A broadcast by cosmonaut Yelena Serova.
Today we conduct one more session of the Coulomb Crystal experiment, which studies the behavior of charged particles in a magnetic trap. This is a unique experiment, it can only be conducted onboard the international space station. It’s impossible to conduct under conditions on the ground. That’s why we, cosmonauts, were entrusted with this responsible task of conducting this experiment. Before observations, I need to assemble the setup from individual units: electromagnet unit, video camera unit, and power and control unit. And that’s what I did.
In the course of the experiment, in accordance with the radiogram received from specialists, I will be controlling currents and voltages per the specified procedure. Let me show you the electromagnetic trap, that’s what it looks like. They (the hardware units) come to us in bags like these, which are called “interchangeable container kits”, and that’s what they look like (she shows a magnetic trap).
Here is our magnetic trap. I have already installed it. For each experiment run, we have its own dedicated container. Today we are going to use interchangeable container No.10. Here we also have a video channel, by which I’ll be monitoring the progress of he experiment, and see what happens inside this unit, thanks to this video camera. The experiment will be monitored, so to say, live, the data will be recorded and will go to Earth, and specialists will be able to see with their own eyes what was happening in the experiment when I was varying currents and voltages.
So, I suggest that we begin this experiment. Now I’m going to switch on the equipment and conduct the experiment. We are hooking up the network... And now we have picture. This image is what the scientists on the ground will see. In the course of the experiment the picture inside the magnet changes, and I keep track of it – I monitor the current flowing through the coils of magnets 1 and 2, record it, making notes of its values. That’s the values which I will report to our specialists. What is now happening inside these coils is the movement of charged particles. We can see this on the screen of the video camera. It’s too bad that I can’t let you see right away what was happening here. But it seems to me that it was something unique and unusual.
So, we have performed an experiment. Here on this slip of paper I have information for our scientists, the video camera has recorded processes? which have been happening here in this magnet unit. And today, in just 5 minutes I'm going to send all this information. And now I’m going to disconnect our equipment, that’s what we always do, when we perform final operations on an experiment. We disconnect magnets, switch off the camera. That’s all. This completes out experiment.
The experiment was jointly prepared by specialists from the United Institute of High Temperatures of the Russian Academy of Sciences (www.jiht.ru) and Rocket and Space Corporation Energia (www.energia.ru). This experiment studies the dynamics of dispersed (dust) charged particles of graphite levitating in non-uniform magnetic field.
Levitation (floating) is containment of the particles in suspended state without contact with container walls.
The Coulomb Crystal experiment on the ISS for the first time demonstrated the feasibility of forming in a magnetic trap under zero gravity (microgravity) stable spatially-ordered structures consisting of several thousand charged particles.
Coulomb crystal or plasma crystal is a system, in which a powerful electrostatic field aligns particles in a certain spatial configuration. A regular structure is formed, where particles take their positions at the nodes, like atoms in a crystal lattice.
The particles in plasma interact with a force described by Coulomb’s law. A stationary electrical charge is not affected by a static magnetic field. But if an electric charge is moving in a magnetic field, it experiences a force described by the Coulomb’s law: The force of interaction in vacuum between two point charges is along the straight line joining these charges, and is directly proportional to the magnitudes of the charges and inversely proportional to the square of the distance between them. The force is attractive if the charges have different sign, and repulsive, if they have different sign/
Plasma is a partially or fully ionized gas with virtually the same densities of positive and negative charges.
Plasma cluster is a group of several uniform elements – ions. That is why plasma clusters are called coulomb clusters and are classified as coulomb systems.
Magnetic traps are magnetic field configurations that are capable of confining charged particles or plasma within a limited volume for long periods of time.
The charging of the particles was performed with the use of a probe (electrodes). In the experiment virtually all the particles become charged, Coulomb repulsion between charged particles results in a cluster of particles in the form of an oblate spheroid.
Scientific equipment COUC (CUlomb Crystal) confines graphite particles within a magnetic well, an area with magnetic field minimum.
The equipment allows to generate in the working area of the electric magnet a magnetic bottle. To generate a magnetic bottle a device is required which has two coaxial coils placed on either side from the center of the magnetic trap (Fig.1). Such arrangement resolves the problem of particles sliding to the edges with the lowest levels of energy intensity.
Fig.1 Schematic drawing of the experimental setup creating a magnetic bottle:
1, 2 – electric magnet coils; 3, 4 – coil cores; 5, 6, 7 – the body of the electric magnet which serves as a magnetic conductor; 8 – working zone of the electric magnet.
The magnetic well is created in the working area of the electric magnet between the two coils with currents circulating in opposite directions. The body of the electric magnet and of the entire setup is a magnetic conductor connecting the cores of the coils. It has a cylindrical shape with diameter of 15 and height of 18 centimeters.
The distance between the cores is 6 cm, their diameter is 5 cm. When current passes through the coils, a magnetic well of the required magnetic-bottle type is created in the working zone. Magnetic field intensity in the trap is adjusted by varying current in the coils. In the course of the experiment the electric magnet consumes up to 200 W, and it is worth noting that a ground-based electric magnet with a stable levitation zone of these dimensions would have consumed tens of thousands times more electric power and would have been much more complex.
When experiment sessions are conducted, an interchangeable container is placed into the working zone of the electric magnet. Inside the interchangeable container there is a glass ampoule (Fig.2) containing the model material – dispersed graphite particles. Graphite (diamagnetic) was chosen because it has the highest diamagnetic susceptibility coefficient in comparison with other materials. Therefore it is easier to make it levitate in magnetic field.
Fig.2 Experimental ampoule with model material
Diamagnetics are the materials that get magnetized in the direction opposite to the externally applied magnetic field, and in the absence of an external magnetic field diamagnetics are unmagnetized.
The experimental ampoules are filled with argon, the diameter of the ampoules is 50 mm, the height is 40 mm. The body of the interchangeable container is made of an aluminum alloy, and therefore it does not distort magnetic field in the working zone.
To observe the motion of graphite particles in the ampoule, a compact CCD-camera (Fig.3) is used. The ampoule is illuminated with a light-emitting diode. To impart charge to the graphite particles, special electrical probes (electrodes) are placed inside the ampoule. At present, there are four interchangeable containers available onboard the ISS containing graphite particles ranging in size from 100 to 400 micron.
Fig.3 A view of an interchangeable container with its cover removed containing one CCD camera.
1 – experimental ampoule, 2 – CCD camera, 3 – slot laser
During the experiment the particles in the ampoule are subjected to forces produced by magnetic and electrical fields. With a potential applied to the central electrode of the probe, the particles get charged when they collide with it. Switching electric magnet on results in graphite particles being pushed out to the magnetic well bottom area (points with zero magnetic field) to form a Coulomb cluster*** of charged particles, a cloud in the shape of an ellipsoid of rotation
Fig.4. Configuration of the Coulomb cluster of 400 micron graphite particles in the magnetic bottle.
To adjust the current in the coils of the electric magnet, to switch on cameras and LEDs in the interchangeable containers, as well as to adjust the electrical voltage at the probe, a power supply and control unit is used. During the experiment, the power supply and control unit is hooked up to an onboard power outlet.
In the course of an experiment session we can observe the processes (Fig.4), occurring inside the ampoule using the camcorder display (Fig.5). The camcorder is used to record all the video information about the experiment, which is handed over to Russian scientists for study.
Fig. 5. Camcorder SONY HVR-Z1J