The effect of cavity tuning on oxygen beam currents of an A-ECR type 14 GHz electron cyclotron resonance ion source

Abstract

The effect of cavity tuning on oxygen beam currents of an A-ECR type 14 GHz electron cyclotron resonance ion source Tarvainen, Olli; Orpana, J.; Kronholm, Risto; Kalvas, Taneli; Laulainen, Janne; Koivisto, Hannu; Izotov, I.; Skalyga, V.; Toivanen, V.Tarvainen, O., Orpana, J., Kronholm, R., Kalvas, T., Laulainen, J., Koivisto, H., . . . Toivanen, V. (2016) The direct injection of intense ion beams from a high field electron cyclotron resonance ion source into a radio frequency quadrupolea) Rev. Sci. Instrum. 85, 02A740 (2014) The efficiency of the microwave-plasma coupling plays a significant role in the production of highly charged ion beams with electron cyclotron resonance ion sources (ECRISs). The coupling properties are affected by the mechanical design of the ion source plasma chamber and microwave launching system, as well as damping of the microwave electric field by the plasma. Several experiments attempting to optimize the microwave-plasma coupling characteristics by fine-tuning the frequency of the injected microwaves have been conducted with varying degrees of success. The inherent difficulty in interpretation of the frequency tuning results is that the effects of microwave coupling system and the cavity behavior of the plasma chamber cannot be separated. A preferable approach to study the effect of the cavity properties of the plasma chamber on extracted beam currents is to adjust the cavity dimensions. were recorded at various tuner positions and frequencies in the range of 14.00-14.15 GHz. It was observed that the tuner position affects the beam currents of high charge state ions up to several tens of percent. In particular, it was found that at some tuner position / frequency combinations the plasma exhibited "mode-hopping" between two operating regimes. The results improve the understanding of the role of plasma chamber cavity properties on ECRIS performances. Published by AIP Publishing.[http://dx

Broadband microwave emission spectrum associated with kinetic instabilities in minimum-B ECR plasmas

“…It was concluded that changing the cavity dimensions which affects the electromagnetic field pattern of an empty cavity, as explained in Ref. 29, does not affect the dominating microwave emission frequencies which suggests that cavity properties play an insignificant role in determining the interaction between hot electrons and the instabilityinduced EM-wave. This conclusion is supported further by the simulated spectral density of TE and TM cavity modes shown in Figure 8.…”

Electron-cyclotron heating and kinetic instabilities of a mirror-confined plasma: the quasilinear theory revised

“…In many cases this is an important channel for the loss of excess energy stored in fast particles [2,3], thereby limiting the achievement of peak plasma parameters in applications. Considering, for example, modern sources of multiply charged ions, particle ejections, which are inherent to the burst regime of cyclotron instability, that cause oscillations of the plasma potential and the beam current accompanied with a significant decrease of the average ion charge [4][5][6][7]. An explosive cyclotron instability of a mirror-confined plasma may be used for generation of high-power microwave and terahertz pulses [8].…”

Controlled turbulence regime of electron cyclotron resonance ion source for improved multicharged ion performance

“…In this source the high charge state Pb ion beam production is maximized with B min /B ECR > 0.9 similar to our experiment in cw mode. In the pulsed operation, the instabilities are avoided at high B min because it takes several tens of milliseconds for them to appear [37] and, thus, periodic instabilities can be avoided with short pulse operation. The GTS at CERN serves as an example to demonstrate that controlling the plasma turbulence at high B min offers a route to intense beam currents of high charge state ions at several kilowatt microwave power and high power density as discussed here.…”

“…The negative effect of increasing the magnetic field was explained by the appearance of periodic, 10 0 -10 3 Hz, development of electron cyclotron instabilities of the plasma that excited when B min /B ECR is above the optimal value defined by (1) [11,12]. Although the ion temperature remains relatively low (about 10 eV), the ECR heating results in formation of a strongly non-equilibrium electron distribution possessing a significant fraction of fast electrons with relativistic energies (up to 1 MeV), in addition to the 'cold' and 'warm' electron populations [13].…”