At the ICARE laboratory of CNRS in Orléans, our team is exploring new ways to study and understand iodine plasma using advanced optical and electrical diagnostic tools. This work is especially important for improving the performance of future space propulsion systems that use iodine instead of traditional fuels. To do that, we need to carefully examine the behaviour of iodine plasma, so we can design better and more efficient thrusters.
To start this research, we built a simple but effective iodine plasma source: a transparent reference cell made entirely from quartz glass. This design allows us to clearly observe the plasma with optical methods. The main part is a 250 mm long cylinder, 30 mm in diameter, with two smaller tubes, called “fingers”, attached to it. One finger connects to a vacuum system, and the other serves as solid iodine reservoir. When we create a vacuum inside the cell using a pumping system, the solid iodine begins to sublimate—that means it changes directly from a solid to a gas without becoming liquid. This iodine gas then fills the cell, creating the conditions needed to generate a plasma discharge. We ignite the plasma using a radio-frequency antenna placed around the glass tube. This antenna is connected to a matching box that adjusts the impedance to make sure the energy goes efficiently into the plasma, minimizing losses. Once the plasma forms, the glowing iodine gas becomes visible through the walls of the cell, which makes it easy to study using light-based techniques. The transparent design of the cell is key. It lets us use optical diagnostics—such emission and laser spectroscopy—to observe the plasma without disturbing it. This setup gives us valuable insights into how iodine plasma behaves.
This is just the beginning. With this setup, we’re laying the foundation for more advanced experiments. The first diagnostic technique we put in place is Optical Emission Spectroscopy (OES). This method involves using a spectrometer to collect the light naturally emitted by the plasma. Every chemical element emits light at specific wavelengths, almost like having a unique fingerprint. By analyzing the wavelength of light coming from the plasma, we can determine exactly which species—atoms, ions, or molecules—are present inside it. In our case, the light from the iodine plasma was directed into a spectrometer, allowing us to identify the different species populating the plasma. This gave us a first look at its composition and helped confirm that the source was working as intended.
The next major step has been the development of a laser spectroscopy bench, a more advanced and powerful tool for studying plasma. Unlike OES, which passively observes the light already emitted by the plasma, laser spectroscopy uses laser beams to actively interact with the particles in the plasma and reveal much more detailed information. One of the most important techniques we're implementing is called Laser-Induced Fluorescence (LIF). With LIF, a laser is tuned to a very specific wavelength to “excite” selected atoms or ions in the plasma. These excited particles then emit light in response, and by analyzing that light, we can learn how fast the particles are moving and in what directions. This allows us to map out what's called the Velocity Distribution Function (VDF)—essentially, a detailed picture of how iodine atoms and ions behave in the plasma. A big advantage of LIF is that it’s minimally invasive. That means it doesn’t disturb the plasma, unlike traditional electric probes that can alter the conditions they’re trying to measure. LIF relies on the Doppler effect—the same phenomenon that makes a passing siren change pitch—to measure the motion of particles. When particles move toward or away from the laser, they interact with the light at slightly shifted frequencies, and that shift tells us their speed. This advanced technique opens the door to a much deeper understanding of iodine plasmas and will play a central role in refining and optimizing our propulsion system.