# Study of RF Stray Currents in ITER Neutral Beam Test Facilities
## 3 Circuital Model
### HVD and ISEPS  
In the following, the representation in the model of each power supply output stage is given. The connections between power supplies and loads are also discussed.  
* ISRF: Only one ISRF unit is considered in these simulations. The oscillator consists of two Thales IQK 35-2 tetrodes, in the push-pull configuration (Figure 2). The tetrodes are implemented using an analytical model [12]; the output power is defined by setting anode voltage (ISRF-TE power supply) and screen grid voltage. In Figure 2, both CVD configurations are represented (voltage divider at secondary and primary side). Since the divider voltage ratio is large, the CVD is modelled only considering its equivalent capacitance, e.g., the smallest capacitance of the divider. Perfect isolation is assumed between the two legs of the CVD. To move the CVD at the primary side of the transformer, its capacitance (e.g., its gain) was modified in order to obtain a voltage measurement comparable with the one performed at secondary side [5]; therefore, it could be fed directly into the control loop with small adjustments. The capacitance for a single leg of the CVDs is:
* CVD II: C = 250 pF
* CVD I: C = 100 pF Since the CVD installed at the primary becomes part of the oscillator resonant circuit, it is represented just by including it in the variable capacitors (\(C_{V}\)). The stray capacitance between primary and secondary windings (\(C_{STRAY}\)) is also represented in the model. The total interwinding capacitance was measured to be 90 pF. In the model, it is represented with two 45 pF capacitors (\(C_{STRAY}\) in Figure 2). A section of RF coaxial line, modeled with the pi-equivalent representation, connects the output of ISRF to the beginning of the TL.
* ISBI: only the output filter capacitors of the power supply are modeled. The cables connecting ISBI to the busbars installed in the transmission line are modeled as lumped inductors. A resistor is installed between PGFB and BI busbars before the beginning of the TL.
* ISPG: only the output filter capacitors of the power supply are modeled. The busbars from ISPG to the beginning of the transmission line are represented as mutually coupled conductors, with inductive and capacitive parameters per unit length computed by means of the finite element method in FEMM. The coupling with other conductors is not considered inside the HVD.
* ISEG: at the output of ISEG, a low-pass R-C filter is installed, to reduce the voltage ripple of the power supply. The inductive component of the filter resistor is considered in the model. ISEG positive pole is locally referred to the HVD by means of a R-C filter. The connection to the transmission line conductors is performed with a shielded medium-voltage cable, which is represented as mutually coupled conductors, with inductive and capacitive parameters per unit length computed by means of the finite element method in FEMM. The inner conductor of the medium-voltage cable is connected to PGFE at the beginning of the transmission line, while the outer shield is connected to HVD potential and to the EG cable of the transmission line.