Spectral measurements of inductively coupled and helicon discharge modes of a laboratory argon plasma source Murat Celik ⁎ Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA 02139, United States abstract article info Article history: Received 26 August 2010 Accepted 12 January 2011 Available online 19 January 2011 Keywords: Helicon plasma Inductively-coupled plasma Spectroscopic measurement Electric propulsion An experimental study was conducted to investigate the effects of several operational parameters in the emission spectra, in the 400–850 nm wavelength region, of a laboratory Argon plasma source. In particular, the emission spectra of the inductively coupled plasma and the Helicon plasma modes of operation were compared. Comparisons of spectra point to a significant increase in the ionization fraction of the plasma for the Helicon mode of operation. The spectral measurements allow one to determine the major trends in the plasma electron density for various parameters such as power delivered to the helical antenna, propellant mass flow rate, and applied external magnetic field intensity. Analysis of a prominent Argon single ion line, at 434.8 nm, points out that the plasma electron density increases linearly with the power delivered to the helical antenna, and that there is an optimum propellant mass flow rate for maximum ionization fraction. Additional analysis of the same line shows that above a minimum applied axial magnetic field intensity, the variation in the magnetic field strength has little effect on the plasma electron density. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The use of helical shaped Radio Frequency (RF) antennas to create high density plasmas (n N 10 18 m -3 ) has been widely studied. In the inductively coupled plasma (ICP) sources, typically, the region of plasma generation is surrounded by a helical shaped coil that creates a time varying magnetic field around it when supplied with RF currents. The time varying magnetic field induces a solenoidal RF electric field which accelerates the free electrons and creates the plasma [1,2]. In the helicon plasma sources, similar to the ICP sources, a radio frequency driven helical antenna is placed around a dielectric cylinder but with a direct current (DC) axial magnetic field applied in the region of the plasma generation allowing the excitation of a helicon wave within the source of the plasma [1]. Because of their efficient high density plasma production, the helicon plasma sources are getting increased attention over the past few decades [3–6]. However, the detailed mechanism of the helicon mode plasma generation is still an ongoing scientific debate [7–10]. The mini Helicon Thruster Experiment, mHTX, has been built to characterize the helicon plasma source in order to gain a better un- derstanding of the plasma generation by helical shaped RF antenna, and identify methods by which plasma parameters can be tuned to accelerate the obtained high density plasma in order to achieve an efficient propulsive system [11–13]. In electric thrusters external energy is used to ionize gas and then accelerate the resulting plasma using electric and magnetic field forces. In thruster concepts such as the Variable Specific Impulse Magnetoplasma Rocket (VASIMR), a helicon source is used to produce high density plasma, while a secondary stage is used to heat the ions by ion cyclotron resonance heating using radio frequency waves and a magnetic nozzle is used to convert azimuthal momentum into axial momentum to accelerate the gas particles [14–16]. For the mHTX concept, the goal is to obtain high density plasma using a helicon discharge and then accelerate it through thermal pressure which creates ambipolar potential gradients. The power is delivered to the particles through wave–particle coupling using the helicon waves. In the current study, emission spectroscopy is used as a means to deduce information about the plasma through the measurement of line radiation emitted from the plasma particles [17]. It is shown that change in the operational parameters significantly affects the ionization fraction of the plasma. 2. Experimental setup and procedures All spectral measurements were conducted at the MIT Space Propulsion Laboratory. The plasma source was placed inside the 1.5 m diameter 1.6 m long vacuum chamber that is equipped with a mechanical roughing pump and two cryogenic pumps with a total pumping capacity of 7000 L/s for Xenon [18]. Spectrochimica Acta Part B 66 (2011) 149–155 ⁎ MIT Space Propulsion Laboratory, currently Assistant Professor at Bogazici University, Istanbul, Turkey. Tel.: +90 212 359 7372; fax: +90 212 287 2456. E-mail address: murat.celik@boun.edu.tr. 0584-8547/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2011.01.003 Contents lists available at ScienceDirect Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab