WDS'12 Proceedings of Contributed Papers, Part II, 93–98, 2012. ISBN 978-80-7378-225-2 © MATFYZPRESS Study of Helicon Double Layer Thruster A. K. Petrov, E. A. Kralkina Lomonosov Moscow State University, Faculty of Physics, Moscow, Russian Federation. Abstract. The present work is devoted to the investigation of the radio-frequency (RF) inductive discharge placed in an external magnetic field. The discharge can be used as a basis for the development of the high-performance ion thruster with flexible control options. In the present thruster model, ions are accelerated in a potential drop of double layer appearing at the output of the thruster in the divergent magnetic field and due to gas-dynamic effect. The highest achieved ion energy is 175 eV at gas flow rate of 2 sccm. Introduction In 1970, Australian physicist R. Bosvell [1] investigating the inductive RF discharge placed in an external magnetic field discovered a resonant increase of the plasma density at some values of the magnetic field induction. Later, F. F. Chen [2] hypothesized that under experimental conditions of Boswell helicons were excited in a discharge whose energy was absorbed by nonlinear collisionless Landau damping mechanism. However, later studies [3, 4, 5] showed that the energy is absorbed not primarily due to helicon, but due to the associated Trievelpiece-Gold wave (or oblique Langmuir wave). The best RF power input can be obtained by placing the discharge in an external magnetic field, whose value corresponds to the conditions of resonant excitation of the waves. In papers [6–8], these statements were analyzed using mathematical modeling and were confirmed experimentally. In the mid-1990s, Bosvell [9–12] showed that when using a divergent magnetic field, the ions with energies of several tens of eV were observed. The reason for the acceleration of ions on the hypothesis of Boswell is a potential drop that occurs at the boundary cells of different diameters. In 1995, a group of Australian scientists led by Boswell suggested to use this effect to generate thrust in a new electrodeless ion thruster. The new thruster was named the helicon double layer thruster (HDLT). Because currently at the forefront of space industry is the demand of increasing thruster’s lifetime (which is of much importance for both deep space exploration and communication systems operations) HDLT became of great interest since it has no direct contact between plasma and vital parts which significantly reduces erosion processes and increases the expected thruster`s lifetime. During last decade, it was discovered that HDLT was able to work with chemically active propellants [13] allowing the use of bingo fuel remaining after chemical engines work. In our experiments, the contraction was made at the end of the thruster model. This allowed us to reach ion energies of 175 eV at gas flow rate of 2 sccm combining gas-dynamic effect with potential drop acceleration mechanism. Aims of the present work were following: investigations of power input dependence on the magnetic field and gas flow rate, research of the discharge spectral characteristics, analysis of Ion Energy Distribution Function (IEDF) depending on the power input, gas flow rate and magnetic field, determination of main ion acceleration mechanisms and as the result revealing the conditions of the highest ion energy and thruster efficiency. Experimental facility Plasma is ignited in a gas discharge chamber which is a glass cylinder 15 cm long, 5 cm in diameter with a gas distributer on the upper end and with a 2 cm constriction on the other end (Fig. 1a). In this article, we describe the results obtained using only this configuration. Alternative discharge chambers such as shorter chambers (10 cm long and 5 cm in diameter) and chambers with a Laval nozzle are represented in Fig. 1b. In the experiment (Fig. 2), a signal from a radio frequency generator processing on 13.56 MHz is applied through the matching system (Fig. 3) to a spiral antenna assembled on the lateral side of the plasma source. The discharge is placed into the external magnetic field provided by electromagnet. The discharge chamber is mounted on a vacuum chamber with operating pressure of 10–5–10–6 Torr created by rotary, booster and diffusion pumps. Measurements are made using Retarding Field Energy Analyzer (RFEA) and Langmuir probes installed inside of the vacuum chamber. To obtain ion energy distribution function (IEDF), the dependence of the ion current vs retarding field is measured. A signal is registered in a computer after the voltage divider and analog- to-digital converter (ADC). Then, the Tihonov regularization method is used. To obtain the value of a potential drop in double layer compensation method is used. Langmuir probes are mounted inside and at the exit of the 93 PETROV AND KRALKINA: STUDY OF HELICON DOUBLE LAYER THRUSTER plasma source and connected to a power supply. Then, we measure the voltage on a power supply at which the current becomes zero. This voltage is equal to a difference between plasma floating potentials in these two points. Due to the difference in electron temperatures inside and out of the discharge chamber (and as a consequence due to different slopes of I–V characteristics of Langmuir probes), it is pretty close but not equal to the potential drop in a double layer (Fig. 4). For the purposes of the present work (to qualitatively estimate some phenomenon) such accuracy is satisfactory. Also, it should be noted that probes are not RF compensated, so the floating potentials may be shifted significantly. On the other hand, no absolute value is used but the difference between floating potentials in two points is applied. Obtained dependencies are interesting (see below in Experimental results, Fig. 11) but should be repeated in the future research. Figure 1. (a) main gas discharge, (b) alternative discharge chambers. Figure 2. General view of experimental facility. Figure 3. Matching system. 94 PETROV AND KRALKINA: STUDY OF HELICON DOUBLE LAYER THRUSTER To estimate the radial component of the external magnetic field, a computer simulation is performed. The results of the modeling for different positions of the magnet relative to the discharge chamber are presented in Fig. 5. In the present work, a configuration with electromagnet mounted on the top of the chamber is used. In this case, an appreciable radial component of the magnetic field necessary for double layer formation appears [9–12]. Experimental results Observations showed that the power absorption efficiency increases significantly at the magnetic field of 75–200 Gs due to the excitation of helicon and Trivelpiece-Gold waves (Fig. 6). Power absorbed by our system (PGEN) is equal to a difference between forward (PF) and reflected (PR) generator power and is equal to power loss in plasma (PPL) and in external load (PE): PGEN=−= PPP F R PL + P E , (1) Figure 4. Error of the Langmuir probe method due to electron temperature differences. Figure 5. Magnetic field configuration of the computer simulation (blue lines – gas discharge chamber, black lines – magnetic field lines.). Figure 6. Efficiency of power absorption depending on the magnetic field. 95 PETROV AND KRALKINA: STUDY OF HELICON DOUBLE LAYER THRUSTER P=⋅= UR I2 ⋅ R, (2) where P – power, U – voltage, I – current, R – resistance. External load resistance can be calculated measuring power absorption without a discharge: 2 REE= PI/ 0 , (3) where I0 – current in system without plasma. Then power absorbed by a discharge: 22 2 PPL= P GEN − RI E ⋅=( PGEN / I00 − PIE /) ⋅ I (4) Radiation intensities of atomic and ionic lines measured versus RF power input are presented in Fig. 7. To demonstrate the connection between these lines, they are scaled. The magnetic field value is chosen so that the RF field penetrates deep into the plasma. While atomic line first increases and then saturates, the ionic line is growing throughout the considered power range. We suggest that the reason of such behavior is plasma ionization degree growth. Figs. 8, 9 and 10 represent a typical ion energy distribution function in arbitrary units depending on gas flow rate, power input and magnetic field, respectively. Conditions were chosen to demonstrate selected dependences. It should be noted that Fig. 8 was obtained using a discharge chamber with the Laval nozzle, while Figs. 9 and 10—using a standard discharge chamber with simple contraction. With gas flow decrease, the number of collisions among ions reduces. This leads to a reduction of energy redistribution, so the number of low-energy ions decreases, while the number of fast ones increases (Fig. 8). A negative part of the graph and the parts where IEDF increases near zero appears due to features of differentiation using the Tikhonov regularization method. An increase of the RF power input leads to an increase of high energy peak (Fig. 9). With the magnetic field increasing up to threshold value where the resonant excitation of helicons and Trivelpiece-Gold waves take place the ions with higher energy appear (Fig. 10). To estimate the role of different ion acceleration mechanisms, an additional investigation is carried out. Double layer potential drop and ion energy are measured using the Langmuir probe method and RFEA, respectively. Subtracting potential drop from the ion energy one can qualitatively learn a contribution of the gas- dynamic effect. A comparison of the DL potential drop and ion energy (Fig. 11) showed that gas-dynamic acceleration mechanism leads to approximately 20 percent energy increase. As it was mentioned above, this experiment is additional and should be revalidated in future works. Figure 7. Intensity of a plasma radiation. F(ε) 0,8 Gas flow rate 2 sccm Gas flow rate 4 sccm Gas flow rate 6 sccm 0,6 0,4 0,2 0,0 0 50 100 150 200 B=0 Gs Ion Energy, eV Power input - 100 W Figure 8.