Wave-driven countercurrent plasma centrifuge

A method for driving rotation and a countercurrent flow in a fully ionized plasma centrifuge is described. The rotation is produced by radiofrequency waves near the cyclotron resonance. The wave energy is transferred into potential energy in a manner similar to the α channeling effect. The countercurrent flow may also be driven by radiofrequency waves. By driving both the rotation and the flow pattern using waves instead of electrodes, physical and engineering issues may be avoided. (Some figures in this article are in colour only in the electronic version) Rotating plasmas have been investigated in a variety of configurations for the separation of elements and isotopes [1–4]. Isotope separation has applications in the production of nuclear fuel [5], and in research and medical isotope production [6]. While most enriched uranium is produced by the gas centrifuge method [7], a significant number of isotopes are still produced by calutrons due to their flexibility [8]. The idea to use rotating plasma to separate isotopes was first suggested by Bonnevier, who performed one of the first experiments to test this theory [1, 9]. In preliminary experiments, significant separation was measured, but it was found that the rotation rate (and therefore separation) was limited by the Alfven critical ionization velocity at the insulator surface along the field lines. The Alfven critical ionization velocity is vc = √ 2eVi/m where Vi is the ionization potential of the neutral species. Although the mechanism is not entirely understood, attempts to exceed this rotation speed at the insulator surface only result in ablation and increased ionization [1, 10–12]. Further experiments revealed that viscous heating fundamentally limits the separation factor in partially ionized plasmas [2]. Both of these limitations were overcome by the introduction of the vacuum-arc plasma centrifuge [3]. These pulsed devices were fully ionized, and did not have enough contact with the wall to be limited by the Alfven critical ionization velocity. However, due to their pulsed nature, throughput is limited, and the concurrent flow pattern limits the separation factor and flexibility of vacuum-arc plasma centrifuges. The primary method for producing the radial electric field of rotating plasmas is by segmented ring electrodes at the end of the centrifuge [13]. However, the Alfven critical ionization velocity limits the speed of plasma rotation at the electrode surface, even if the plasma is fully ionized [10, 13]. The ionization and density must also be carefully controlled to maintain an electrical connection to the end electrodes [14]. Since both the product and waste exit along the field line, these electrodes will be subject to a significant particle flux. A mechanism for removing product and waste from and around the electrodes would be advantageous. It has also been found that some elements of interest, such as uranium, react strongly with electrode materials [15]. These issues suggest that an electrodeless configuration would significantly reduce the engineering difficulty of a plasma centrifuge. In this paper, we will describe such an electrodeless plasma centrifuge. First we will summarize advantages of the plasma centrifuge over the gas centrifuge. Next, we will give an overview of the device design and its operating parameters. Then we will describe techniques to generate the radial electric field and the countercurrent flow using radiofrequency waves. 1. Comparison with gas centrifuge The separation in a centrifuge (plasma or gas) is produced by the force balance between the pressure and centrifugal forces. Since the centrifugal force is different for each species, there will be different pressure profiles. For a constant rotation frequency ", the equilibrium profile is (n2/n1)r = (n2/n1)0 exp ( #m"r/2Ti ) , (1) where #m = m2 − m1 is the mass difference between the isotopes or elements being separated, which are assumed to 0963-0252/09/045003+06$30.00 1 © 2009 IOP Publishing Ltd Printed in the UK Plasma Sources Sci. Technol. 18 (2009) 045003 A J Fetterman and N J Fisch have equal charge states. In gas centrifuges, the peripheral speed is limited by the stress tolerance of the rotating shell, and so only limited values of the local separation factor α0 = exp(#m"r/2Ti) can be achieved. However, in plasmas there is no such limitation. In order to compare separation methods, we can consider three quantities to be the most important: the single stage separation factor α, the single stage separative power δU and the energy cost per unit separative power [6]. If the fraction of the desired isotope in the waste of a separation element is x and in the product is y, the separation factor is α = y(1−x)/x(1−y). This quantity in general is independent of the concentration of the desired isotope in the feed. The separative power δU is the change in Fφ(z) between the input and output streams, where F is the flow rate, φ(z) is the separative potential, φ(z) = (2z − 1) ln(z/(1 − z)), and z is the concentration of the desired isotope [16]. A plasma centrifuge with length L, rotation frequency ", column radius a and constant temperature T has the maximum separative power, by analogy with gas centrifuges [16], δUmax = 2π 〈nD⊥〉L ( #m"2a2