Online tuning of impedance matching circuit for long pulse inductively coupled plasma source operation—An alternate approach

Abstract

Impedance matching circuit between RF generator and the plasma load, placed between them determines the RF power transfer from RF generator to the plasma load. The impedance of plasma load depends on the plasma parameters through skin depth and plasma conductivity or resistivity. Therefore, for long pulse operation of ICPs, particularly for high power (~ 100kW or more) where plasma load condition may vary due to different reasons (e.g. pressure, power, thermal etc.), online tuning of impedance matching circuit is necessary through feedback. In fusion grade ion source operation such online methodology through feedback is not present but offline remote tuning by adjusting the matching circuit capacitors and tuning the driving frequency of the RF generator between the ion source operation pulses is envisaged. The present model is an approach for remote impedance tuning methodology for long pulse operation and corresponding online impedance matching algorithm based on RF coil antenna current measurement or coil antenna calorimetric measurement may be useful in this regard. Introduction:

Solid state generator for powerful radio frequency ion sources in neutral beam injection systems

“…The transformed plasma resistance has been calculated to 1.8 Ω from the parallel capacitance which is necessary to achieve 50 Ω at the output of the amplifier or SWR=1 respectively. This value is consistent with the results achieved by transformer model calculations for inductive discharge in [12].…”

Can we estimate plasma density in ICP driver through electrical parameters in RF circuit?

“…Therefore, it is clearly understood that major contribution of error comes from R p . The value of R p is estimated in the process of impedance matching calculation [15] and it is experimentally observed that ~ 70% matching is only possible in reality, as reflected in the experimental cos values in the fig.8 of reference 15. Remaining ~ 30% can be considered as the uncertainty of the R p estimation due to the simplified assumption based on solenoid approximation of the RF coil.…”

“…The term L mp is due to the discharge current path and is inductively coupled to the primary coil through mutual inductance M . The figures 3b -3c [15] depict only the coupling phenomena between the coil antenna and the plasma, based on transformer model [15] and do not include the electrical parameters of the impedance matching circuit.…”

“…In the transformer model, a low-pressure inductively coupled plasma discharge is regarded as the secondary coil of an air core transformer as shown in figure-3a [15]. The primary of the transformer is the antenna coil itself, which is having n turns.…”

Note: Measuring capacitance and inductance of a helical resonator and improving its quality factor by mutual inductance alteration

“…To understand the above mechanism, i.e., the dependencies of Q with the separation between the primary antenna and the helical coil, we have modelled the resonator along with the primary antenna as air core transformer. 13,14 The primary antenna, electrically connected with the RF source, is considered to be transformer primary coil and the helical coil as the secondary coil (Fig. 4(a)).…”