance reaches 29% under the discharge current of 18 mA and the magnetic field of 260 G with the cathode length of only 20 mm. A direct comparison of two kinds of FADOFs shows that the performance of the HCL-FADOF under the discharge current of 15-20 mA is equivalent to a VC-FADOF with a 20 mm long saturated vapor cell which is heated to 60-65 \({}^{\circ}\)C. However, the peak transmittances of the HCL-FADOF are lower 1-2% than that of the VC-FADOF and the FWHM (full width at half maxima) of the HCL-FADOF is larger than that of the VC-FADOF. These further prove that the HCL can be used to increase the atomic density for building up a FADOF at room temperature. Moreover, an empirical formula, which indicates the relationship between the atomic density and the discharge current of the HCL, is fitted.