## 4 Conclusions  
In this work, the addition of heat load B was adopted to obtain a higher threshold critical power for the heat pipe systems. Two kinds of heat pipes, including thermal switch heat pipe (TSHP) and traditional grooved heat pipe (TGHP), were investigated experimentally, respectively. The thermal performance of TSHP and TGHP was also investigated experimentally, including the threshold critical power, starting characteristic, recovery characteristic, surface temperature distribution, thermal resistance, etc. The optimized TSHP exhibited a larger threshold critical power range with low heat load B input (from 55 W to 65 W, almost 20% increase), low starting heat load B input (2 W), good recovery characteristic (less than 150 s at 75 \({}^{\circ}\)C after overload), low thermal resistance with heat load B input (less than 0.13 \({}^{\circ}\)C/W). These results indicate that the optimized TSHP can offer great potential for the overload operation situation with the addition of heat load B. Moreover, the addition of heat load B provides another method for adjusting the steam pressure distribution along the necking heat pipe to obtain a better heat pipe system of overload, stability, and low thermal resistance. Such distinction between primary and additional heat sources in the experimental investigations of multiple heat source systems can also provide a method for thermal resistance adjustment. Furthermore, the variable cross-section of the heat pipe can provide another method for adjusting steam pressure and thermal resistance of it. Such a novel necking heat pipe is very suitable and has the potential for a real situation where the waste heat of a low-power chip can be utilized to drive a high-power chip working under the overload situation on the same circuit board. Moreover, the addition of an extra heat load or condensation module can be utilized in the heat pipe systems to obtain different thermal performances for complicated working situations, especially the overload operation.  
Figure 9: (**a**) The thermal resistance of TSHP-1 with different heat load A from 10 W to 55 W; (**b**) the thermal resistance of TSHP-1 with different heat load B from 0 to 12 W under a certain heat load A input of 45 W; (**c**) the thermal resistance of TGHP-1 with different heat load A from 15 W to 50 W; (**d**) the thermal resistance of TGHP-1 with different heat load B from 0 to 10 W.  
J.L.: Conceptualization, Investigation, Methodology, Writing--original draft. S.Z.: Data curation, Writing--review and editing. H.T.: Investigation, Visualization. Y.L.: Visualization, Writing--review and editing. S.W.: Supervision. G.Z.: Writing--review and editing. Y.T.: Resources, Writing--review and editing. Funding acquisition. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China (No. 52105444, No. 51735004), Natural Science Foundation of Guangdong Province (2022A1515010375), S&T Innovation Projects of Zhuhai City (ZH01110405180034 PWC) and Foshan Science and Technology Innovation Project (1920001000339).  
Not applicable.  
The authors declare no conflict of interest.