Alcator C-mod interior

A view inside the Alcator C-Mod tokamak.

Robert Mumgaard/MIT

Alcator C-Mod pressure record: FAQ


On September 30th 2016 the Alcator C-Mod team set the plasma pressure record for a magnetically confined fusion device. For the first time anywhere the team obtained plasma pressures higher than 2 atmospheres. This represents an important milestone in the quest for controlled fusion energy as the high pressures are similar to those expected in future fusion reactors. The experiments were conducted on the Alcator C-Mod tokamak located at MIT in Cambridge, Massachussets on the last day of planned operation of that facility. Below are some questions and answers about fusion, Alcator C-Mod, and the record setting experiments.

Fusion in general
Alcator C-Mod
The record setting experiments
The pressure record in context
The future


Fusion in general

Q: How does the pressure relate to fusion energy?

A: In order for the fusion reaction to take place, the constituent nuclei of the fuel (isotopes of hydrogen) must be very hot, and they must collide with each other frequently enough. Fusion thus requires high temperature and density simultaneously, which when multiplied together is defined as pressure. In fusion’s case the pressure is in a plasma unlike air where it is in a gas, but the principle holds the same way. Thus, high pressures (>2atm) and temperatures (>50 million degrees, several times hotter than the center of the sun) are required to obtain fusion reactions in a magnetic confinement device. For the fusion reactions to make more power than it requires to sustain the temperature the plasma must also contain the heat. Thus, in much the same way a house has insulation, the plasma has insulation from the magnetic field.

Q: Why does the plasma need to be hotter than the center of the sun?

A: The sun operates at temperatures significantly lower than what is required for a fusion reactor but overcomes this by having very high pressures and by being very large. Even at the core of the sun, where the plasma is hottest and densest the fusion energy production per volume is similar to energy production from microbes in a compost pile. To make fusion a reality on Earth the reactor must therefore be several times hotter than the center of the sun while obtaining high enough pressures.

Q: Why is pressure important for fusion reactors?

A: In addition to being required for obtaining fusion, the pressure determines the rate of fusion reactions taking place inside the fusion reactor once the desired temperature is reached. The reaction rate, and thus the fusion power of the device, goes approximately as the pressure squared, so a doubling of pressure leads to a quadrupling of the fusion power. Therefore, techniques that enable increased pressure or that obtain high pressure using cost-effective technologies, improve the overall economics of a fusion reactor. The economics require that magnetic fusion reactors achieve pressures of 3 to 10 atmospheres.

Q: What fusion devices achieve the highest performance?

A: To date the highest performing devices, meaning the ones with highest pressure multiplied by confinement time, is a configuration called a tokamak.

Q: What are the obstacles to obtaining high plasma pressures in fusion devices?

A: As the plasma pressure is increased inside the magnetic bottle the plasma becomes more and more prone to slowly leaking heat, thus taking more input power to sustain the pressure and temperature. Eventually the pressure becomes large enough for the plasma to suddenly become unstable and the plasma pressure is lost, like popping a balloon. The worst form of this is termed a disruption and can do damage to the internal components of the machine, but does not affect the safety of the device. Additionally, as pressure is increased the plasma can develop turbulence that transports energy from the center of the plasma, cooling it.

Q: How are these obstacles overcome?

A: Generally, the threshold for these problems arising can be increased by changing the shape of the magnetic bottle or by heating the plasma at the correct locations. However, the most fundamental method to raise the threshold of these phenomenon is to increase the stabilizing magnetic field that winds the long-way around the tokamak. This magnetic field is provided by large electromagnets or “coils”. The higher the field the coils provide the more stable the plasma becomes.

Q: What is a tokamak?

A: A tokamak is a type of magnetic bottle designed to confine a superheated fusion-relevant plasma. The tokamak is in the shape of a donut, with strong electromagnets used to hold the plasma away from the walls of the chamber containing it. Several good descriptions exist for a tokamak:

Q: How many tokamaks are there?

A: Tokamaks are the most common type of fusion device. Since their invention in the USSR in the late 1960s over 170 have been built around the world. The current generation, including Alcator C-Mod, are the highest performing type of fusion reactor and are extensively researched. There are approximately a dozen large tokamaks at major fusion research centers around the world. The United States operates three major tokamaks, Alcator C-Mod at MIT in Cambridge, MA; NSTX-U at the Princeton Plasma Physics Laboratory in Princeton, NJ; and DIII-D at General Atomics in San Diego, CA. Other major tokamaks around the world include JET in the UK, EAST in China, KSTAR in South Korea, ASDEX-Upgrade in Germany, SST-1 in India, WEST in France and many others. Various notable tokamaks are shown to scale below.


Q: What other methods are there for containing fusion plasmas?

A: There are a variety of methods to confine a fusion relevant plasma using magnetic fields. The Stellarator is similar to a tokamak as it uses strong magnets but in a more complex shape and has demonstrated the next highest fusion performance. Wendelstein W7-X is an example recently commissioned in Germany. Other geometries include systems that generate their magnetic fields via the plasma itself. Additionally, lasers can be used to compress small pellets of fuel in what is termed inertial confinement fusion of which the NIF (National Ignition Facility) at Lawrence Livermore National Laboratory is an example.

Q: How is fusion research funded?

A: In the United States, the majority of fusion research is funded by the Federal government. The funding comes from the Department of Energy Office of Science, which is the largest funder of basic research in the US. This is the same office which funds the large particle accelerators, supercomputers, and x-ray light sources. In the US, Alcator C-Mod is the smallest of the three major tokamaks and operates with a budget approximately 1/4th and at approximately 1/3rd  of the cost per science user of the other two major federally funded tokamaks; DIII-D and NSTX-U.

Q: Is there private funding for fusion research?

A: Several private companies are pursuing fusion research. Most utilize concepts other than the tokamak and currently have significantly lower demonstrated performance and are largely unexplored. It is hoped by investors that these concepts can be scaled to the performance required for a reactor. Many of these experimental devices are similar size to Alcator C-Mod, but much smaller than other tokamaks.


Alcator C-Mod

Q: What is Alcator C-Mod?

A: Alcator C-Mod is a compact, high-field fusion device called a “tokamak” operated at the MIT Plasma Science and Fusion Center. It is a type of device capable of confining plasma, a super-heated gas, at temperatures and pressures prototypical of a fusion reactor by utilizing strong magnetic fields. It is the 3rd such fusion device constructed and operated at MIT. An engineering drawing and photo of the interior are shown below.

Alcator C-Mod

Q: What is special about Alcator C-Mod?

A: The approach with Alcator C-Mod is to use a very high magnetic field, more than double that typically used in other tokamak designs, which more than quadruples the bottle’s strength to resist instabilities driven by high pressure. With this strategy the pressure increases, the amount of fusion power per unit volume increases dramatically, and one can then design a very compact device at less cost. C-Mod has the highest magneticfield for a tokamak with its shape in the world. The high magnetic field and compact size leads to a very cost-effective research device that is small enough to be housed at a university instead of a large national laboratory. The small size of the device has enabled Alcator C-Mod to rapidly incorporate innovations and provide time-sensitive answers to problems involved in developing fusion science and technology.

Q: What types of scientists, students, engineers, and technicians are involved in C-Mod?

A: It takes a wide variety of talented individuals to successfully operate a tokamak. The C-Mod team includes engineers and technicians experienced with high power electronics, power plant equipment, high-vacuum, radio frequency and radar equipment. The team includes scientists and students from disciplines such as physics, nuclear engineering, computer science, mechanical engineering, electrical engineering, and aerospace engineering. Over the course of the Alcator C-Mod program over 150 graduate students have performed research contributing to their PhD. These students come from all over world to gather data. Additionally, the machine has changed configurations many times as upgrades are constantly undertaken to increase the device’s performance, its flexibility and to field better instruments to measure the plasma properties.

Q: How does Alcator C-Mod decide which experiments should be undertaken?

A: Alcator C-Mod was funded as a user facility, meaning it accepted proposed experiments from any user who promised to publish the results in an academic journal. A committee of users selects the experiments with the most merit (there are approximately 3x more experiments proposed than time available on the facility) and these are then conducted using the tokamak. The facility has served hundreds of scientist per year from academic institutions and national laboratories around the world.

Q: What does a C-Mod experiment look like?

A: The experiments are done as a series of plasmas, each about two seconds long, taken 15 minutes apart. The plasma is initiated, established, and then heated. The plasma conditions are probed using different measurement techniques and then the plasma is ramped down. The copper magnets and internal components are then cooled in preparation for the next plasma. Each plasma is different from the one before to enable the scientific hypothesis to be tested. Dozens of researchers and engineers are on hand to setup the conditions and analyze results. Each plasma discharge generates nearly 20 Gigabytes of data that is then analyzed and archived. Learn more about the C-Mod device using the 360deg video tour below.


The record setting experiments

Q: How were the experiments on Alcator C-Mod planned?

A: The record setting experiments were chosen because they explore several techniques to obtain high pressure that would only be available on Alcator C-Mod. The teams of scientists used previous results from C-Mod, experience on other machines, computer simulations, and theory to postulate methods to obtain higher plasma pressures using the machine.

Q: What strategies were used to obtain the high pressure?

A: With the clock ticking toward the midnight deadline three different strategies for breaching the pressure record were tried. Two came very close to reproducing the previous mark, and the third set the ultimate record. Two of the approaches, including the record-setting one, were led by scientists from other laboratories in the US, including the Princeton Plasma Physics Laboratory, the Oak Ridge National Laboratory, and General Atomics in San Diego. Each approach was thoroughly planned before hand and attempted sequentially throughout the day. The record setting approach had a modest temperature (only 35 million degrees) and a very high density. The other two, near record, approaches used higher magnetic fields of 7.8 Tesla and currents and less heating power or higher temperatures of over 70 Million degrees and lower plasma density to reach the high pressures. Interestingly, all three strategies achieved high performance despite approaching the problem in very different ways, demonstrating the breadth of the understanding of the requisite plasma physics and machine operation. The resulting maximum pressures for each attempt throughout the day are shown in the figure below. The dark red diamonds correspond to the highest current plasmas, the blue diamonds to the highest density plasmas, and the orange diamonds to the highest temperature plasmas.

Pressure performance during last day

Q: Who was involved in setting the pressure record?

A: A very large team was involved in the experiments with contributions from around the world. Key personnel included Earl Marmar (MIT) and Jim Irby (MIT) who operated the machine, Jerry Hughes (MIT), Phil Snyder (General Atomics), and Steve Scott (PPPL) who led the high temperature approach, Steve Wolfe (MIT) who led the high field and current approach., and Matt Reinke (Oak Ridge National Laboratory) who led the record setting high density approach. The experiments incorporated previous results and insight from a very wide variety of personnel in advance. The team’s reaction to the record setting plasma is shown in the video below.

Q: What role did the high magnetic field play in setting the record?

A: In addition to enabling such a compact high performance machine, the high magnetic field enables the record setting plasma to be obtained without any instabilities that can damage the interior components or lead to excess heat leakage. The plasmas ran the full desired length of 1.8 seconds without any deleterious effects.

Q: What were some of the properties of the plasma?

A: The record setting plasma was in many ways a typical plasma on C-Mod. It was established and then heated with radio frequency waves (like how one microwaves popcorn but to 35 Million degrees). It then obtained what is termed an H-mode, which is a configuration that has less heat leakage and high density. At this point it reached peak pressure but maintained high pressure until the heating was stopped, the fusion reactions decayed and the plasma was terminated by the operator before the magnets got to hot, after about 1.8 seconds. Traces of plasma properties are shown below.

Traces of plasma properties

A video of the plasma using a video camera external to the machine is below.

Q: How much fusion power did Alcator produce in these experiments?

A: The experiments on Alcator C-Mod did not use fuels that produce large amounts of fusion power as these are difficult to handle. As such the C-Mod experiments only made 150 watts of fusion power. If actual fusion fuel had been used this would be much higher.

Q: Why was the experiment conducted on the last day?

A: The experiment was deferred to the last day of C-Mod operation in order to leave ample time for other experiments of high scientific value to be completed. Earlier in the week Alcator C-Mod sessions were used to fill in needed data to complete several high priority research tasks, including graduate student projects. Due to its unique character, the final campaign was conducting experiments that could not be conducted at any other facility in the world now or in the foreseeable future and the machine deadline was a hard deadline. Therefore, it was critical to obtain as much research as possible before the machine performance was pushed into new territory.

Q: Why were the experiments conducted in the evening?

A: Typically, Alcator C-Mod ran from 9am to 5pm to conduct experiments. However, because it was the final day of operation it was decided to operate to midnight to maximize the time available. The entire team participated. The device’s performance was slowly increased throughput the day in a systematic manner. For the parameter pushing experiments in the evening the majority of the team gathered in the control room under a cheerful, but purposeful atmosphere, complete with off-site and international collaborators joining via video conferencing. . By building from the experience of each plasma’s performance the team charted a path to higher and higher pressure.

Q: Why was this the last day for Alcator C-Mod’s planned operation?

A: The Department of Energy and the US Congress had previously decided to cease funding Alcator C-Mod to conserve funding for the construction of ITER.

Q: Where will these results be presented?

A: The results will be presented by Earl Marmar, head of the Alcator project, at the 26th IAEA conference on fusion energy at Kyoto Japan on October 14th 2016. This is the premier conference organized by the international atomic energy agency, the world’s nuclear energy forum every two years. The conference will highlight the recent accomplishments of fusion researchers from around the world. The results will be published in a peer-reviewed journal article shortly afterwards.


The pressure record in context

Q: What is meant by pressure

A: The value being referred to is the volume averaged plasma pressure. Plasmas are constituted of two distinct species: the very light electrons and the relatively heavy ions; Alcator is so hot that basically all the ions have all their electrons stripped and are really bare nuclei. What we quote is the summed pressure from the ions and electrons averaged across the entire volume of the plasma.

Q: What is a layman context for the pressure record?

A: The plasma pressure inside Alcator C-Mod was approximately 2 atmospheres. This is a fairly low pressure by human standards, about the pressure of an overinflated football. However, the plasma was about 100,000 times hotter than air and 100,000 times less dense, which gives it much different properties than gases at similar pressures. The pressures obtained on other tokamaks would lead to very deflated footballs since the pressures are just above atmospheric pressure.

Q: What was the previous record?

A: Prior to the current campaign, the record of 1.79 atmospheres had stood since April 2005, also set on Alcator C-Mod. Prior to Alcator C-Mod operation, the record was 1.6 atmospheres set in the high magnetic field device Alcator C, which preceded Alcator C-Mod.

Q: Where else are plasmas found and at what pressures?

A: Low pressure plasmas are found inside plasma TVs fluorescent light bulbs, the Aurora Borealis i.e. the Northern Lights, and in material processing lines. These are all a small fraction of a percent atmospheric pressure. The solar wind is a plasma with pressures in the 10 millionths of an atmosphere at the Earth’s location that cause problems with satellites. The sun is made of plasma with pressures of hundreds of billion atmospheres (it is very large!).

Q: How did the Alcator C-Mod pressure compare to the big fusion reactor: the sun?

A: The center of the sun is at hundreds of billions of atmospheres of pressure. This is enabled by large gravitational forces from such a massive body. Such pressures cannot be sustained on Earth and only occur at the center of nuclear explosions or experimental configurations designed to be similar to nuclear explosions.

Q: How does the Alcator C-Mod pressure compare to other tokamaks?

A: The next highest plasma pressure is similar between many large tokamaks from around the world. This is at approximately 1.2 atmospheres. This is the highest pressure reached on devices such as JET, DIII-D, TFTR, and JT-60U, and is 60% of the pressure record set on C-Mod despite C-Mod being 20 – 100 times smaller in volume than these devices. A bar chart showing the record pressures and the highest pressures obtained on other tokamaks is shown below.

Highest pressures obtained at various tokamaks

Q: How did the record shot compare to other shots on Alcator C-Mod?

A: The vast majority of experiments on Alcator C-Mod are aimed at other areas of plasma physics and thus high pressure in and of itself is not typically a goal.  However, the record setting discharge was certainly high performance even for C-Mod. The time history of the pressures for all the approximately 33,000 plasmas on C-Mod is shown below. The record setting plasmas and the recent high-performance campaign are apparent on the very top right of the graph.

Performance of all shots on Alcator C-Mod

The histogram of C-Mod shots showing the maximum pressure is shown below.

Histogram of Alcator C-Mod maximum pressures

Q: How does the Alcator C-Mod pressure compare to other fusion configurations?

A: The pressure is a factor of 10 above the highest achieved on stellerators to date and is a factor of approximately 50 above that achieved in magnetic confinement experiments other than tokamaks or stellarators. It is significantly lower than that achieved in inertial (laser) confinement experiments which require astronomically high pressures to operate unlike a tokamak.

Q: How long will the pressure record stand?

A: Unless there is a breakthrough that increases performance a factor of several hundred times on non-tokamak devices, the pressure record for a magnetic fusion device is likely to stand until the mid-2030s after ITER is operational. This is because no new tokamaks are planned in the meantime that would achieve Alcator’s magnetic field and performance.

Q: What is ITER?

A: ITER is a large multi-national tokamak being constructed in the South of France. It will produce 500 million watts of fusion power using the correct fusion fuels for up to 400 seconds. The experiment is funded by a multi-national organization consisting of seven partners: The US, EU, China, India, South Korea, Russia and Japan. The tokamak is planned to begin operation in 2025 but will not start high performance fusion physics experiments until after 2032. ITER will be by far the largest tokamak built to date and will weigh as much as a WWI battleship and tower seven stories high. More can be learned at the ITER website.

Q: What is the highest pressure ITER is expected to obtain?

A: Despite its large size ITER it is anticipated that ITER will only obtain approximately 30% higher pressure (2.6 atm) than the C-Mod record. However, it will do so at about six times the temperature and with longer confinement time (about factor of 20) due to its large size. It will operate at 5.3T, which is significantly lower than the C-Mod field and will have a volume 800x the C-Mod volume.


The future

Q: What is the future of the Alcator C-Mod device?

A: Though the experiment is no longer operating, the device is still intact. It is hoped that major components can be re-used in a new experiment that utilizes high magnetic fields to study the interactions of plasmas and the exhaust ports required for fusion reactors. It is possible that it could also be donated to a fusion program elsewhere, a fate that happened to the tokamak that preceded Alcator C-Mod at MIT.

Q: What comes next for the Alcator C-Mod team?

A: The MIT fusion team is currently working on developing high temperature superconductors that would enable high-field tokamaks to be built without the use of energy consuming copper magnets such as used in C-Mod. This could enable fusion reactors at a much smaller size than previously envisioned, speeding development. Read more about these developments at MIT News and on the PSFC website. In addition, this might enable small fusion demonstration devices with performance similar to reactors at a scale slightly larger than Alcator C-Mod. Until then the Alcator C-Mod team will continue to collaborate on other fusion experiments around the world to further discovery of fusion-relevant plasma physics.

Topics: Magnetic fusion energy, Alcator C-Mod tokamak