Thrust Stepping

Step wise thrust variation was demonstrated for the MiDGIT breadboard thruster by incrementing the RF power level set by the signal generator. The thruster was operated by QinetiQ at various flow rates with beam and accelerator grid voltages of 950V/-95V [88]. Large positive steps in thrust could be achieved of the order 100N - 200N for a +1dB change in forward RF power (an example is given in Figure 5.17).

Thrust resolution and repeatability of the MiDGIT thruster will be dependent on the resolution and accuracy of the RFG and flow control unit (FCU). The resolution of the IFR 2031 signal generator used during the MiDGIT breadboard characterisation was limited to 0.1dB. Thrust resolution was observed to be of the order 15N - 25N for the SE1 and SE2 configurations due to a 0.1dB change in RF power, as summarised in Table 5-6. At this level of performance, a thrust resolution of < 0.5N would demand an RF supply with resolution of the order 0.01dB. Lower thrust resolution could obviously be achieved by reducing the number of apertures of the extraction grids but this would impact the maximum thrust level.

(a)

(b)

Figure 5.17. Thrust stepping of the SE2 configuration of the MiDGIT breadboard thruster

operated at 0.051 mgs-1 with Vb = 950V, VACC = -250V. Thrust level and forward RF power

(mgs-1) (W) due to 0.1dB step (N) SE1 0.037 14.5 20.3 0.040 14.5 18.2 0.043 14.8 15.6 0.047 15.2 15.1 0.050 17.0 15.7 SE2 0.047 14.8 25.4 0.051 15.9 19.7 0.052 15.9 15.4 0.053 16.3 17.8 0.058 17.8 14.2

Table 5-6 – Minimum thrust step at various flow rates for the SE1 and SE2 configurations of

the MiDGIT breadboard thruster due to a 0.1dB change in forward RF power [88].

5.8 S

Characterisation of the two single-end configurations of the MiDGIT breadboard thruster was performed in order to determine preliminary performance with regards to coarse thrust requirements. SE operation of the thruster has initially demonstrated thrust levels between 200N-500N and specific impulse of 400s-1100s with beam and accelerator grid voltages of 950V/-95V respectively. Beam profiles were obtained in order to provide verification of the measured electrical thrust and to determine a thrust correction factor to account for losses due to beam divergence. The extracted ion beams were observed to be affected by CEX interactions at low beam currents due to the high background pressure during operation. Operation of the thruster within a higher vacuum environment will be essential for performance and lifetime testing of any future engineering or qualification models of the MiDGIT thruster to reduce sputtering of the accelerator grid due to CEX ions. Further testing of the MiDGIT breadboard thruster by QinetiQ has demonstrated that the maximum thrust, limited by direct ion impingement on the accelerator grid, can be increased to approximately 780N by increasing the beam voltage to 1300V. The thruster was determined to be operating at approximately 70% perveance at maximum thrust for Vb = 950V and approximately 51%

perveance at maximum thrust for Vb = 1300V. Maximum thrust could be increased further

still by operating at even higher beam voltages or increasing the number of grid apertures, however these changes will impact the minimum thrust level. Thrust resolution was observed to be of the order 25N worst-case, limited by the 0.1dB resolution of the signal generator

controlling the RF power level. Electrical efficiency was observed to be low and discharge loss observed to be high, indicating poor power coupling to the plasma. Significant improvements to electrical efficiency could be achieved by operating the thruster with an optimized RF supply located directly next to the thruster. The design of the induction coil should also be improved to minimize coil resistance. Heating of the coil was observed to be an issue due to the effect on coil resistance. An increase of up to approximately 50% in coupling efficiency would be required to reduce power levels and bring specific power more in line with the requirement of 50W/mN. This may also permit reductions in flow rate and the possibility of lowering the minimum thrust level.

A summary of single-end performance of the MiDGIT breadboard thruster is provided in Table 5-7, along with performance data of similar sized miniature RF ion thrusters as a comparison. It can be seen that the current configuration of the MiDGIT thruster is unable to meet the requirements specified for coarse thrust control. However, the RIT-4 and RIT-2.5 thrusters, at a more advanced stage of development, have demonstrated suitable performance for coarse thrust mode [27, 41, 93], and therefore, implementation of the specified changes to a MiDGIT engineering model should allow the requirements for coarse thrust mode to be achieved.

 Maximum thrust increased to ~780 µN with Vb = 1300V

* Flow rate corrections applied to SE2



Estimated from Gaussian fit to central beam distribution, not accounting for effects on the beam due to CEX collisions

Table 5-7 – Summary of the performance of the SE1 and SE2 configurations of the MiDGIT breadboard thruster.

(neglecting neutralizer power and flow rate)

(Performance data of the similar sized RIT-4 and RIT-2.5 thrusters are provided as a comparison [27, 41, 93]).

SE1 SE2 RIT-4/7 RIT-4/151 RIT-2.5/37

Thrust range 175µN - 500µN  200µN - 550µN  10µN – 200µN 150µN – 3.5mN 50µN - 575µN

Specific impulse 500s – 1100s 400s – 1000s 3850s @ 250µN >3700s @ 3.5mN 363s – 2861s

Flow rate * 0.030 mgs-1 - 0.050 mgs-1 0.046 mgs-1 - 0.056 mgs-1 0.004 mgs-1 – 0.008 mgs-1 - 0.014 mgs-1 – 0.021 mgs-1

Discharge power (Pfw) 13W – 17.5W 13W – 17.5W - - 12.4W – 18.25W

Mass utilization efficiency 13% - 29% 11% - 27% 25% - 50% 80% @ 3.5mN 14.7% - 51.5%

Electrical efficiency 18% - 33% 22% - 35% <50% 60% @ 3.5mN 4.2% - 46.5%

Specific Power 100 W/mN – 58 W/mN 85 W/mN – 55 W/mN - 40W/mN @ 3.5mN 262 W/mN – 59.8 W/mN

Minimum Discharge Loss 1916 eV/ion 1770 eV/ion - 769 eV/ion 2287 eV/ion

Beam divergence <13o  <13o  <15.2o - -

Operating frequency 5.25 MHz 1 MHz ~2 MHz 2.9 MHz

Grid apertures 55 7 151 37

Beam potential 950V 1100V - 1600V 1400V 372V – 1980V

Chapter Six