Effect of Exhaust Magnetic Field in a Helicon Double-Layer Thruster Operating in Xenon Christine Charles and Rod W. Boswell

Transcription

1 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 5, OCTOBER Effect of Exhaust Magnetic Field in a Helicon Double-Layer Thruster Operating in Xenon Christine Charles and Rod W. Boswell Abstract A xenon ion beam is spatially characterized by using a retarding-field energy analyzer positioned 7 cm downstream of a helicon double-layer thruster (HDLT) operating at 500-W radiofrequency power, 0.07-mtorr ( Pa) gas pressure, and with an exhaust magnetic field diverging from a maximum of about 142 G ( T) inside the thruster to about 26 G (0.026 T) at the probe location. The beam is formed by acceleration through the potential drop of a double layer (DL). It is found that, for constant operating pressure, increasing the maximum exhaust magnetic field from about G ( T) induces an increase of both the ion-beam energy and the ion-beam-to-downstream-plasma-flux ratio, both indicators of an increased thruster efficiency. Hence, the specific impulse can be controlled by using the exhaust magnetic field in the HDLT. Index Terms Double layer (DL), electric propulsion, helicon, ion beam, plasma thruster. I. INTRODUCTION THE DEVELOPMENT of electric propulsion over the past few decades has been strongly associated with two main types of thrusters: the gridded ion thruster [1] and the Hall effect thruster [2], [3]. These have been successfully used in space missions, and their characterization in the laboratory and by computer simulation has also been extensive. The dc magneticfield configuration associated with Hall effect thrusters plays a crucial role in the thruster efficiency [2], [4], [5] [7]. More recently, there has been increasing interest in the development of electrodeless magnetoplasma thrusters with anticipated longlife characteristics for high-power future deep-space missions. The plasma source can be an inductive source, a helicon source, an annular helicon source, or an electron cyclotron resonance source [8]. These low-pressure high-density sources have been successfully operated with nonreactive and reactive gases for applications in the microelectronics industry [9], [10]. Plasma flow systems with no applied magnetic field are also investigated for electric-propulsion applications [11], [12]. Ion acceleration downstream of low-pressure high-density magnetized plasma sources has long been proposed as a possible source of thrust for electric propulsion [13]. Of particular interest is the expansion associated with the formation of a double layer (DL) near the source exit which produces a lowdivergence ion beam with a well-defined energy [14] [16]. In Manuscript received November 4, 2007; revised February 10, First published October 31, 2008; current version published November 14, The authors are with the Space Plasma, Power and Propulsion Group, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT 0200, Australia ( christine. charles@anu.edu.au; Rod.Boswell@anu.edu.au). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TPS Fig. 1. HDLT attached to the Chi Kung s diffusion chamber, showing major components and diagnostics. The solenoid centered at z = 21 cm is called the exhaust solenoid and defines the exhaust magnetic field. the case of a helicon plasma source, the proposed concept has been named the helicon DL thruster (HDLT) [17], [18]. Recently, such a DL has been experimentally investigated in a number of laboratory devices with various geometries and magnetic-field configurations and using various diagnostics (electrical probes, optical emission spectroscopy, laser-induced fluorescence) [16], [18], [19]. The DL has also been characterized in pulsed argon plasmas [20] [22]. Results from a recently developed model [23], [24] and Particle-In-Cell simulations [16], [25] have also been compared to the experimental results. Other models including a diverging magnetic field have been recently proposed [26], [27], but the detailed mechanism involved in the DL formation and the role of the magnetic field remains unclear. In this paper, we report on an experimental characterization of the ion beam formed with the HDLT operating in xenon, the high-mass propellant used for space missions. The effect of the exhaust magnetic field on the ion-beam energy and ion-beam-to-downstream-plasma-flux ratio is investigated using an energy analyzer. II. EXPERIMENTAL SETUP AND RESULTS The HDLT is mounted on Chi Kung s 30-cm-long 32-cmdiameter earthed aluminum diffusion chamber described previously [28] and shown on Fig. 1. The HDLT consists of a 15-cm-diameter pyrex tube with a middle section defining a plasma-cavity length of 31 cm, and z = 30 cm is defined at thruster/chamber interface. Xenon gas is injected using nylon tubing on the closed end of the helicon source cavity via a hole in the middle section of the pyrex tube. Hence, the entire thruster walls are insulating. The 18-cm-long double-saddle field antenna that surrounds the pyrex tube is fed from an RFmatching network/generator system operating at MHz, /$ IEEE

2 2142 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 5, OCTOBER 2008 Fig. 2. B z component of the dc magnetic field measured along the z-axis for a constant current of 6 A in the solenoid centered near the closed end of the thruster (z =1.5 cm) and a varying current in the HDLT exhaust solenoid: (open triangles) 2, (crosses) 4, (black circles) 6, (open circles) 8, and (open squares) 10 A; z = 30 cm is the thruster/chamber interface. and the RF power is maintained at 500 W. Two solenoids surrounding the HDLT generate a divergent magnetic field. The Chi Kung vacuum chamber is pumped down to a base pressure of about torr using a 2000-L s 1 turbomolecular/rotary pumping system connected to the closed end of the diffusion chamber and facing the HDLT. The effective pumping speed measured for Xe is about 700 L s 1. The pressure is measured using a baratron gauge and an ion gauge attached to a sideport of the chamber. A retarding field energy analyzer (RFEA) positioned at z = 37 cm and which can be moved radially ( r =0 13 cm) and on its axis (facing the HDLT or facing the chamber sidewalls) is used to measure the collector current versus discriminating voltage characteristic I(V d ) [29]. The discriminator voltage is referenced to the grounded chamber walls as is the grounded RFEA. The derivative of I(V d ), called the ion-energy distribution function (IEDF), is fitted by a sum of Gaussians and normalized using a method previously described [28]. Previous measurements with the RFEA have shown the presence of a low-divergence xenon ion beam at z = 37 cm for an RF power of 500 W, an operating pressure of 0.07 mtorr ( Pa) corresponding to a xenon mass flow rate of 4 sccm, and for equal currents of 6 A in both solenoids [18]. This magnetic-field configuration shown by black circles on Fig. 2 corresponds to a local maximum of about 142 G at z = 21 cm, where the exhaust solenoid is centered, to about 10 G in the middle of the pumping vessel (z = 45 cm), and has been correlated with the formation of an electric DL near the thruster exit (z 25 cm) for a variety of propellants (argon [14], hydrogen [30], xenon [18]). In this paper, the current in the solenoid centered at z =1.5 cm (near the closed end of the HDLT) is kept constant at 6 A which induces a local maximum magnetic field of about 150 G (Fig. 2). Only the exhaustsolenoid current will be changed from 2 to 10 A, generating the various exhaust-field configurations shown on Fig. 2. The exhaust-field divergence is not changed and corresponds to a B z (21 cm)/b z (37 cm) ratio of about 5.4. The xenon ion beam is initially characterized along the chamber radius with the RFEA positioned at z = 37 cm and facing the thruster: Fig. 3 shows the normalized IEDF measured across the chamber diameter [color scale from 0 (blue) to 1 (red)] for an RF power of 500 W, an operating pressure of 0.07 mtorr ( Pa) corresponding to a xenon mass flow rate of 4 sccm, and for equal currents of 6 A in both solenoids (black circles on Fig. 2). The results are similar to those obtained for argon [31]. Outside the projection of the helicon source tube ( r > 8 cm), a single-peak IEDF around the local plasma potential V p ( V) is measured. In the projection of the source ( r 8 cm), a two-peak distribution is measured with the high-energy peak ( V) corresponding to an ion beam of velocity about 6 km s 1 which is generated in the HDLT and accelerated through a DL potential drop of 18.5 V [18]. The plasma potential has been previously measured along the z-axis in argon using a radial RFEA (aperture hole facing radially) showing the potential drop of the DL [14], and the results have been correlated to the ion-beam measurements downstream of the DL using an axial RFEA (aperture hole facing the plasma source and DL) [28]. Simulation in the diverging magnetic field of the orbits of the ions accelerated by a 25-V DL have shown plasma detachment at about z = 39 cm with a simulated divergence of 2 for xenon [17], [32], [33]. Experimentally, with the RFEA positioned at 37 cm, i.e., near the simulated point of detachment, the beam edge is measured for a radius r edge of 8 cm (Fig. 3), and the beam divergence angle α has been previously estimated [18] using tan α =(r edge r tube )/(z RFEA z DL ), where r tube is the source tube internal radius (6.8 cm) and z RFEA and z DL are the position in the magnetic nozzle of the RFEA and of the DL, respectively. The experimental ion-beam divergence is 5.7 for an 18.5-V DL [18], [32], [33]. The xenon ion beam has been previously characterized and modeled as a function of operating pressure, showing a DL pressure range between 0.02 and 2 mtorr ( Pa) with the present geometry [18]. A decrease in operating pressure leads to an increase of the ion-beam energy, hence, of the thruster specific impulse. A set of experiments is carried out by keeping a constant current (6 A) in the solenoid near the closed end of the thruster (6 A) and by increasing the exhaust-solenoid current from 2 to 10 A (Fig. 2) for an operating RF power of 500 W and a slightly higher pressure of 1.25 mtorr (0.166 Pa) half way in the DL pressure range and corresponding to a xenon mass flow rate of 7 sccm. The measured IEDFs corresponding to the magneticfield configurations of Fig. 2 are shown on Fig. 4. Each IEDF is fitted by the sum of two Gaussians: G beam associated with the high-energy ions and G downstream associated with the lowenergy ions (downstream plasma) G beam = A beam exp (V d V beam ) 2 B beam G downstream = A downstream exp (V d V downstream ) 2 B downstream

3 CHARLES AND BOSWELL: EFFECT OF EXHAUST MAGNETIC FIELD IN AN HDLT OPERATING IN XENON 2143 Fig. 3. (Color bar) Normalized IEDF versus Chi Kung s chamber radius measured with the RFEA positioned at z = 37 cm for an RF power of 500 W, an operating pressure of 0.07 mtorr ( Pa) corresponding to a xenon mass flow rate of 4 sccm, and a current of 6 A in each solenoid (black circles on Fig. 2). Fig. 4. Normalized IEDF versus exhaust-solenoid current. (Solid line peaking at 19 V) 2, (dotted dashed line) 4, (dashed line) 6, (dotted line) 8, and (solid line peaking at 47.7 V) 10 A for an RF power of 500 W, an operating pressure of 1.25 mtorr (0.166 Pa) corresponding to a xenon mass flow rate of 7 sccm, and a constant solenoid current of 6 A near the closed end of the thruster. The corresponding B z is shown in Fig. 2. The discriminator voltage is referenced to the grounded chamber walls as is the grounded RFEA. where B beam and B downstream are fitting constant related to the width of the peaks, respectively. The respective Gaussians are not shown on Fig. 4 for better clarity. For exhaust-solenoid currents of 2 and 4 A, the largest peak in the IEDF is the low-energy peak corresponding to the downstream plasma (G downstream ), and V downstream is about 19 V. The smallest peak is the high-energy peak corresponding to G beam, and V beam is about 32 V. A drastic change in the IEDF shape is observed when the exhaust-solenoid current is increased from 4 to 6 A: The IEDF shifts to higher energies (27 37-V range for V downstream and V range for V beam ) and the highenergy peak becomes the dominant ion population. The change of IEDF shape from 4 to 6 A may result from a transition from a simple expansion to a DL-containing plasma [34]. Fig. 4 shows that (V beam V downstream ) is about 13 V at 1.25 mtorr (0.166 Pa) for the 6-A case (dashed line) which is lower than the value of 18.5 V obtained for 0.07 mtorr ( Pa) and 6 A (Fig. 3). This results from the decrease of the DL potential with increasing pressure [18]. The variation of the ion-beam energy ev beam and the downstream plasma potential V downstream is plotted versus the exhaust-solenoid current shown in Fig. 5. Both parameters are similar for the 2- and 4-A cases. When the exhaust-solenoid current is increased from 4 to 10 A, both V beam and V downstream increase by about 18 V, and the difference (V beam V downstream ) does not change much: It is 12 V for 2-A exhaust-solenoid current, 11.7 V for 6 A, and 10.9 V for 10 A, respectively. Hence, the exhaust magnetic field can be used to control the ion-beam energy, hence, the specific impulse. The maximum ion-beam energy measured for an exhaust-solenoid current of 10 A is 48 V at 1.25 mtorr (0.166 Pa). Detachment of a 50-V ion from a thruster operating in space yields a specific impulse of 860 s 1 in xenon and 1550 s 1 in argon. The ionbeam energy corresponds to the plasma potential inside the thruster just upstream of the DL [34] [36]. The change of magnetic field mostly occurs in that region (z = cm) when the exhaust-solenoid current is varied (Fig. 2). In the present experimental system, which may differ from an HDLT operating in a more realistic space environment, a downstream plasma is always present in the vacuum chamber. It has been previously shown that the ion-beam density will decay exponentially with increasing z as a result of ion neutral collisions, drastically reducing A beam and A beam /A downstream along the thruster s main axis [20]. Still, the variation with exhaust magnetic field of the measured A beam /A downstream

4 2144 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 5, OCTOBER 2008 presented here because of the very large turbo-molecular pump and of the gas inlet location. Some idea of the thrust from the ions (T ions ) can be obtained from previous experiments in argon carried out under similar operating conditions (an RF power of 400 W and a pressure of 0.3 mtorr [ Pa]) using electrostatic probes positioned at about z = 20 cm. T ions is given by T ions = d(mv) dt = v ex dm dt v ex(v B Anm) Fig. 5. (Open circles) Ion-beam energy ev beam and (open triangles) downstream plasma potential V downstream versus exhaust-solenoid current; same operating conditions as in Fig. 4. Fig. 6. Maximum beam Gaussian over maximum downstream Gaussian A beam /A downstream ratio versus exhaust-solenoid current; same operating conditions as in Fig. 4. ratio should give some indication of the respective role of the ion beam and downstream plasma and of the thruster efficiency. The variation of the A beam /A downstream ratio versus exhaustsolenoid current is shown in Fig. 6. It is about 0.2 for 2 and 4 A and increases to 2.6, i.e., by over an order of magnitude, when the exhaust-solenoid current is increased from 4 to 10 A. Hence, the downstream plasma density decreases when the exhaust magnetic field is increased. The measured increase of both V beam and A beam /A downstream should lead to an improved thruster efficiency. Direct measurements of the plasma parameters inside the thruster have not been carried out for the xenon case study where v is the ion velocity, m is the argon ion mass ( kg), v ex is the ion exhaust velocity, n is the measured ion density ( m 3 ) [34], A is the source-tube area (0.015 m 2 ), and v B is the Bohm velocity (v B = kt e /m 4400 m s 1 ) calculated using a measured electron temperature T e of 8 ev ( K) [35]. In calculating the ion exhaust velocity, we assume that, in space, the potential at infinity equals the potential at the wall, and therefore, the ions exiting the thruster gain the full plasma potential. Here, the ion exhaust velocity is obtained using the measured plasma potential V p of 60 V (v ex = 2eV p /m m s 1, where e is the electron charge) [34]. The thrust in argon with the present geometry is about 8 mn for 400 W which yields about 10 mn for 500 W, assuming a linear increase of the ion density versus RF power in this inductively coupled plasma. Here, it is assumed that the ions enter the DL at the Bohm velocity, although the actual velocity might be up to twice the Bohm velocity as previously discussed in [8]. It has also been previously shown that reducing the plasma cavity length by a few centimeters leads to an argon ion-beam flux increase by a factor of three [36], suggesting that the HDLT can be further optimized in terms of thrust and efficiency. Recent measurements of the electron-energy probability function (EEPF) in argon upstream and downstream of the DL [35] have shown that, upstream, the EEPF is Maxwellian up to an energy determined by the DL potential (these are the trapped electrons ) and shows a depletion for higher energies. Downstream of the DL, the EEPF is close to a Maxwellian and resembles the shape and magnitude of the depleted upstream population. These are the high-energy free electrons measured upstream, which can overcome the electrostatic potential barrier of the DL and propagate downstream while being decelerated by the DL. Hence, a neutral plasma is emitted from the HDLT, and no neutralizer is needed [35]. Those free electrons which are not scattered or lost on the end wall of the diffusion chamber will be reflected by the end-wall sheath and return to the upstream area, regaining the energy they have lost to the DL structure and forming a half Maxwellian beam of accelerated electrons entering the trapped population upstream. Although not the case for all recent DL systems [8], here, the HDLT walls are insulating, and the DL is defined as current-free, as no current can flow through the wall at the closed end of the thruster. In anode DL experiments [37], with and without any applied dc magnetic field, the production of the anode plasma relies on ionization produced by electrons accelerated from the plasma to the anode plasma (the DL separates the two plasmas),

5 CHARLES AND BOSWELL: EFFECT OF EXHAUST MAGNETIC FIELD IN AN HDLT OPERATING IN XENON 2145 and the DL potential drop is typically V in argon, as it must slightly exceed the ionization potential ([8], [37], and references therein). In inductively coupled plasma systems [8], the potential of the DL formed in the expanding region can be much greater than the ionization potential of the background gas (15 V in argon) with values of up to 40 V measured in argon [38]. The inductively coupled plasma upstream of the DL does not rely on ionization produced by electrons accelerated from downstream to upstream, although these will obviously contribute to the global power balance. The EEPFs recently measured upstream of such a DL [35] show that both the free and the trapped electrons (with an energy exceeding the ionization potential) will contribute to the ionization upstream [23], [24]. III. CONCLUSION The HDLT is a new type of magnetoplasma thruster with potential applications for space missions where propellant consumption, safety, and lifetime are of concern, i.e., for interplanetary travel or large Earth-orbit raising maneuvers. The HDLT is simple, has no electrodes and no moving parts, and can operate with a variety of propellants. It has no need of an external neutralizer. This paper demonstrates that the HDLT operates well in xenon and that the specific impulse can be controlled by varying the exhaust magnetic field. REFERENCES [1] R. Walker, C. Bramanti, O. Sutherland, R. Boswell, C. Charles, D. Fearn, J. G. Del Amo, P. E. Frigot, and M. Orlandi, Initial experiments on a dual-stage 4-grid ion thruster for very high specific impulse and power, presented at the 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conf. and Exhib., Sacramento, CA, Jul. 9 12, 2006, Paper AIAA [2] V. V. Zhurin, H. R. Kaufman, and R. S. 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6 2146 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 5, OCTOBER 2008 Christine Charles received the French Engineering degree in applied physics from the National Institute of Applied Physics, Rennes, France, the Ph.D. degree in plasma physics and the French Habilitation thesis in materials science from the University of Orléans, Orléans, France, and the B.Mus. degree in jazz from the Australian National University, Canberra, Australia. She is currently an Associate Professor with the Space Plasma, Power and Propulsion Group, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT, Australia. For the past 20 years, she has been working on experimental expanding plasmas and their applications to electric propulsion, microelectronics and optoelectronics, astrophysical plasmas, and, more recently, to the development of fuel cells for the hydrogen economy. She is the inventor of the helicon double-layer thruster, a new electrodeless magnetoplasma thruster, which applications include satellite station keeping or interplanetary space travel. The thruster concept is based on her discovery in 1999 of the current-free double layer in an expanding radiofrequency plasma. Rod W. Boswell received the undergraduate degree from Adelaide University, Adelaide, Australia, and the Ph.D. degree from Flinders University, Adelaide. He is currently a Professor with The Australian National University, Canberra, Australia, where he is the Head of the Space Plasma, Power and Propulsion Group of the Research School of Physical Sciences and Engineering. He is active in the fields of plasma processing of surfaces for microelectronics and optoelectronics, plasma thrusters, fuel cells, as well as basic linear and nonlinear processes in plasmas. Over the past 15 years, he has published over 100 papers in major international journals, given about 50 invited lectures in international conferences, and presented his group s work to many industrialists in many countries. He is the holder of seven patents. He is interested in discovering interesting phenomena and using them in practical ways. His helicon reactor is well known as a fascinating research experiment and an effective processing tool in the microelectronics industry. In recent years, he has become interested in applying electric double layers to astrophysical phenomena and to space propulsion. His group will be contributing to the hydrogen economy by deposition of nanoagregates of catalysts and new proton-conducting membranes. He is the coinventor of the WEDGE virtual-reality theater, a number of which are now installed outside the university in museums, etc.