**2. Experimental setup and results**

Experiments were carried out on a laboratory stand, the schematic diagram of which is shown in **Figure 1** on the left. Vacuum chamber 1 contained a test mockup of the accelerator with open walls. The accelerator consisted of a magnetic core with permanent magnets 2 and an electrode system with cylindrical anode 3 and cathode 4 formed by a system of pins. The experimental installation allowed a controlled gas input, by usinCHA-2 system. The gas pumping was performed with the use of a vacuum unit with an oil-vapor pump. The photo of the accelerator with anode layer and open wall is shown in **Figure 1** on the right.

As can be seen from the **Figure 1** on the left, the cathode is composed of two parts that are separated in space. The ends of the cathode pins coincide with the magnetic surfaces that in area between them are parallel to the anode surface. The application of the plasmaoptics principles to the design of a cylindrical accelerator with anode layer and open walls allowed to create an accelerator with virtual cathode that is parallel to the anode plane for its entire width and with cathode that is several centimeters wide. That allows to form a wide flow of accelerated ions to the system axis of symmetry. Due to that each part of the cathode is made in the form of pins, the collecting surface area is significantly reduced, that reduces the contribution of the cathode material in the flow.

The discharge in the system burns due to the working gas (argon) ionization by the electrons. Electrons are magnetized and formed stable negative space charge. The created ions are accelerated from the ionization zone towards the cathode.

The formation of the magnetic field is provided by a system of permanent magnets located in the magnetic circuit. By selecting the number and polarity of the *The Emerging Field Trends Erosion-Free Electric Hall Thrusters Systems DOI: http://dx.doi.org/10.5772/intechopen.99096*

**Figure 1.**

*Left: Experimental setup; vacuum chamber (1), magnetic system (H= 650–750 Oe) (2), anode (3), cathode (4). Right: Photo of the accelerator with anode layer and open walls.*

magnets in each system layer, it was possible to change the magnetic field geometry in a wide range. In experiment the system of permanent magnets was arranged in such a way that the magnetic field in the gap between the cathode and the anode was parallel to system's axis as much as possible. It was owing to this configuration of the magnetic field that a system with open walls was created.

### **2.1 Experimental results**

As mentioned above, the cathode is composed of two parts separated in space and their edge coincide with magnetic field lines that are parallel to anode surface. The magnitude of the magnetic field is such that Larmor radius of the electron is much smaller than the system radius. Due to this, the principle of the equipotentialization of magnetic field lines with accuracy up to the electron temperature works [11]. Accordingly, a cylindrical virtual surface is formed between these parts, which potential is close to cathode potential. Through the electrons drift in the crossed electric and magnetic field ExH there is a closed Hall current and corresponding space charge, that creates a layer near the anode surface where the main potential drop occurs [12]. With the appearance of the Hall current in the perpendicular direction, the electron current along E becomes small. In turn, the ions are almost unaffected by the magnetic field, because the Larmor radius of the ion is more than characteristic system size [2, 13]. Ions under the electric field influence, moving in the cathode direction, converge to the center and are pushed out of the system along the axis. The electrons exit speed from the system along the axis is quite small and is compensated by the working gas ionization by these electrons. At low pressure, ionization occurs between the anode and virtual cylindrical cathode. With increasing pressure, the anode layer size can reach the radius of the system. The operation modes of the accelerator also change [9].

The dependence of the current strength on the applied potential to the anode for different working gas pressure were obtained experimentally. The results of the effect of the change in the working gas pressure in the source volume on the discharge current density for the low-current mode are shown in the **Figure 2**. For the presented curves for different voltages, it can be seen that the current density little depends on the gas pressure in the system for potentials up to 1,5 kV in the discharge gap.

This result can be explained by the assumption that the ionized particles concentration under these conditions does not depend on the working gas pressure. This corresponds to the obtained theoretical results also.

### **Figure 2.**

*Dependence of current density on the pressure the chamber at different values of the voltage applied to the discharge gap.*

Note that the operation modes of this kind accelerator are very close to those of the classical accelerator with an anode layer. There are two operation modes of this kind accelerator. The first is low-current, with a clearly visible narrow radiating layer between the anode and cathode, in the range of 10<sup>4</sup> Torr. The current increases monotonically with increasing applied voltage (see **Figure 3a**). With increasing pressure at a constant applied voltage, this layer gradually occupies the entire volume of the accelerator. During this mode, the ions are mostly formed in a narrow anode layer and move towards the system axis, hardly experiencing the influence of a magnetic field. Accumulating on the system axis, they are pushed out of the volume along it. The study of the distance changing influence between the virtual cathode and anode showed the existence of the optimum. The maximum current on the system axis is fixed at a distance d=10 mm between electrodes, as one can see from **Figure 3a**.

In the second mode – high current, the discharge extends to the entire internal system volume. When the voltage reached a certain value (U > 1,8 kV), the

### **Figure 3.**

*Volt-ampere characteristic of the accelerator in low-current mode at a) different distance between the anode and cathode at pressure 10<sup>4</sup> Torr and b) at different values of pressure in chamber.*

### *The Emerging Field Trends Erosion-Free Electric Hall Thrusters Systems DOI: http://dx.doi.org/10.5772/intechopen.99096*

discharge current increased in a jump-like manner (see **Figure 4a**), and the discharge transited into the high-current mode, in which the distinct anode layer was absent. In this mode, a typical discharge current was several orders of magnitude higher (up to 2 A, as can be seen from **Figure 4**) than in the low-current mode. Thus, It is evident that the transition into the high-current mode occurs under the influence of two factors: the working gas pressure (see **Figure 3b**) and the voltage applied across the discharge gap (**Figure 4**).

Another characteristic feature of the high-current mode is the formation of a plasma torch. At the discharge voltage U > 1,8 kV bright radiation is observed from the system volume from the ends of the cylindrical channel along the rotation symmetry axis (see **Figure 5a**). In the discharge concerned, ions are accelerated along system's radius towards system's axis. The torches at the ends, on the contrary, are observed along the axis, perpendicularly to the radius and the direction of initial ion acceleration.

Thus, owing to the discharge geometry, in space limited by electrodes of the accelerator there is an accumulation of ion space charge like a lens with a positive space charge, that was proposed earlier for negatively charged particles beam focusing [14–17]. The generated ions reach the system axis and accumulate in the region around it. Ions are stored in the cylinder volume until their own space charge creates a critical electric field. This field forces ions to leave the volume. The main part of the generated ions escapes from the system perpendicularly to its radius. Due to this plasma torches are formed at the edges of the device, which are clearly

**Figure 4.** *Volt-ampere characteristic of the accelerator at different pressure in a) low-current mode b) high-current mode.*

### **Figure 5.**

*a) Plasma jet from volume of thruster in high current operation mode. b) Floating potential dependence on the pressure at the system edge.*

### *Plasma Science and Technology*

visible in high-current mode. The results of measuring the floating potential along the axis of the system show that under certain conditions along the plasma torch axis there may be a potential drop (see **Figure 5b**), which can be used to accelerate the generated ions and form a charged particles beam.

Radial studies of the plasma flow coming out along the system axis in this device at different pressures revealed a significant increase in current density on the axis (see **Figure 6**). That fact may indicate the plasma acceleration in this direction. The study of the dependence of the uncompensated current density on the accelerator volume shows the existence of a maximum for a pressure of 6<sup>10</sup><sup>4</sup> Torr. This operation mode of the accelerator is mostly interesting for use as a prototype of the ion-plasma small rocket engine.

Determination of the ion energy distribution function (see **Figure 7a**) in this accelerator was performed by retarding potential method using a 3-line analyzer (discharge current 1.5–2 A; voltage 1.5–2.1 kV; pressure in the range 10<sup>4</sup> –10<sup>3</sup> Torr). The research results showed the formation of a sufficiently monoenergetic beam with of FWHM (as can be seen from **Figure 7b**), at the level of 10% of the average value which reached two thirds of the anode discharge potential (in particular, at Uanode = 1.8 kV, E = 1.2 keV).

### **Figure 6.**

*a) Distribution of the current density along the system radius at the accelerator output. b) the dependence of the current density on the system axis on the pressure in chamber at different values of potential at the anode.*

### **Figure 7.**

*a) Ion energy distribution function ion energy distribution function (voltage 1.8 kV; pressure 3\*10<sup>4</sup> Torr). b) the dependence of FWHM (full width at half maximum) on the voltage applied to the anode.*
