Thruster operation mode I Ratio of erosion rates of insulators ΔVR, % Direct method 33

by the following expression:

Symbol definitions of Figure 14.

Hall Thruster Erosion

DOI: http://dx.doi.org/10.5772/intechopen.82654

erosion measurements

insulators (Figure 14).

Experimental results with the direct methods.

Table 2.

107

Table 2.

Table 1.

where ERAD\_OUT and ERAD\_IN are changes in thickness of the edges of the external and internal insulators after the thruster tests, mm.

The radial erosion rate is defined as:

$$V\_{RAD\\_OUT} = \frac{E\_{RAD\\_OUT}}{t\_N}, \quad V\_{RAD\\_IN} = \frac{E\_{RAD\\_IN}}{t\_N},\tag{8}$$

where VRAD\_OUT and VRAD\_IN—radial erosion rate of outer and inner insulators, mm/h; tN—duration of the thruster test phase, h; and N—thruster test stage number.

Figure 13. Scheme of radial erosion measurements.


#### Table 1.

5. Comparison of the erosion rate of insulators in different modes of HT. The ratio of the erosion rates of edges obtained by the OESSC method (ΔEr) was

6.Development of recommendations regarding the optimal operating mode with

After taking measurements by the OESSC method and selecting the operating mode, the thruster is tested on a time base of 50 hours, and measurements of radial erosion by a direct method are performed (Figure 13 and Table 1). As a basis for measuring the insulator edge thickness, the outer diameter of the outer insulator

Measurements are provided with the help of an instrumental measuring microscope with an error of less than 0.0005 mm. To eliminate errors in the installation of the DCh relative to the microscope, each measured position is photographed.

Radial erosion is defined as the difference between the coordinates of the edges

ERAD\_OUT ¼ eOUT\_INITIAL � eOUT\_FINAL ERAD\_IN ¼ eIN\_INITIAL � eIN\_FINAL

where ERAD\_OUT and ERAD\_IN are changes in thickness of the edges of the exter-

where VRAD\_OUT and VRAD\_IN—radial erosion rate of outer and inner insulators, mm/h; tN—duration of the thruster test phase, h; and N—thruster test stage number.

, VRAD\_IN <sup>¼</sup> ERAD\_IN

tN

� 100% if ð Þ Erinner\_ceramic>Erouter\_ceramic or

(6)

(7)

, (8)

� 100% if ð Þ Erouter\_ceramic>Erinner\_ceramic

determined like:

Propulsion Systems

<sup>Δ</sup>Er <sup>¼</sup> Erinner\_ceramic � Erouter\_ceramic Erinner\_ceramic

<sup>Δ</sup>Er <sup>¼</sup> Erouter\_ceramic � Erinner\_ceramic Erouter\_ceramic

the maximum lifetime of the thruster DCh edges.

and the inner diameter of the inner insulator were chosen.

of the insulators before and after the thruster operation.

nal and internal insulators after the thruster tests, mm.

VRAD\_OUT <sup>¼</sup> ERAD\_OUT

tN

The radial erosion rate is defined as:

Figure 13.

106

Scheme of radial erosion measurements.

6.2 The method of the radial erosion measurements

Symbol definitions of Figure 14.

Comparison of erosion rates of external and internal insulators was carried out by the following expression:

$$
\Delta V\_R = \frac{V\_{RAD\_{-}max} - V\_{RAD\_{-}min}}{V\_{RAD\_{-}max}} \cdot 100\,\%,\tag{9}
$$

where VRAD\_max—erosion of the insulator for which the wear is greater, mm/h; VRAD\_min—erosion of the insulator for which the wear is lower, mm/h.

Results of the erosion measurements with direct methods are presented in Table 2.

Thruster operation modes: I—mode with the lowest discharge current, current of inner coil—5 A, and current of outer coil—5 A.

As it is easy to see from the results on the mode with the lowest discharge current, erosion of ceramic walls is not uniform.

#### 6.3 Comparison of results accepted by the OESSC method and direct radial erosion measurements

Figure 14 shows the results of the erosion rate measurements by the OESSC method. It was found that in the operating mode of the thruster with the minimum discharge current (mode I), the difference in the erosion rate of insulators was about 32%, which corresponded to the data obtained with direct measurements. After diagnostics and comparing the irregularity of erosion in 25 regimes, it was found that at the operating mode II, the difference in the erosion rate of the edges does not exceed 0.3%. The results were confirmed in subsequent tests and measurements by a direct method. The time spent on the study of the thruster DCh wear by the OESSC method on 25 operating modes, taking into account the expectation of the time for stabilizing the erosion rate, was 7 hours, which made it possible to reduce significantly the search for optimal current of the coils of the magnetic system from the point of view of uniform wear of the edges of the insulators (Figure 14).


#### Table 2. Experimental results with the direct methods.

mechanical damage to their surfaces. The design of the thruster allows ensuring

The measurements of the ceramic mass are carried out as follows:

2. degassing of insulators in a vacuum oven at a temperature of 200° C for

The results obtained were used to measure the weight of the esterified substance at the end of each test stage. After stopping the tests, the thruster should be cooled. The temperature of the thruster was controlled by the voltage on the coil. Then the thruster was reinstalled from the chamber, was sorted out, and the ceramic insulators were mounted on the scales. Measurement of the insulator mass was carried out for 40 minutes. Counting of time began from the

3. reweighing of the samples till the original weight value was restored to 95%

It was found that the samples with the composition of pure BN, as well as with 5 and 10% impurities of SiO2 reduced in mass for half an hour when they were in the atmosphere. It took more than 2 hours to restore the original mass of the

As you can see from Figure 15, the lowest erosion parameters have pure nitride

The result of the tests: black—inner ceramic erosion rate; red—outer ceramic erosion rate.

For each of the ceramic samples, the test time was 68 hours. The whole period of time was divided into four stages of 17 hours each. At the end of each stage of the test, the measured mass for each of the ceramic inserts was suspended and mea-

unambiguous installation of ceramic inserts in the DCh.

sured using a laboratory scale WA-21.

DOI: http://dx.doi.org/10.5772/intechopen.82654

1. insulator weighing;

Hall Thruster Erosion

30 minutes; and

boron ceramic.

Figure 15.

109

7.1 Ceramic insulator mass measuring method

remaining samples (20, 35, and 50% SiO2).

moment of air intake into the vacuum chamber. Figure 15 shows the results of the experiment.

#### Figure 14.

Dependence of the erosion rate from the magnetic system coil currents.


#### Table 3.

Experimental results of erosion measurement with the OESSC method and direct method on two thruster regimes.

The erosion rate data for each of the insulators were approximated and extrapolated, in order to find regimes with the same erosion of the insulator edges. It is obtained that in the investigated range of coil current variation, there are five modes, under which the wear of the DCh edges will be uniform. However, the general wear of the material in other modes is greater. Therefore, mode II was selected for thruster operation.

After getting the results with the OESSC method, the experiment with direct measurements was provided for the thruster regime II. Table 3 presents the results.

As it is seen from Table 3, results accepted by direct measurement were totally confirmed with the results of OESSC method measurements.

Conclusion was done to operate the thruster on the regime II, because on this regime, thruster lifetime is significantly higher.

#### 7. Results of the investigation of several types of ceramic for the Hall thruster

Resource tests of the hollow thruster of power 100 W with one coil were conducted in the STC SPE of National Aerospace University named after N. Ye. Zhukovsky "Kharkiv Aviation Institute," Ukraine. The test purpose was to study the rate of ceramic insert erosion of BN with a different percentage of SiO2 impurities.

To carry out the experiment, a thruster design was developed that allowed the rapid extraction of the outer and inner ceramic insulators without causing

#### Hall Thruster Erosion DOI: http://dx.doi.org/10.5772/intechopen.82654

mechanical damage to their surfaces. The design of the thruster allows ensuring unambiguous installation of ceramic inserts in the DCh.

For each of the ceramic samples, the test time was 68 hours. The whole period of time was divided into four stages of 17 hours each. At the end of each stage of the test, the measured mass for each of the ceramic inserts was suspended and measured using a laboratory scale WA-21.

#### 7.1 Ceramic insulator mass measuring method

The measurements of the ceramic mass are carried out as follows:


It was found that the samples with the composition of pure BN, as well as with 5 and 10% impurities of SiO2 reduced in mass for half an hour when they were in the atmosphere. It took more than 2 hours to restore the original mass of the remaining samples (20, 35, and 50% SiO2).

The results obtained were used to measure the weight of the esterified substance at the end of each test stage. After stopping the tests, the thruster should be cooled. The temperature of the thruster was controlled by the voltage on the coil. Then the thruster was reinstalled from the chamber, was sorted out, and the ceramic insulators were mounted on the scales. Measurement of the insulator mass was carried out for 40 minutes. Counting of time began from the moment of air intake into the vacuum chamber.

Figure 15 shows the results of the experiment.

As you can see from Figure 15, the lowest erosion parameters have pure nitride boron ceramic.

Figure 15. The result of the tests: black—inner ceramic erosion rate; red—outer ceramic erosion rate.

The erosion rate data for each of the insulators were approximated and extrapolated, in order to find regimes with the same erosion of the insulator edges. It is obtained that in the investigated range of coil current variation, there are five modes, under which the wear of the DCh edges will be uniform. However, the general wear of the material in other modes is greater. Therefore, mode II was

Experimental results of erosion measurement with the OESSC method and direct method on two thruster
