From a young age I have been fascinated with nuclear reactions that could transform atoms of one element into another. At some point this fascination led me to the conclusion I'd need to build a fusion reactor of my own, and so began quest to explore these reactions leading to a series of fusion devices I built over the next several years. With these reactors I would be able to investigate many types of confinement techniques and fusion fuels, from the lukewarm and diffuse plasmas of tabletop devices, to incredibly hot and dense pulsed power devices that shred metal to pieces, from Deuterium plasmas to those of even heavier elements like Helium and Boron. With these devices, I gained the ability to not only study the physics of fusion plasmas, but utilize the particles freed by these reactions to transmute stable isotopes into radioactive ones, and analyze the properties and composition of materials. I'd start when I was 14, by using electricity to create a potential well for hot Deuterium ions; the design derived from a type of fusion device invented in the the 1960s by the inventor of the television.
Inertial Electrostatic Confinement
The first successful fusion device (pictured at right) I built, utilized the principle of Inertial Electrostatic Confinement (IEC). IEC conceptually can be viewed as creating a electrostatic potential well for ions using high voltage electricity, in which positively charged ions want to fall into the negatively charged well and fuse. The voltage potential, or 'depth' of this potential well is on the order of tens of thousands of electron volts (eV).
Most of the structure of the reactor consists of an ultra high vacuum (UHV) system used to remove the vast majority of air molecules from the fusion chamber. The air is pumped out with a series of vacuum pumps providing the high vacuum conditions. Small amounts of fusion fuel enter the reactor and become ionized, where a powerful high voltage supply creates the electrostatic potential that heats (accelerates) and attracts the Deuterium ions (Deuterons) to the center of a grid made of a special high melting point refractory alloy of 98% Tantalum and 2% Tungsten. This device can also utilize auxiliary ion sources capable of more effectively fueling and heating the plasma discharge for a variety of fuel including Deuterium, Helium-3, and Boron-11.
The Deuterium (Hydrogen-2) fuel that I used in this reactor is a stable, naturally occurring isotope, and has been isolated and enriched from natural water where it occurs with an abundance of about 0.0156%. Due to isotope effects in these extremely light isotopes of hydrogen, electrolysis of water allows for a preferential enrichment of the heavier 2H from 1H due to the stronger chemical bond experienced between the 2H and Oxygen. This Deuterium fuel is a unique isotope- its lineage can be traced back all the way to the Big Bang, where it was synthesized in the primordial heat from protons. Unlike many of the nuclides found in nature, Deuterium atoms aren't made in active cosmic process, they existed in their current state since the beginning of time- at least until I came along with my reactor and converted them into Helium.
Inertial Electrostatic Confinement is unlike traditional approaches to fusion in that it is not thermonuclear, meaning the fusion reactions take place out of thermal equilibrium (not a Maxwellian distribution of ion temperatures). The ions that fuse are at high temperatures whereas the majority of the surround gas/plasma is relatively cool. While originally hoped to have a deep potential well that promoted ion recirculation until ions fuse, in practice losses created by electrons, the cathode, etc. mean that many of the ions do not collide with each other before colliding with some other species and losing their high energy.
For this reason, it is unlikely traditional IEC schemes would be able to produce breakeven conditions or useful amounts of fusion power. There are alternatives to traditional IEC devices, such as the Polywell which attempts to replace the inner grid with a virtual grid of electrons circulating around magnetic field lines at high potential. However, to date, these approaches haven't shown much success.
While my IEC devices produced an incredibly small amount of fusion power compared to that pushed into the reactor, the byproducts made it very powerful for experiments I would soon conduct. The neutrons liberated by the fusion reactions of Deuterium inside the reactor allowed me to probe the heart of matter. With these neutrons I could make new atoms that were unstable, and remotely identify unknown materials from the radiation signature they release under neutron bombardment.
Research on Inertial Electrostatic Confinement really had it's start with Philo T. Farnsworth, inventor of (among many other things) the television. My first reactor was a derivation of the "Fusor" design he developed in the 1960's. Farnsworth had great hope for the energy producing potential of these devices but for the reasons alluded to above, these never materialized. However, a talented group of engineers and hobbyists have formed a community around such devices, which you can take a look at here.
A "Demo Fusor" that demonstrates plasma at a lower temperature and higher pressure than a real fusion device but with no actual fusion reactions has become a popular science project for students and hobbyists and you can find a plan for making one at Make Magazine. These Demo Fusors provide an excellent experimental introduction to the physics of plasmas as well as to constructing physics experiments.
Another way I have explored nuclear fusion is with Pulsed Power devices. The IEC devices above are similar to Magnetic Confinement in their operating parameters- characterized by long discharge times and diffuse confinement. These pulsed power devices share more similarities with Inertial Confinement Fusion approaches, combining all the action into one big bang.
The physics of circuits provide an exciting possibility. Typically we don't have access to Gigawatts of power in the laboratory (power is simply energy over time). If we are able to take large amounts energy and store them up over long periods of time, then rapidly discharge them, we can access very large peak power. The use of capacitors and carefully shaped circuits allows us to to take kilojoules (kJ) of energy and discharge them in fractions of a second (10s of microseconds down to 100s of nanoseconds). These large pulses of power shred through cold matter in an incredibly destructive event, raising temperatures and pressures to the conditions required for nuclear fusion.
Dense Plasma Focus
In a Dense Plasma Focus (DPF), this power is dumped into a set of coaxial electrodes in a higher pressure gas environment than that of an IEC device, generating a hot, turbulent plasma filled with complex magnetohydrodynamic (MHD) instabilities, compact balls and toroids of plasma, and energetic plasma beams.
The growth of the plasma pinch can be imaged in a variety of wavelengths from x-rays to visible light and a plot of the current breakdown can be captured on an oscilloscope using devices known as B-dots or Rogowski coils, that measure the changing induced magnetic field. Even with diagnostics such as these, the Plasma Focus remains a poorly understood phenomenon. Along with studying the basic physics of these devices with different parameters and fuels, my primary interest in them came from the chaotic instabilities they created. Beams of MeV charged particles fly out of the pinch and I utilized these ion beams to create large quantities of radioisotopes.
Like IEC approaches, while early hopes were high for DPF's power potential, much of the fusion neutron yield of these devices has been shown to be from "Beam-On-Target" neutrons as opposed to thermonuclear neutrons and the yields have been demonstrated to flatten out at higher capacitor bank energies. Even though the energy producing potential of Plasma Focuses themselves doesn't seem strong, similar pulsed power coaxial discharge systems serve the injectors for other concepts like Magnetized Target Fusion. With the DPF, like my other early fusion experiments, I was able to apply the concept to a technology other than power production. It was out of my research with DPFs that I developed a system to producing medical isotopes. By optimizing the instabilities that prevent Plasma Focus devices from optimally confining a fusion plasma, I was able to coax out of such a pulsed power device beams of charged particles of sufficient energy to be useful in the production of short lived medically relevant radioisotopes.