# Study of RF Stray Currents in ITER Neutral Beam Test Facilities
## 5 Rf Stray Currents Simulations
### Benchmark--Time Domain Simulations  
In SPIDER, oscillators cannot work in perfectly matched conditions, due to the frequency flip phenomenon [14]. The power measurements from ISRF diagnostic and control system are not reliable when the load is unmatched to the generator; thus, a correct estimation of the equivalent RF driver impedance is difficult to obtain. On the other hand, the \"unloaded\" or \"vacuum\" impedance of the RF driver, namely the impedance of the RF driver when no plasma is ignited inside the source, is well known. For this reason, only the vacuum pulses are considered to obtain a comparison between experimental results and  
Figure 5: Common mode current path with CVD installed at RF transformer primary side.  
Figure 6: Complete scheme of the circuital model. In red, the stray elements, and in blue, the position of the voltage and current measurements (\(V_{OUT},I_{OUT},I_{ISEG-F}\)).  
circuital model. With reference to Figure 6, the load, matching network and transmission line parameters considered for the vacuum simulation are the following:  
* \(Rd=1.95\,\Omega\)
* \(Ld=20.3\) uH
* \(Cs=3\) nF
* \(Cp=10\) nF  
For the ISRF unit considered, the length of the RF coaxial line from oscillator to load is 43 m. The load, matching network and RF coaxial line parameters are obtained by fitting the impedance measurement performed at the beginning of the RF coaxial line, over the operating frequency range, to the equivalent RF circuit model. The ISRF representation used for the time domain simulations is the one of Figure 2, with the oscillator modeled in its entirety. The experimentally measured screen grid voltage, anode voltage and tuning capacitance (\(C_{V}\)) are used as simulation inputs. Table 1 collects the results obtained from current measurements (RMS values) performed at the ISEG output filter branch (\(I_{ISEG-F}\), see Figure 6), and from simulations. The simulation is performed in time domain, and the results reported are relative to oscillator steady-state conditions (constant frequency and output power).  
The differences between measured and simulated oscillator output voltage and current that we can observe in Table 1 are mainly due to the imprecise characterization of the output RF transformer; additional dedicated measurements are performed to refine the representation of primary, secondary and mutual inductances of the transformer. Despite the challenges in representing correctly also ISEPS conductors not intended for transmission of RF power and RF stray connections in the beam source, the model provides reasonable results, when comparing to the specific operating condition defined in this section. To test the limitations of the model and to understand if additional components need to be introduced in the simulation to provide a correct depiction of the RF stray currents behavior, it would be necessary to perform measurements in several locations of SPIDER ISEPS (e.g., current measurement for all the conductors of the transmission line) and in the whole operating frequency range. The experimental activities are underway to assess the RF stray currents frequency response in the operating range.