The growing utilization of High-Voltage Direct Current (HVDC) systems in the transmission grid leads to an increased demand for reliable DC cable systems where overhead lines can not or shall not be used. Prominent examples of the latter are the planned SuedLink and SuedOstLink HVDC corridors in Germany. By decision of the federal government in 2015, these shall be constructed using mostly HVDC cables in order raise public acceptance. A HV cable is a costly asset that can be critically damaged if an electric breakdown occurs during its lifetime. To ensure the HV cable’s withstand capability to voltage stress, different high voltage tests are defined in the relevant standards which can be applied during development, production, installation and operation of the cable system. For HVAC cables, partial discharge (PD) measurement proved itself to be a valuable addition to AC voltage tests by drastically increasing the probability of finding localized insulation defects. As HV cables themselves are generally manufactured to a high quality standard, insulation defects mostly arise as a result of bad workmanship during assembly on-site. Prior laboratory investigations on HVDC cables with artificial assembly defects led to the conclusion, that the DC-PD activity of these defects is very limited. It is to be expected, that PD measurements on polymer insulated cables under DC voltage are generally less effective than under AC voltage of comparable voltage magnitude. When applied correctly, ionizing radiation is known to significantly increase the PD activity of void-type defects in cast gas-insulated switchgear (GIS) insulators when tested with AC voltage. It can thereby increase the effectiveness of an ongoing PD measurement. This approach was adopted in order to increase the PD activity of a defective HVDC cable system, subjected to DC voltage stress under laboratory conditions. The cable was prepared with different assembly defects which reflected severe mistakes that could occur during the on-site jointing process. Low dose X-ray pulses were applied by radiating through the joint body with a portable X-ray generator. Vital parameters like the PD inception voltage, PD repetition rate and single pulse magnitude were recorded. To put the performance of X-ray stimulation as a technique to increase PD activity into perspective, these measurements were furthermore conducted at different cable temperatures. Additionally, the time between X-ray and PD occurrence was analyzed in order to define a time window in which PDs can be considered as being triggered by X-ray. It was shown that it is possible to force an instantaneous discharge of a suitable pre-charged insulation defect with a X-ray pulse. It was also shown that this pre-charged state must be relatively close to the point of spontaneous PD occurrence. A temperature rise of 10 °C generally had a stronger effect on PD inception voltage and PD repetition rate than X-ray stimulation.