Plasma characterization of a microwave discharge ion source with mirror magnetic field configuration

Abstract

Microwave coupling to plasma through cavity dependent resonant modes is one of the key aspects in a microwave discharge ion source (MDIS) for improving the ion beam qualities as well as plasma dynamics. Knowing these concerns, a MDIS is designed, fabricated, and developed at Institute for Plasma Research to produce high current and a low emittance ion beam for accelerator applications. The present manuscript reports the development of MDIS and the characterization results of the first plasma produced by launching a microwave (MW) of frequency 2.45 GHz. The plasma is characterized at a particular distance away from the ion extraction aperture, using three diagnostics tools such as a Langmuir Probe (LP), Optical Emission Spectroscopy (OES), and a microwave spectrum analyzer. The required mirror-B magnetic field is created by placing four high power ring magnets (each magnet pole strength is 1.38 T) around the cylindrical source chamber. All diagnostics measurements are performed under an operating pressure of range 2 × 10−4–1 × 10−3 mbar and the plasma absorbed power of 30–160 W. The measured cold electron temperature and density varies in the range of ∼1.5-11.8 eV and 5.6 × 1016 m−3–6 × 1017 m−3, respectively, within the source volume. The electron population has distinct hot and cold plasma temperature. The hot electron temperature changes from ∼20 to 70 eV within the above absorbed power range. The LP and OES measurements witnessed the density jumps from under-dense (∼7.3 × 1016 m−3) to over-dense (∼2.9 × 1017 m−3) for the change in absorbed power from 50 W to 80 W. This density jump is accompanied by the sideband generation around the cavity resonant mode (including the launched MW) frequencies which range from 238 kHz to 873 kHz and is recognized as ion waves from the dispersion relation. The ion temperature, estimated from these observed low frequency instabilities, changes from 0.095 to 1.25 eV. The influences of these instabilities on beam emittance growth are of paramount importance in future studies.

Evolution of Microwave Electric Field on Power Coupling to Plasma during Ignition Phase

“…Since the electrostatic wave does not face any density barrier, it is capable of penetrating the core dense plasma and transferring the energy through damping to the plasma particles. As demonstrated by other groups [23,28], this change in the power coupling mode from the electromagnetic to electrostatic case is a signature of the UHR heating. Due to the heating at UHR, the plasma electron temperature and also the density are possible to enhance further.…”

“…Experiments are carried out in a microwave ion source system that has similar system configuration, magnetic field distribution, MW conditions and also the operating conditions. The simulated temporal plasma parameters, such as the plasma density and hot electron temperature, are validated with the experiment [23]. In the present experiment, MW-plasma reactor of the experimental set-up is a cylindrical cavity (Figure 11) of 107-mm length and 88-mm diameter.…”

“…The plasma in the reactor is generated by coupling microwave through the electron cyclotron resonance (ECR) heating as well as off-ECR heating methods, as discussed before. The complete experimental set-up consists of a cylindrical cavity, microwave system, ion beam extraction system and two pairs of ring magnets (each magnet has pole strength $1.38 T) assembly [23]. The plasma cavity/reactor is surrounded by two pairs of ring magnets to generate a mirror-type magnetic field to confine the plasma inside the cavity.…”

“…The assumption is adapted from the Boltzmann equation that is multiplied by some weighing function and then integrated the resulted function over the velocity space. This exercise yields completely three-dimensional and time dependent equations [10,23]. During the modeling, FEM assumes that ion motion is negligible with respect to the electron motion in the timescale (ns) of MW.…”

“…This normalization helps in convergence of the solution and avoids any disproportionate absorption of the power by the plasma. Here, a fixed plasma absorbed power of 70 W is chosen to benchmark its results with the experimental findings that are reported taking the boundary conditions [23,24] similar to the computation system and the operating conditions.…”