Collective Modes Associated with Rarefied Populations of Heavy Nuclei*

Plasmas in which low density populations of heavier nuclei than those of the main population are a frequent occurrence [1] and an important case is that of “impurities” whose cyclotron frequencies, Ω_I , differ from that, Ω_i, of the main population. “Ω_I- frequency” modes are found to be sustained by the impurity density gradient. In the limit of “short” transverse wavelengths (k^{2}d_{i}^{2}>1, where d_{i}=c/omega_{pi}) these modes are nearly electrostatic while in the relatively long wavelength limit (k^{2}d_{i}^{2}<< 1) they involve significant magnetic field fluctuations [2]. The relevant transverse phase velocities are in the direction of the local impurity diamagnetic velocity and produce a transport of this population across the magnetic field. Growth rates are found which depend on pre-existing thermal energy transport processes for the same population such as those associated with the excitation of the lower frequency “impurity drift modes” introduced in Ref. [3]. A programmed immission of non-reacting minority populations is suggested in order to increase the total reactivity of a fusion burning plasma column, for instance, by conditioning the spatial density profiles [1] of the reacting populations. *Sponsored in part by the Kavli Foundation (MIT and by ENEA (Italy).

[1] C. Mazzotta, et al., Paper IAEA-CN-899 MFE, Fusion Energy Conference (I.A.E.A., Vienna, 2021).

[2] B. Coppi, S. Cowley, et al., Phys. Fluids 29, 4060 (1986).

[3] B. Coppi, H. Furth, M. Rosenbluth and R. Sagdeev, Phys. Rev. Lett. 17, 377 (1966).