Electron-cyclotron plasma generation and spectrum characterization

Author: Victor Etxebarria, professor, Dept. Electricidad y Electrónica. Fac. Ciencia y Tecnología, Universidad del País Vasco-UPV/EHU

Ernest Lawrence was awarded the Nobel Prize in Physics in November 1939 “for the invention and development of the cyclotron and for results obtained with it, especially with regard to artificial radioactive elements”. His invention was based on generating a spiral accelerated trajectory of protons governed by a simple alternating radio frequency voltage together with a constant magnetic field. The angular frequency of this circular motion depends on the charge-to-mass ratio of the particles and is proportional to the magnetic field.

The electron cyclotron resonance (ECR) is a physical effect based on the same ideas but applied to electrons. The natural rotation frequency of electrons in a magnetic field can be tuned to coincide to the frequency of some incident microwave radiation we introduce in a gas container. By means of this simple concept, the resonance is very efficient to add energy and heat to the gas, turning it into plasma.

The IZPILab Beam Laboratory from UPV/EHU has designed and built ¹ a new generation compact multi-species plasma reactor capable of efficiently producing hydrogen plasma under less than 100 watts microwave power. This reactor is conceived as a key component of a new multi-species ECR ion source (ECRIS) used to mainly extract proton beams (PIT30), whose basic schematic is shown in Figure 1.

The basic components of the new PIT30 design include (a) the gas inlet port where Hydrogen is introduced by means of a mass flow gas controller, (b) the coaxial power microwave coupler to the chamber, (c) the plasma chamber itself, embraced by (d) a Halbach type permanent magnet array structure to generate the tuned magnetic field to produce a resonance situation. If efficient Hydrogen plasma is produced by means of the ECR phenomenon, then the remaining basic components of the ion source include (e) the extraction and focusing triplet high voltage electrodes, (f) the ceramic insulating beam pipe finally connecting to the beam diagnostics ports (g) which are connected to ground potential.

Plasma produced in laboratory is typically measured by means of different kind of probes. Amongst them, Langmuir probes and its variants, are the most widely used. However, these direct measurements are not always convenient or even possible during the real-time operation of an ion source, and what’s worse, they are intrusive by nature and modify the plasma itself because they have to be inserted into the chamber, and the interpretation of their current to voltage curves can often be difficult and their results become rather unreliable.

The Beam Laboratory of UPV/EHU proposed a simple optical emission spectroscopy technique developed to characterize the Hydrogen plasma obtainable in the ECRIS. Instead of reproducing full standard optical emission methods, the approach adopted has been to simplify and avoid as much as possible the use of expensive spectrometric equipment. Among its advantages it is worth mentioning that the method itself is non-invasive, therefore not affecting the plasma being measured. Also, in ion sources, where microwave and magnetic fields as well as high potentials are typically used, the recording of spectra is not disturbed by these. Moreover, only a line-of-sight through the plasma is required to take appropriate measurements by means of a standard Charged Coupled Device (CCD) camera.

The direct CCD captures provide basic snapshots of the Hydrogen plasma luminescence, which can greatly vary depending on the microwave power injected to the plasma chamber and the supplied Hydrogen mass flow. If a low-cost diffraction grating (100 lines/mm) is mounted as a standard filter on the CCD camera lens, instead of capturing the direct image of the plasma and its luminosity, we can very easily perform a basic spectral analysis of the emitted light which can give us valuable information on the plasma formation.

Figure 2 displays the Hydrogen plasma emission spectra for a range of 15.7 to 105 W of absorbed microwave power and its Gaussian fitting for the case corresponding to 93 W. (3 sccm Hydrogen mass flow, 3 GHz microwave frequency). It is readily seen that in all cases the peaks are centered approximately around 𝜆=656 nm 𝜆=486 nm, corresponding, as expected, to the wavelengths of the Hydrogen spectral lines H_𝛼 and H_𝛽 in the Balmer series. This is a remarkable method for appropriate plasma characterization for ion sources, similar in many respects to the well-known astronomical observation of the optical spectra of stars.