SPARC

SPARC is a tokamak-type fusion experiment being built by the American private company Commonwealth Fusion Systems (CFS) together with the Plasma Science and Fusion Center at the Massachusetts Institute of Technology (MIT). In contrast to ITER, a massive state-led megaproject, SPARC aims to demonstrate fusion “smaller, faster, and with private money.” This page walks through why a compact device can still target high performance, what the high-temperature superconducting coils at the heart of it are, and what the privately led development model means.

SPARC in One Sentence (High School)

A tokamak is a device that confines an ultra-hot plasma of about 100 million degrees (a gas in which atomic nuclei and electrons fly around separately) inside a donut-shaped vessel to make fusion happen. The plasma is held in an “invisible cage” made of powerful magnets. The stronger this cage, the more tightly it can squeeze the hot plasma.

Earlier large devices could not make their magnets very strong, so they made the whole device bigger to make up for it and gain performance. This is why ITER becomes a huge building nearly 30 meters across.

SPARC’s idea is exactly the opposite. If you can make the magnets far stronger, the device can stay small. What made those strong magnets possible is a new material, a high-temperature superconductor, whose electrical resistance drops to zero even at relatively high temperatures. SPARC has only about one-seventieth the volume of ITER, yet it targets a comparable fusion power output. Think of it as “doing the work of a heavy truck in the size of a compact car.”

The name SPARC comes from the initials of the development team’s motto, Soonest / Smallest Private-funded Affordable Robust Compact.

Why a High Field Allows Miniaturization (Undergraduate)

The core of SPARC lies in the scaling that “fusion power is proportional to the fourth power of the magnetic field strength.” Grasping this intuition is the most important thing on this page.

How much pressure a tokamak plasma can withstand is determined by the beta value $\beta$, the ratio to the magnetic pressure.

$$\beta = \frac{p}{B^2 / (2\mu_0)}$$

Here $p$ is the plasma pressure, $B$ is the magnetic field strength, and $\mu_0$ is the vacuum permeability. This equation expresses “how much of the pressure produced by the magnetic field can be used as plasma pressure.” Note the $B^2$ in the denominator. Each device has roughly an upper limit on the beta value $\beta$ it can achieve, so if we treat it as constant, the plasma pressure scales as $p \propto \beta B^2$, meaning it can be increased with the square of the magnetic field.

Meanwhile, the fusion power density (heat produced per unit volume) is roughly proportional to the square of the plasma pressure. Since the product of fuel density and temperature is the pressure, the likelihood of reactions occurring scales with the square of the pressure.

$$P_{\text{fus}} / V \propto p^2 \propto (\beta B^2)^2 \propto B^4$$

Combining these two, the power density is proportional to the fourth power of the magnetic field. Doubling the field increases the power density by $2^4 = 16$ times. That is exactly why, if you can make the field strong, you can obtain sufficient output without making the device large. This is the foundation of the design philosophy of the high-field compact tokamak.

Let us look at SPARC’s main design values alongside those of ITER.

Parameter	SPARC	ITER
Plasma major radius	~1.85 m	6.2 m
Plasma minor radius	~0.57 m	2.0 m
Central field	~12 T	5.3 T
Plasma current	~8.7 MA	15 MA
Fusion power	tens of MW class	500 MW
Pulse duration	~10 s	hundreds of s

SPARC’s central field is about 12 tesla (T), more than double ITER’s 5.3 T. Since a field twice as strong makes the power density more than 10 times higher, even with a volume about one-seventieth that of ITER, it can fully compete in fusion power. The short pulse duration of about 10 seconds is because SPARC is designed not for steady-state operation but as an experimental device to first confirm the physics of burning plasma.

The Key: High-Temperature Superconducting Coils (Graduate)

Saying “double the magnetic field” is easy, but as coil technology there is a big wall. This is the technical heart of SPARC.

Superconductors have an upper limit on the magnetic field (upper critical field) beyond which superconductivity breaks down. In low-temperature superconductors such as the $\text{Nb}_3\text{Sn}$ (niobium-tin) used by ITER, even when cooled to an absolute temperature of 4 K (about minus 269 degrees), the practical limit is around 12 to 13 T, and the field usable as a coil is even lower. This is why ITER’s central field stops at 5.3 T.

SPARC adopted a high-temperature superconductor called REBCO (rare-earth barium copper oxide). REBCO is processed into a tape form and can carry a large current in a strong field far exceeding 20 T even at the somewhat higher temperature of 20 K (about minus 253 degrees). The higher the operating temperature, the more margin there is for cooling, and the more thermal stability against disturbances can be gained.

CFS developed a cable called VIPER, in which dozens of these REBCO tapes are stacked and housed in a copper jacket through which coolant flows. This achieves both a high current density and the stability that superconductivity does not break down in a chain reaction even when local heating occurs.

In September 2021, CFS and MIT succeeded in generating a 20 T field with a full-scale model coil of a toroidal field coil (the main field coil surrounding the plasma) made with this technology. This demonstration, showing that a 20 T-class high-temperature superconducting coil works at the scale of a fusion reactor, is positioned as the single most important milestone in the entire SPARC program. It was the moment the magnetic field that determines the device’s performance was backed up not by desk calculations but by a real coil.

To assemble this coil into a fusion device, elements such as the vacuum vessel, shielding, heating and current drive, and the divertor (the exhaust port for heat and particles) must fit into a limited small space. Precisely because it is compact, the thermal and neutron loads on the components become large per unit volume, making it a challenging design from the standpoint of materials engineering as well.

Research Frontier (PhD)

The scientific goal SPARC aims for is the demonstration of fusion gain $Q$. $Q$ is the value obtained by dividing “the energy produced by fusion” by “the energy poured in from outside to heat the plasma.”

$$Q = \frac{P_{\text{fus}}}{P_{\text{heat}}}$$

$Q > 1$ is the state in which more energy comes out of fusion than is put in, and $Q = \infty$ means ignition, the point at which the plasma keeps burning on its own heat alone without external heating. By design, SPARC is built with the margin to potentially reach a $Q$ of around 10, and it sets as its demonstration goal to reliably achieve at least $Q > 2$.

From around the point where $Q$ exceeds 5, self-heating, in which the alpha particles (helium nuclei) produced by fusion themselves heat the plasma, becomes dominant. This is the regime of the burning plasma, in which the reaction’s heat sustains the next reaction without relying on external heating. How the plasma in this state behaves, and how the waves and instabilities caused by alpha particles develop, still have scarce experimental data and are frontier problems of fusion physics. SPARC is expected to be one of the world’s earliest devices to step into this regime.

Among the main topics currently being researched are the following. How to predict and mitigate the disruptions in which the plasma suddenly collapses, where prediction using machine learning and mitigation by injecting pellets (small ice bullets) are being considered. Also, how to receive at the divertor the heat loads that become orders of magnitude larger in a compact device is an important challenge.

Beyond SPARC lies the ARC concept, which aims to demonstrate power generation. ARC stands for Affordable, Robust, Compact, and it is a plan to develop the high-field, high-temperature superconducting technology confirmed with SPARC into an actual electricity-generating power plant. In the ARC concept, a blanket surrounding the outside of the coils with the molten salt FLiBe (a molten salt of lithium fluoride and beryllium fluoride) is being considered. FLiBe is intended to catch the neutrons flying out from fusion and convert them into heat, while at the same time the lithium it contains reacts with neutrons to produce the fuel tritium (tritium breeding). Being a liquid, it is also aimed at making maintenance and replacement of the device easier.

Here are some keywords, with English notation, that appear frequently when reading papers: high-field pathway, burning plasma, HTS magnet, demountable coil, molten salt blanket, and so on.

Check Your Understanding

Q1. Why can SPARC target a comparable fusion power output even with about one-seventieth the volume of ITER?

A Because making the device smaller keeps heat from escaping and raises the temperature B Because fusion power density is proportional to the fourth power of the magnetic field, and SPARC uses a field of about 12 T, more than double that of ITER C Because a smaller device can drive a larger plasma current D Because using REBCO increases the fuel density itself

Q2. How is the relationship that power density is proportional to the fourth power of the magnetic field derived?

A The plasma pressure is proportional to the field, and the power density is proportional to the pressure, so together it becomes the square of the field B It has been measured experimentally that the power density is directly proportional to the fourth power of the field C The plasma pressure is proportional to the square of the field, and the power density is proportional to the square of the pressure, so together it becomes the fourth power of the field D Because the field is made with four coils, the output becomes four times larger

Q3. Why did SPARC adopt a REBCO-based high-temperature superconductor rather than a low-temperature superconductor?

A Because low-temperature superconductors are practically limited to fields of about 5 T, while REBCO can handle strong fields exceeding 20 T and large currents even at 20 K B Because REBCO needs no cooling at all and operates at room temperature C Because REBCO is cheaper and easier to obtain than low-temperature superconductors D Because REBCO gives the same output even with a weaker field

Q4. What is the burning plasma that appears when the fusion gain Q becomes large?

A A state in which external heating is maximized to forcibly keep the plasma burning B A state in which neutrons heat the plasma and sustain the reaction C A state in which self-heating, where the alpha particles produced by fusion themselves heat the plasma, becomes dominant D A stable state in which the plasma cools and the reaction proceeds slowly

Q5. What is the role of the molten salt FLiBe blanket being considered in the successor ARC concept?

A To cool the coils and keep them in a superconducting state B To catch neutrons and convert them into heat, while at the same time the lithium it contains reacts with neutrons to breed the fuel tritium C To directly touch the plasma and measure its temperature D To electrically insulate the outside of the device

Related Topics

• Tokamak - The basics of the magnetic confinement method SPARC adopts
• Superconducting Coils - The technology of high-temperature superconducting REBCO coils
• Private Fusion Ventures - Trends among private companies worldwide, including CFS