The PPU uses three types of control: discharge oscillation control, power control, and discharge current control. The power control and the discharge oscillation control are necessary for the stable in-orbit operation of the 6 kW Hall thruster. Discharge current control improves the propulsion efficiency of Hall thruster systems and enhances their market competitiveness.

Severe discharge current oscillations are not acceptable for the PPU or the 6 kW Hall thruster. Therefore, the thruster must be operated within the discharge current amplitude limit. Discharge current oscillations with amplitudes beyond the filter's capability to handle can cause noise that can cause satellite interface signals to deviate and, at worst, damages the PPU, thrusters, or the entire spacecraft. For the PPU and Hall thruster, the amplitude of the discharge oscillation during nominal operation must be less than 80% peak-to-peak (e.g., the limit for an average discharge current of 20 A is 16 A peak-to-peak). The behavior of discharge current oscillations is closely related to the magnetic field generated by the coils of the thruster. Therefore, in the discharge oscillation control algorism, the PPU controls the coil current to decrease the amplitude of the oscillations in the discharge current.

The in-orbit power consumption of the thruster system must not exceed the amount of power the satellite's power system can supply. Since the discharge current of the Hall thruster is highly dependent on the propulsion gas flow rate, the PPU controls the gas flow rate to move the power consumption of the thruster toward the target value.

For a given gas flow rate, JAXA's thruster tends to have a high thrust efficiency at low discharge currents. The higher the propulsion efficiency, the greater the thrust-to-power ratio, and the larger the propellant fraction that can be saved. The PPU can improve the thrust efficiency by changing the coil current to reduce the discharge current.

Figure 6 is a flowchart of the thruster control algorithm that integrates the power control, discharge oscillation control, and discharge current control. The gas flow rate and coil current increments in the flow chart are fixed (highlighted in yellow), so the PPU digital controller does not need to calculate them. The algorithm's computational complexity is minimal. This algorithm runs with a cycle time of 1 s while the thruster is running. The algorithm has a sequence number (Seq No.), which is initially Seq No.=1, which determines the algorithm's behavior for each control cycle. The operator can choose to enable or disable power control. The operator can also enable or disable oscillation control, discharge current control, or neither. If discharge current control is selected and the amplitude of the discharge current exceeds the upper specification due to a changed coil magnetic field, the algorithm transitions to oscillation control to stabilize thruster operation. When neither control function is selected, the algorithm runs with Seq No. = 2 and continues to monitor the amplitude of the discharge current. When both power control and discharge current control are selected, the behavior of the algorithm is as follows:

1) If the discharge current \({I}_{d}\) oscillation amplitude exceeds \({I}_{limit}\), the PPU turns off the thruster; otherwise, the power control sequence starts.

2) The PPU compares its power consumption with the threshold \({P}_{max}\), and, if the total power is greater than \({P}_{max}\), the gas flow rate is reduced by \(\varDelta m\); otherwise, the flow rate is increased by \(\varDelta m\). The Seq No. is updated to 2. \({T}_{gas}\) is the wait time after the gas flow rate changes.

3) After \({T}_{gas}\) s, the PPU updates the Seq No. to 21, selecting discharge current control.

4) When the amplitude of the discharge current oscillation is smaller than \({I}_{ste}\) (i.e., when the thruster is running stably), the coil magnetic field is changed by \({S}_{c}\bullet \varDelta {I}_{c}\). and seq No. is set to 22. \(\varDelta {I}_{c}\) is the amount of change in coil current. \({S}_{c}\) can be 1 or -1, corresponding to an increase or decrease in coil current. The wait time after the coil current change is \({T}_{mag}\).

5) After \({T}_{mag}\) s, When the amplitude of the discharge current is less than \({I}_{ste}\), the PPU2 compares the value of the discharge current with that of the previous cycle.

6) Then Seq no. is set to 1 if the discharge current is less than that of the previous cycle; otherwise, the coil current is changed by -2\({S}_{c}\bullet \varDelta {I}_{c}\) (i.e., the coil current increment opposite to the previous cycle).

In discharge current control, if the amplitude of discharge current oscillation exceeds the upper specification limit, the algorithm sets Seq. No to 11 and starts the oscillation control. The discharge current control is not restarted unless the amplitude falls below the upper specification limit in the oscillation control. The algorithm for calculating the amplitude of the oscillation of the discharge current is omitted in Fig. 6 and is as follows:

1) The discharge currents are acquired over 100 ms and divided into 1000 intervals.

2) The amplitude for each interval is calculated from the maximum and minimum values of \({I}_{d}\) in the interval.

3) Count the intervals in which the amplitude value is greater than \({I}_{ste}\).

In oscillation control, when the amplitude of \({I}_{d}\) is larger than \({I}_{ste}\), the coil current is changed by \({S}_{c}\bullet \varDelta {I}_{c}\). This operation is repeated until the amplitude of \({I}_{d}\) becomes smaller than \({I}_{ste}\). \(\varDelta {I}_{c}\) is the amount of change in coil current. \({S}_{c}\) can be 1 or -1, corresponding to an increase or decrease in coil current. When the amplitude of the discharge current \({I}_{d}\) increases by a changing coil current, the PPU reduces the coil current by \(2\varDelta {I}_{c}\).