#ion source

After two days of nearly non-stop tests, I believe I have isolated the problem with the cyclotron's ion source. While this problem can definitely be fixed, it will take significantly more time than I have here (today is my last day) to do so. 

The ion source of the cyclotron ionizes hydrogen atoms by pushing them through a sea of high-energy electrons. These electrons are emitted from a hot filament (in a process very similar to boiling water) that rests at the center of the source. However, for some reason, right now these electrons are not leaving the filament surface. Without the sea of electrons, the ion source cannot produce protons and, of course, we cannot produce beam.

I have a number of theories as to why the filament might not be emitting, but they all come down to physical details of the source's construction. A careful analysis will have to be done, as well as some tests on the ion source in different physical configurations. 

Yesterday we experienced another filament burnout, requiring us to open up the chamber again so that I could repair the problem. Luckily, this repair only took about an hour and allowed me to snap the cool pictures above!

The first picture above shows the broken filament before it was repaired. Notice that the white ceramic around the filament is now a sort of black graphite-color rather than it's original white. I suspect this is a thin layer of tungsten metal that has been vaporized off the filament and deposited onto the surfaces around it, a process called "sputtering". We also saw a small dent in the bottom of the filament holder which looks like it might have been formed during the small explosion that destroyed the filament!

This photo shows (via an inspection mirror) the glow of the ion source filament during it's first test. The light is mostly blocked by the copper chimney that sits on top of the filament, so what you see here is a very small fraction of the light the filament is actually producing. 

The most challenging part of preparing for this test was finding a quick, cheap, and simple way to power the filament. The filament requires 12A of current, which is not difficult to provide with a standard lab power supply. However, the filament must also be "biased" 200V below ground! This presents more of a problem. 

In physics, "ground" is taken to be a sort of zero point for a circuit. While the origin of the name lies in the use of the ground beneath our feet as a reference point for electrical measurements, it has come to represent any common point in a circuit; the physical voltage of the earth is now referred to as "earth ground". In most circuits, the negative terminal of the battery or power supply is considered the ground, and all voltage measurements will be made relative to this point. 

 In the cyclotron, the metal of the chamber is used as the "ground" for the entire machine. However, this presents a problem. When electrons leave the filament, we want them to see a positive charge so that they will be slightly accelerated before they hit the hydrogen molecules. However, the metal ion source chimney is at ground, making it a potential of 0V! This problem is solved by shifting (or "biasing") the entire filament circuit -200V relative to the ground of the chamber. This is done by simply putting attaching the negative wire of a 200V power supply to the circuit where I would normally connect the circuit to ground. Imagine that the filament has 12V across it; in a normal, grounded circuit this would translate to 12V on one side and 0V on the other. However, if we bias the filament -200V, these voltages become -188V and -200V. The voltage difference across the filament is still the same (12V) so the same current flows. However, now the electrons the filament produces see the walls of the chimney as if they had +200V applied to them!

The second picture above shows the circuit diagram for the completed filament power supply. on the left, a simple loop between a 12V car battery and a couple of resistors provides the 12A current the filament requires to run. This alone would be equivalent to the 12V, 0V case discussed above. The component to the far right, however, is the bias supply. When this supply is turned on, it shifts the entire circuit -200V relative to ground. The little triangle at the bottom of the bias power supply represents a connection to ground. Because the circuit appears electrically to be sitting on top of the bias power supply, this is often called "floating" a circuit above ground. 

For the past week or so, much of the activity on the cyclotron has been at a stand-still due to one, crucial, missing part; the tungsten filament for the ion source. Today, this piece of wire arrived and was quickly installed!

During operation, a high electrical current will be placed over the filament, causing it to violently eject electrons in all directions. As hydrogen (H2) gas flows into the ion source and over the filament, these electrons will split the hydrogen molecules into individual atoms and strip those atoms of their electrons. The protons that remain are then extracted out of the ion source and accelerated to form the cyclotron's beam.

The filament's design is very simple; it's just a short length (3/4'') of a special tungsten wire, coated in the radioactive element Thorium. This special coating makes the filament last longer before eventually burning out. The small amount of radiation emitted by the filament is harmless to humans, as the low-energy alpha particles it emits cannot penetrate our skin. In fact, some incandescent light bulb filaments use the same material!