Plasma Physics & Magnetic Reconnection (0:29)
0:29-12:07 (Scott Hsu reflects on his early interest in energy and how it led him to study plasma physics and magnetic reconnection)
Q: Where did your interest in fusion first come up?
A: Scott Hsu remembers the oil crisis of the 1970’s even as a very young child growing up in the Los Angeles areas. In high school, Scott did a research project on fusion, which flipped the switch on his interest in energy and fusion. Scott later took multiple physics classes taught by plasma physicists during his time at UCLA, leading him to work in the lab of Dr. Francis Chen. The discipline of plasma physics was largely invented and developed because of the chase to do fusion. Plasma is a collection of charged particles in which the electrons are stripped off the nucleus, forming a sea of electrons and ions. Enough of the electrons need to be stripped off so that the charged gas has its own set of collective behavior governed by the electromagnetic field. Typically, a temperature close to 10,000 degrees Kelvin is needed for a plasma to exist. Usually, plasmas have much lower densities than solids and even air.
After deciding he wanted to pursue fusion while at UCLA, Scott proceeded to study fusion at Princeton, the top plasma physics program in the country, if not the world. Early on, he had a big interest in the theoretical aspects of fusion heating systems. The year after Scott got to Princeton, the Tokamak Fusion Test Reactor (TFTR) was on the page of The New York Times for setting the world record in fusion energy produced. Eventually, TFTR got to about 10 megawatts of thermal energy power production, if only for a very short duration. The TFTR used deuterium and tritium in the fusion process. The lightest elements are the most acceptable; deuterium-deuterium (DD) and deuterium-tritium (DT) are the most common elements used in these experiments. Tritium is hard to come by because it has a 12-year half life and it more difficult to handle. Tritium can take the place of hydrogen and form water molecules, so it cannot get into water systems or into a person’s body, but the legitimate dangers of it are not that severe.
Scott’s area of focus during his PhD was in magnetic reconnection, a very fundamental piece of plasma physics. If magnetic fields are brought together in a way that they have opposing directions, they will cancel each other, but the energy contained in a magnetic field will then be converted into plasma motion. Scott took direct measurements of what was going on in that layer as the fields cancel each other out. The canceling of the magnetic fields provide heating of the plasma, which can be harnessed in good ways, but it takes a lot of energy to slam the plasmas together. This is typically not enough to get to where you need to be for net gain fusion energy. The instantaneous, impulsive heating must be maintained for a certain duration at a certain density to have a chance at net gain fusion.
Fusion Programs at ARPA-E (12:07)
12:07-26:40 (How the fusion program at ARPA-E was formed and how it has impacted the development of fusion technology)
Q: What became of your studies with magnetic reconnection?
A: Scott Hsu focused on the effects of magnetic reconnection during his PhD studies at Princeton, leading to some very good scientific results. One of the luminaries in the field of fusion, Dr. Russell Kulsrud, stopped by Scott’s lab during a measurement of the profile of the magnetic field across the reconnection layer. He called Scott’s work the most exciting result in plasma physics in 20 years. Scott then went to Caltech to do a post-doc on an alternate fusion concept called a spheromak. Like a tokamak, a spheromak has a donut-shaped magnetic configuration, but it doesn’t have a solid center rod and it requires internal plasma dynamics to create the field structure, rather than strong applied fields from toroidal field magnets. However, spheromaks have not shown the heat confinement properties as a tokamak. After his post-doc, Scott went to Los Alamos, where he has now spent 17 years. Since the mid-1990’s, Los Alamos is not allowed to do integrated tests of the full system. There are international agreements to abide by which set the rules for this testing.
Pat McGrath, at ARPA-E (Advanced Research Projects Agency-Energy), decided he wanted to formulate a fusion program around 2013. At the time, the breadth of things being studied in fusion was winnowing due to pressures from different directions. Some concepts reached their performance limit with the current physics knowledge, but there were also new concepts that didn’t have a well-developed scientific understanding until much later. There is an effort to compile the information about fusion concepts as a working history of the technology. Scott has been going around to the different fusion communities to identify legitimate opportunities. ARPA-E’s Alpha program funded three different areas, including integrated concepts to push fusion performance. One of these projects, the Stabilized Z-Pinch, made tremendous progress and got an award in the 2018 ARPA-E Open program, allowing them to continue their work. Z-Pinch was one of the earliest concepts ever studied: driving a current through a plasma or a wire, creating an electromagnetic force that pinches it. ARPA-E was able to push performance of this project by increasing current.
Experiments in Electromagnetic Physics (26:40)
26:40- (A review of some past and present fusion experiments and how they are being used in costing studies)
Q: Are we learning new things about fundamental particles themselves or physics itself?
A: These fusion experiments are not necessarily providing new information about fundamental particles or physics. Plasma physics is classical electromagnetic physics, a complicated N-body system. In a plasma, all the particles cannot be followed for a meaningful experiment and one can’t predictively calculate what is going to happen. In the Z-Pinch project, the difference in flow between the center and outside of the wire stabilizes the “kink” in the “sausage”. Other opportunities include the concept called LINUS. The Naval Research Lab (NRL) studied LINUS in the late 1970’s, a spinning liquid metal that also compresses a plasma to fusion conditions. The problem at the time was the ability to complete the operation at a certain speed with the required precision. Later on, the use of advanced electronics understanding can be used to power the implosion and an improved plasma physics understanding used to get the right plasma inside the compression.
Through the years, a series of government-sponsored reactor costing studies called ARIES were performed. Scott finds himself arguing fusion people down in cost and energy advocates up in cost, since it’s not a fair comparison to pit fusion, or nuclear, against the cheapest natural gas since they provide different benefits. There is a key difference between fission and fusion. On a fusion power plant, there should not be any fissile materials present, which is a binary advantage. Fission plants use fissile material as the fuel. The microreactor class-size concept has not been questioned enough, because it would be difficult to track the widespread fissile materials.
ARPA-E’s Alpha program focused in general on the class of magnetized pulsed concepts, called magneto-inertial fusion. The main approach at Sandia National Labs is based on this idea of imploding concepts with a magnetic field. With the role of ARPA-E in the energy funding ecosystem, ARPA-E is tasked with higher risk, higher reward approaches and going back to looking at fundamental ways of making fusion more attractive. This could be with advanced fuels or finding legitimate ways to control the fusion cross-section. There is evidence that the nuclear fusion cross-section in a condensed matter environment. Two bare hydrogen ions or isotopes, which have the same charge, need to be slammed with enough energy that they get close enough that the probability of quantum tunneling is big enough to cause them to fuse. This effective area provides the probability.
Technical and Economic Feasibility of Fusion (40:04)
40:04-53:53 (How the marriage between technical and economic feasibility of fusion can bring change to the world)
Q: If an isotope of an element has a lot more neutrons than protons, why can’t it be put towards another element or isotope that also has a lot more neutrons to hold each other together?
A: Neutrons do not keep everything together, but instead a subatomic particle called a gluon. If two neutral atoms are close together, there is no electromagnetic repulsion; this may be a gas. However, if they were in a solid, a lattice holds everything together through chemical bonds. The understanding of nuclear fusion cross-sections has been largely developed in a high energy, two-body picture. One question is if this picture is applicable in a low energy, condensed matter environment. A recent Google study aimed to do careful, rigorous, scientific measurements of loading hydrogen into palladium samples and doing careful calorimetry. There is very sparse data on the fusion cross-section, whether it is in condensed matter or not, below a few keV of energy. The cross-section gets so small that it is hard to get a meaningful signal. ARPA-E insists on a really good, credible science basis, even if they don’t understand it yet.
LENR, Low Energy Nuclear Reaction, looks at whether nuclear reactions can happen at chemical relevant energies and whether nuclear binding energies, which are typically MeV, with only eV level energy inputs. LENR doesn’t necessarily have to be fusion, but could also be other nuclear transmutations of nuclear capture processes. Scientists and physicists know fusion works in a star; it’s a matter of harnessing it on Earth in an economic way. Fusion has an opportunity right now due to a spread of opinions. One side just wants to demonstrate technical feasibility at all costs; the other side wants to focus on making it commercially and economically attractive. The opportunity right now is for the U.S. as a country, and for the whole world, to decide how urgent and bold they want to be to find the right spot in the spectrum. The world is focused on technical feasibility, but the private companies are looking for economic feasibility. Scott Hsu went to ARPA-E to accelerate the meeting of the minds of the two ends to allow fusion to have an impact in a timescale that matters for our problems this century.