SPARC

SPARC is an innovative tokamak-type fusion experimental reactor being jointly developed by MIT’s Plasma Science and Fusion Center (PSFC) and Commonwealth Fusion Systems (CFS). By adopting high-temperature superconducting (HTS) magnets, it aims to realize a significantly more compact and higher-performance device than conventional approaches, accelerating the commercialization of fusion power generation.

The name SPARC is derived from the acronym “Soonest/Smallest Private-funded Affordable Robust Compact,” expressing the project’s essence of rapid and economical fusion development through private funding.

Historical Background

The Legacy of MIT’s Alcator Series

SPARC’s technical foundation lies in the accumulated knowledge from MIT’s high-field tokamak research spanning over half a century. MIT’s Plasma Science and Fusion Center (PSFC) has consistently pursued the “high-field, compact” approach since the 1970s.

Alcator A (1973-1979)

The first Alcator featured an innovative design for its time, with a major radius of only 0.54 m and an on-axis magnetic field of 10 T. Using copper Bitter-type magnets, it experimentally verified the effects of high magnetic fields on plasma confinement. The confinement data obtained from Alcator A formed the basis for subsequent scaling laws.

Alcator C (1978-1987)

Alcator C achieved the world’s highest confinement performance with a major radius of 0.64 m and magnetic fields up to 13 T. By combining peaked density profiles through pellet injection with high magnetic fields, it set world records for the fusion triple product.

Alcator C-Mod (1993-2016)

Alcator C-Mod was designed as the “ultimate compact tokamak” with a major radius of 0.67 m and a magnetic field of 8 T. It was the first in the world to adopt an all-metal (molybdenum) first wall and achieved high-density H-mode operation.

Key achievements of Alcator C-Mod:

• Plasma pressure of 2.05 atmospheres (world record for magnetic confinement, 2016)
• Discovery of I-mode (improved confinement mode)
• Elucidation of H-mode physics at high density
• Heat load research on high-field divertors

Until the end of its operation in 2016, Alcator C-Mod was the tokamak operating at the highest magnetic field in the world. The knowledge gained from this device has been directly applied to SPARC’s design.

History of HTS Technology Development

The discovery of high-temperature superconductors dates back to 1986, when Bednorz and Muller at IBM Zurich Research Laboratory discovered copper oxide superconductors. This discovery brought a paradigm shift to physics, and both researchers received the Nobel Prize in Physics the following year.

First-Generation HTS (BSCCO-based)

Bi-Sr-Ca-Cu-O (BSCCO) superconductors were discovered in 1988 and commercialized as tape conductors. However, the decrease in critical current density under high magnetic fields remained a challenge, making them unsuitable for fusion magnets.

Second-Generation HTS (REBCO-based)

REBCO (REBa₂Cu₃O₇₋δ) thin-film conductors developed in the 1990s overcame the challenges of the first generation. From the 2000s through the 2010s, continuous manufacturing technology for long tape conductors was established, and costs decreased dramatically.

REBCO tape price trends:

Era	Price ($/kA·m)	Notes
2010	~400	Research use only
2015	~150	Mass production technology established
2020	~50	CFS long-term contracts
2025	~25	Mass production effects

This price reduction is the biggest factor that made SPARC possible. While HTS magnets were technically possible earlier, they only became an economically viable option from the late 2010s.

Founding of CFS

In March 2018, a group of researchers from MIT PSFC founded Commonwealth Fusion Systems (CFS). The founders included Professor Dennis Whyte (then PSFC Director), Dr. Bob Mumgaard (CEO), and Dr. Brandon Sorbom.

The founding of CFS was driven by a sense of urgency that the development speed difficult to achieve in academic research was needed. The shutdown of Alcator C-Mod due to budget cuts highlighted the vulnerability of fusion research dependent on government funding. A new model of rapid development through private funding began here.

Mission and Objectives

SPARC’s main goal is to demonstrate the feasibility of commercially meaningful fusion energy. Specifically:

• Achieving Q > 2 (fusion output more than twice the input energy)
• Ultimate goal of Q = 10 or higher (50-100 MW fusion output)
• Demonstration of high-temperature superconducting magnet technology
• Technical bridge to the commercial reactor ARC
• Verification of compact and economical fusion reactor design

Significance of Scientific Breakeven

The most important metric for evaluating fusion reactor performance is the fusion gain Q. Q is defined as the ratio of fusion power $P_\text{fusion}$ to heating input $P_\text{heat}$:

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

Q = 1 is called scientific breakeven, meaning a state where the fusion energy obtained equals the energy input. For commercial power plants, considering plant auxiliaries and energy conversion efficiency, Q > 10 is required.

SPARC’s design target is Q > 2, but this is not merely a milestone. By achieving Q > 2, full-scale research of burning plasma physics becomes possible. Burning plasma is a plasma state where self-heating by alpha particles is dominant, and understanding the physics of this regime is key to commercial reactor development.

Alpha particle heating power $P_\alpha$ is approximately 20% of fusion output:

$$P_\alpha = \frac{1}{5} P_\text{fusion} = \frac{Q}{5} P_\text{heat}$$

When Q = 5, $P_\alpha = P_\text{heat}$, and the plasma enters the burning plasma regime where self-heating is dominant. SPARC’s ultimate goal of Q > 10 is intended to sufficiently explore this burning plasma regime.

Strategic Positioning

CFS’s strategy is clear: a two-stage approach of demonstrating burning plasma physics with SPARC and directly applying that knowledge to the commercial reactor ARC. In conventional fusion development, multiple intermediate stages were anticipated for the transition from experimental to commercial reactors, but CFS is attempting to significantly shorten this process.

At the core of this strategy is high-temperature superconducting magnet technology. The high magnetic fields enabled by HTS magnets make high performance in a compact device possible, leading to significant reductions in development time and cost.

Technical Specifications

SPARC’s design parameters are the result of pursuing both compactness and high performance.

Key Parameters

Parameter	Value	Notes
Plasma major radius $R_0$	1.85 m	About 1/3 of ITER
Plasma minor radius $a$	0.57 m	Aspect ratio 3.25
Aspect ratio $A = R_0/a$	3.25	Similar to conventional tokamaks
Magnetic field (on-axis) $B_0$	12.2 T	2.3 times ITER
Maximum magnetic field (at coil)	21.5 T	Achieved with HTS
Plasma current $I_p$	8.7 MA	High current density
Number of toroidal field coils	18	Pancake structure
Number of poloidal field coils	8	For position control
Central solenoid peak field	17 T	For current ramp-up
Fusion power $P_\text{fusion}$	50-100 MW	Predicted
Fusion gain Q	> 2 (design), > 10 (ultimate)	Burning plasma
Fuel	Deuterium-tritium (D-T)	Optimal reaction
Cryostat diameter	~7.3 m	24 feet
Cryostat height	~8.5 m	28 feet
Total weight	~1,000 tons	1/30 of ITER
Pulse duration	10 seconds	Flat-top
Repetition period	~2 hours	Cooling cycle

Plasma Parameters

SPARC’s plasma is designed to have the following characteristics:

Parameter	Value	Physical Meaning
Electron density $n_e$	$3.1 \times 10^{20}$ m$^{-3}$	High-density operation
Electron temperature $T_e$	21 keV	~240 million degrees
Ion temperature $T_i$	18 keV	~210 million degrees
Normalized beta $\beta_N$	1.0	Conservative design
Toroidal beta $\beta_T$	1.6%	MHD stable regime
Poloidal beta $\beta_p$	0.44	Bootstrap current contribution
Greenwald density fraction $f_{GW}$	0.37	Large margin to density limit
Confinement enhancement factor $H_{98}$	1.0	Standard H-mode
Safety factor (95% flux surface) $q_{95}$	3.1	Kink stability
Elongation $\kappa$	1.8	Shape optimization
Triangularity $\delta$	0.54	Edge stability

The Greenwald density limit is empirically given by:

$$n_{GW} = \frac{I_p}{\pi a^2} \times 10^{20} \text{ m}^{-3}$$

For SPARC with $I_p = 8.7$ MA and $a = 0.57$ m:

$$n_{GW} = \frac{8.7}{\pi \times 0.57^2} \times 10^{20} = 8.5 \times 10^{20} \text{ m}^{-3}$$

The operating density $n_e = 3.1 \times 10^{20}$ m$^{-3}$ is 37% of this, providing sufficient margin against the density limit. This margin ensures tolerance for density fluctuations and impurity accumulation during burning plasma operation.

Confinement Performance Predictions

SPARC’s confinement performance predictions use scaling laws based on the ITER physics basis. The energy confinement time $\tau_E$ in H-mode is given by the IPB98(y,2) scaling:

$$\tau_E^{IPB98(y,2)} = 0.0562 \times I_p^{0.93} B_T^{0.15} n_{19}^{0.41} P^{-0.69} R^{1.97} \varepsilon^{0.58} \kappa_a^{0.78} M^{0.19}$$

Where:

• $I_p$ is plasma current [MA]
• $B_T$ is toroidal magnetic field [T]
• $n_{19}$ is line-averaged electron density [$10^{19}$ m$^{-3}$]
• $P$ is loss power [MW]
• $R$ is major radius [m]
• $\varepsilon = a/R$ is inverse aspect ratio
• $\kappa_a$ is elongation
• $M$ is effective ion mass [atomic units]

Substituting SPARC’s parameters predicts $\tau_E \approx 0.77$ seconds. With this confinement time and high density combination, the fusion triple product becomes:

$$n_i T_i \tau_E \approx 3.1 \times 10^{20} \times 18 \times 0.77 = 4.3 \times 10^{21} \text{ keV} \cdot \text{m}^{-3} \cdot \text{s}$$

This exceeds the $3 \times 10^{21}$ keV$\cdot$m$^{-3}\cdot$s required for ignition conditions.

Pedestal Predictions

In H-mode plasma, a pedestal structure with high pressure gradients forms at the edge. The pedestal height significantly affects confinement performance. SPARC’s pedestal predictions use the EPED model:

$$p_\text{ped} = c \times \beta_{N,\text{ped}}^2 \times B_T^2 \times \left( \frac{W_\text{ped}}{a} \right)$$

Where $W_\text{ped}$ is the pedestal width. High magnetic field affects pedestal pressure quadratically, so SPARC can achieve a higher pedestal than a same-sized low-field device.

SPARC’s predicted pedestal temperature is approximately 6 keV, which serves as the foundation supporting the high core plasma temperature (over 20 keV).

Innovation in High-Temperature Superconducting Magnets

SPARC’s greatest technological innovation is the adoption of high-temperature superconducting (HTS) magnets. This technology has the potential to fundamentally change the paradigm of fusion development.

Basic Physics of Superconductors

Superconductors are materials whose electrical resistance becomes zero below a critical temperature $T_c$. According to BCS theory, in the superconducting state, electrons form Cooper pairs and behave as a coherent quantum state.

The maximum magnetic field at which a superconductor can maintain its superconducting state in a magnetic field is called the upper critical field $B_{c2}$. For type-II superconductors, $B_{c2}$ has temperature dependence, approximately:

$$B_{c2}(T) = B_{c2}(0) \left[ 1 - \left( \frac{T}{T_c} \right)^2 \right]$$

Materials with higher $B_{c2}(0)$ are better suited for high-field applications.

Limitations of Conventional Low-Temperature Superconductors

Low-temperature superconductors (LTS) used in conventional fusion devices have inherent limitations:

Material	$T_c$ [K]	$B_{c2}(4.2\text{K})$ [T]	Characteristics
NbTi	9.8	11	Good workability, low cost
Nb$_3$Sn	18.3	24	High-field capable, brittle

NbTi has excellent workability but reaches its limit at magnetic field strengths around 8 T. Nb$_3$Sn can handle higher fields but, being a brittle material, requires heat treatment after winding, making the manufacturing process complex.

ITER uses Nb$_3$Sn, but its on-axis field is limited to 5.3 T. This is the result of considering safety margins and structural constraints.

REBCO Superconducting Tape

The REBCO (Rare Earth Barium Copper Oxide) adopted by SPARC, with the chemical formula REBa$_2$Cu$_3$O$_{7-\delta}$ (RE = Y, Gd, etc.), is a high-temperature superconductor whose properties far exceed LTS:

Parameter	REBCO	Nb$_3$Sn	Ratio
Critical temperature $T_c$	90 K	18 K	5×
$B_{c2}(4.2\text{K})$	> 100 T	24 T	4×
Engineering current density (20T, 20K)	1000 A/mm$^2$	Not practical	-
Operating temperature (SPARC)	20 K	4.5 K	4×
Temperature margin	70 K	14 K	5×

REBCO’s high critical temperature and upper critical field enable stable generation of 20 T-class magnetic fields. Additionally, operation at 20 K has lower cooling costs than liquid helium (4.2 K), and the larger temperature margin improves operational stability.

REBCO tape is manufactured using thin-film technology. The structure involves depositing a REBCO layer on a substrate through intermediate buffer layers and protecting it with a copper stabilization layer. A typical cross-sectional structure is:

1. Substrate (Hastelloy): 50 μm
2. Buffer layers (MgO, LaMnO$_3$, etc.): several μm
3. REBCO layer: 1-2 μm
4. Silver layer: 2-3 μm
5. Copper stabilization layer: 20-40 μm

Tape widths typically range from 4-12 mm, and continuous manufacturing technology for long tapes has been established.

VIPER Cable Structure

The VIPER (Vacuum Pressure Impregnated, Insulation Removed) cable developed by CFS is an innovative conductor structure optimizing REBCO tape for fusion magnets.

VIPER Structure

VIPER cable is a structure containing multiple REBCO tapes within a copper cable space:

Component	Specification
REBCO tapes/cable	12-16
Cable width	~30 mm
Cable thickness	~4 mm
Critical current (20 K, 20 T)	~40 kA
Operating current	~20 kA

Technical Features

1. High current density: Achieves several times the current density of conventional LTS cables
2. Mechanical robustness: Copper jacket protects the tape
3. Thermal stability: Large heat capacity of copper suppresses quench
4. Manufacturability: Modular manufacturing possible

VIPER cable development was published in Superconductor Science and Technology journal in 2020 and has been academically recognized.

Historic Achievement in 2021

In September 2021, CFS achieved a magnetic field strength of 20 T in large-scale HTS magnet testing. This was a world record for high-temperature superconducting magnets and a significant milestone toward commercial fusion.

The magnet used in this test was a full-scale prototype of one SPARC toroidal field coil, with the following specifications:

Parameter	Value
Magnetic field strength	20.0 T (center)
Stored energy	41 MJ
Total weight	9 tons
REBCO tape length	~270 km
Operating temperature	20 K
Cooling time	~4 hours
Energization time	~2 hours

A magnetic field strength of 20 T is unachievable with conventional LTS magnets. This success demonstrated that HTS magnets are practical at fusion reactor scale.

The stored magnetic field energy $W_B$ in the magnet is given by the volume integral of the field:

$$W_B = \frac{1}{2\mu_0} \int B^2 dV$$

The energy of 41 MJ is equivalent to approximately 10 kg of TNT explosive. The technology to safely control this energy is also an important element of SPARC development.

Magnet Structure

SPARC’s toroidal field coils adopt an innovative “pancake” structure. Each coil consists of 16 pancakes, and each pancake is a flat structure with REBCO tape wound in a spiral pattern.

The advantages of the pancake structure are:

1. Manufacturability: Individual pancakes can be manufactured and inspected separately
2. Cooling efficiency: Cooling channels can be placed between pancakes
3. Maintainability: Individual replacement possible during failures
4. Electrical insulation: Turn-to-turn insulation is easy

On the back of each pancake, grooves for helium gas cooling are machined. Supercritical helium (20 K, several atmospheres) flows through these grooves to cool the superconducting tape.

Approximately 10,000 km of REBCO tape is required for all of SPARC. This is comparable to Earth’s diameter. CFS has partnered with tape manufacturers to achieve this mass production.

Quench Protection

The greatest risk in superconducting magnets is “quench.” Quench is a phenomenon where the superconducting state is broken for some reason and transitions to the normal conducting state. In the normal conducting portion, rapid heating occurs due to electrical resistance, potentially damaging the magnet.

The rate of temperature rise during quench can be estimated by:

$$\frac{dT}{dt} = \frac{J^2 \rho(T)}{C(T)}$$

Where $J$ is current density, $\rho(T)$ is electrical resistivity, and $C(T)$ is volumetric specific heat. In magnets operating at high current density, the time from quench detection to protective action is set extremely short.

SPARC’s quench protection system consists of the following elements:

1. Voltage sensors: Quench detection (detection time < 50 ms)
2. Fiber optic temperature sensors: Local temperature monitoring
3. Rapid power supply shutdown: Within hundreds of milliseconds
4. Dump resistors: Release of stored energy
5. Emergency helium cooling: Suppression of temperature rise

HTS magnets have the advantage of higher thermal stability compared to LTS magnets. This is because the specific heat at higher operating temperatures is larger, making local heat generation less likely to convert to temperature rise.

Physics of High-Field Compact Tokamaks

What enables SPARC’s compact design is the physical law relating high magnetic fields to fusion performance.

Relationship Between Fusion Output and Magnetic Field

The D-T fusion reaction power density, using ion density $n_i$ and reactivity $\langle \sigma v \rangle$, is:

$$p_\text{fusion} = \frac{1}{4} n_i^2 \langle \sigma v \rangle E_\text{fusion}$$

Where $E_\text{fusion} = 17.6$ MeV is the energy released per D-T reaction.

In tokamak plasma, from an MHD stability perspective, the beta value $\beta$, the ratio of plasma pressure $p$ to magnetic field pressure $B^2/(2\mu_0)$, has an upper limit:

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

The upper limit of toroidal beta is empirically given by:

$$\beta_N = \frac{\beta}{I_p/(aB_T)} \lesssim 3$$

(toroidal beta in percent, $I_p$ in MA, $a$ in m, $B_T$ in T).

Under ideal conditions, $p \propto \beta B^2$, and $n \propto \beta B^2/T$. Since fusion power density is proportional to $n^2$:

$$p_\text{fusion} \propto \beta^2 B^4$$

This $B^4$ dependence is the physical basis for the superiority of high-field tokamaks. Doubling the magnetic field increases fusion power density by 16 times at the same beta value.

Trade-off Between Device Size and Magnetic Field

Total fusion power $P_\text{fusion}$ is the product of plasma volume $V \propto R_0 a^2$ and power density:

$$P_\text{fusion} \propto \beta^2 B^4 R_0 a^2$$

The device size required to achieve the same fusion output has the following relationship with magnetic field strength:

$$R_0 a^2 \propto \frac{1}{\beta^2 B^4}$$

Comparing ITER ($B_0 = 5.3$ T) and SPARC ($B_0 = 12.2$ T), the magnetic field ratio is about 2.3 times, and the volume required to achieve the same output at the same beta value is:

$$\frac{V_\text{SPARC}}{V_\text{ITER}} \approx \left( \frac{5.3}{12.2} \right)^4 \approx 0.036$$

In other words, SPARC can potentially achieve equivalent power density in about 4% of ITER’s volume. In reality, differences in operating modes and confinement characteristics exist, so this simple comparison is only a qualitative guide, but it demonstrates the power of the high-field approach.

Lawson Criterion and High Magnetic Field

The Lawson criterion gives the conditions for power output to exceed input in a steady-state fusion reactor. The simplified Lawson criterion for D-T reactions is:

$$n_i T_i \tau_E > 3 \times 10^{21} \text{ keV} \cdot \text{m}^{-3} \cdot \text{s}$$

In high-field tokamaks, higher density can be achieved within the beta limit:

$$n \propto \beta B^2 / T$$

Also, from the energy confinement time scaling law:

$$\tau_E \propto I_p^{0.93} B_T^{0.15} \cdots$$

High magnetic field can drive higher current (from $I_p \propto aB_T/q$), resulting in improved confinement time as well.

The combination of these effects puts high-field compact tokamaks in an advantageous position to satisfy the Lawson criterion.

Bootstrap Current

The current that flows spontaneously due to plasma pressure gradients is called bootstrap current. This current arises because charged particles selectively move between trapped and passing orbits due to density and temperature gradients.

The bootstrap current fraction $f_{BS}$ is:

$$f_{BS} = \frac{I_{BS}}{I_p} \propto \frac{\beta_p}{\sqrt{\varepsilon}}$$

Where $\beta_p$ is poloidal beta and $\varepsilon$ is inverse aspect ratio.

A high bootstrap current fraction reduces the need for external current drive and facilitates steady-state operation. SPARC has a modest $f_{BS} \approx 0.1$, but the commercial reactor ARC aims for $f_{BS} \approx 0.5$ or higher.

Plasma Heating Systems

SPARC’s plasma heating is accomplished primarily by three methods.

Ohmic Heating

Joule heating by plasma current is ohmic heating. The heating power is:

$$P_\Omega = R_p I_p^2$$

Where $R_p$ is plasma resistance, and using Spitzer resistivity:

$$R_p \propto \frac{Z_\text{eff} \ln\Lambda}{T_e^{3/2}}$$

At high temperatures, resistivity decreases rapidly, so ohmic heating alone cannot reach burning temperatures (above 10 keV). In SPARC, ohmic heating is used for initial heating and current ramp-up.

ICRH (Ion Cyclotron Resonance Heating)

SPARC’s primary external heating system is ICRH. By injecting radio frequency waves tuned to the ion cyclotron frequency in the toroidal magnetic field:

$$\omega_{ci} = \frac{q_i B}{m_i}$$

ions are selectively heated.

SPARC ICRH system specifications:

Parameter	Value	Notes
Frequency	~120 MHz	High-field compatible
Total input power	25 MW	4 systems
Number of antennas	2-3	Port arrangement
Main scenario	Minority ion heating ($^3$He)	Optimized
Power conversion efficiency	~60%	To plasma

The cyclotron frequency of deuterium ions in a 12.2 T magnetic field is:

$$f_{cD} = \frac{eB}{2\pi m_D} = \frac{1.6 \times 10^{-19} \times 12.2}{2\pi \times 3.34 \times 10^{-27}} \approx 93 \text{ MHz}$$

In practice, the frequency and minority ion species ($^3$He, etc.) are selected considering the radial variation of the magnetic field and optimization of resonance conditions.

LHCD (Lower Hybrid Current Drive)

SPARC also plans research on lower hybrid current drive (LHCD) for future steady-state operation studies. Lower hybrid waves are electromagnetic waves with frequency $\omega$ in the range:

$$\omega_{ci} \ll \omega \ll \omega_{ce}$$

They propagate through the plasma while transferring energy to electrons, driving non-inductive current.

Current drive efficiency is defined as:

$$\eta_{CD} = \frac{n_e R I_{CD}}{P_{CD}} \quad [\text{A/W} \cdot 10^{19} \text{m}^{-2}]$$

LHCD is known for its high current drive efficiency and is an essential technology for steady-state operation in commercial reactors.

Plasma Control and Diagnostic Systems

Control of burning plasma is essential to SPARC’s success.

Real-time Control System

SPARC’s plasma control system consists of the following hierarchy:

Low-level control (response time < 1 ms)

• Vertical position stabilization
• Plasma current feedback
• High-speed digital control loops

Mid-level control (response time 1-100 ms)

• Plasma shape control
• Density control
• ICRH power adjustment

High-level control (response time > 100 ms)

• Scenario management
• Disruption prediction
• Operating mode optimization

Main Diagnostic Equipment

Approximately 30 types of diagnostic equipment will be installed in SPARC:

Diagnostic	Measurement Target	Spatial Resolution
Thomson scattering	Electron temperature and density	~1 cm
Charge exchange spectroscopy	Ion temperature and rotation	~5 cm
Bolometer array	Radiated power distribution	~2 cm
Interferometer	Line-integrated density	-
Magnetics	Magnetic field and current distribution	~5 cm
Soft X-ray camera	Internal structure and instabilities	~1 cm
Neutron detectors	Fusion power	Integrated
Gamma-ray detectors	Fast particle losses	Integrated
ECE (Electron Cyclotron Emission)	Electron temperature profile	~1 cm

A unique diagnostic challenge for burning plasma is the radiation environment from 14.1 MeV neutrons. Many diagnostic instruments will be installed in shielded rooms away from the plasma, with signals transmitted via optical fibers and mirrors.

Disruption Mitigation

The greatest operational risk in tokamaks is disruption (plasma collapse). During disruptions, stored magnetic energy is rapidly released, placing large electromagnetic forces and heat loads on structures.

SPARC’s disruption mitigation system:

Prediction System

• Machine learning-based instability prediction
• Real-time disruption warning (> 30 ms advance)
• Multivariate sensor fusion

Mitigation Actuators

• Shattered Pellet Injection (SPI): Rapid injection of large amounts of gas
• Massive Gas Injection (MGI): Auxiliary gas injection
• Fast poloidal field control

Thermal load during disruption $Q_\text{th}$ is:

$$Q_\text{th} = \frac{W_\text{th}}{A_\text{wetted} \times \tau_\text{TQ}}$$

Where $W_\text{th}$ is thermal energy, $A_\text{wetted}$ is wetted area, and $\tau_\text{TQ}$ is thermal quench time. By causing radiative collapse of the plasma through SPI, the wetted area is increased and heat load is distributed.

Divertor and First Wall

The divertor handles heat exhaust and particle pumping from the high-performance plasma. SPARC’s divertor design is an important technical challenge directly related to commercial reactors.

Heat Load Evaluation

Power flowing through the scrape-off layer (SOL) $P_\text{SOL}$ is:

$$P_\text{SOL} = P_\text{heat} + P_\alpha - P_\text{rad,core}$$

Where $P_\text{rad,core}$ is core radiation loss. For SPARC, $P_\text{SOL} \approx 50$ MW is expected.

Heat load on the divertor target $q_\text{div}$ is:

$$q_\text{div} \approx \frac{P_\text{SOL}}{4\pi R_\text{div} \lambda_q f_\text{exp}}$$

Where $\lambda_q$ is SOL width (several mm) and $f_\text{exp}$ is flux expansion factor.

Empirical scaling for SOL width:

$$\lambda_q \approx \frac{1.35}{B_p^{1.1}} \text{ [mm]}$$

Where $B_p$ is poloidal magnetic field [T]. High-field, high-current density tokamaks tend to have narrower SOL widths, concentrating heat load.

Detachment Operation

To mitigate excessive heat loads, SPARC will employ detachment operation. This involves injecting impurity gas (nitrogen, neon, etc.) into the divertor region, lowering plasma temperature through radiative cooling.

The detachment condition is reached when the divertor plasma temperature drops to a few eV or below and recombination becomes dominant. At this point, most of the heat load is distributed as radiation over a wider area.

First Wall Materials

Tungsten is the primary material used for SPARC’s first wall and divertor. Tungsten’s excellent properties include:

• High melting point (3422°C)
• Low sputtering rate
• High thermal conductivity
• Low tritium retention

However, tungsten impurities in the plasma cause strong radiation losses, making impurity transport control important. Radiation power loss from tungsten is:

$$P_\text{rad,W} = n_e n_W L_W(T_e)$$

Where $L_W(T_e)$ is the cooling rate function, which has maxima around 100 eV and several keV. Edge transport barrier and divertor optimization are needed to prevent tungsten from entering the core plasma.

Safety Systems and Tritium Handling

Tritium Management

SPARC plans to use D-T fuel, making safe handling of tritium (T) essential.

Physical Properties of Tritium

Property	Value
Half-life	12.33 years
Decay mode	Beta decay
Maximum beta energy	18.6 keV
Specific activity	358 TBq/g

Tritium is a beta emitter with low external exposure risk, but internal exposure through inhalation or dermal absorption is a concern.

Tritium Systems

SPARC’s tritium system consists of:

1. Fuel supply system: Tritium-deuterium mixing and injection
2. Exhaust processing system: Exhaust gas treatment from plasma
3. Water treatment system: Processing and recovery of tritiated water
4. Storage system: Safe storage in metal hydride beds
5. Accountancy system: Tracking of tritium inventory

Safety Design

• Gloveboxes: Sealed enclosure of tritium handling areas
• Double/triple containment: Multiple barriers to prevent leakage
• Negative pressure management: Prevention of contamination spread
• Monitoring: Real-time radiation surveillance

Radiation Shielding

D-T fusion generates 14.1 MeV fast neutrons. SPARC has strong shielding to protect workers and equipment from these neutrons.

Shielding materials and characteristics:

Material	Purpose	Thickness
Water (borated)	Neutron moderation and absorption	~1 m
Concrete	Gamma-ray shielding	~2 m
Lead	Gamma-ray shielding	Local
Boron	Neutron absorption	Additive

Regulatory Compliance

SPARC will be under the regulation of the U.S. Nuclear Regulatory Commission (NRC). In 2024, the NRC began consideration of a regulatory framework for fusion facilities.

Since fusion reactors are fundamentally different from fission reactors, directly applying existing nuclear regulations would be inappropriate:

• No risk of runaway reactions
• Does not produce long-lived radioactive waste
• No risk of nuclear weapons proliferation

CFS is actively engaging with the NRC and DOE, contributing to the development of a rational regulatory framework appropriate for fusion.

Construction Progress

Key Milestones

Time	Milestone	Details
March 2018	CFS founded	Spinoff from MIT
March 2018	Series A raised	$50 million (Eni, Breakthrough Energy)
May 2019	Series B raised	$500 million (Tiger Global, etc.)
September 2021	20 T magnet success	World record achieved
November 2021	Series B2 raised	$1.8 billion
February 2024	Site selected	Devens, Massachusetts
November 2024	Central solenoid demonstration	Prototype completed
April 2025	Cryostat base installed	Assembly begins
August 2025	Series B2 additional raised	$863 million
September 2025	DOE milestone achieved	$8 million grant
October 2025	Vacuum vessel delivered	48 tons, half completed
2026	First plasma	Planned
2027	Q > 1 demonstration	Planned

Current Status (2025)

Assembly of SPARC is underway in Devens, Massachusetts. The cryostat base, over 7 m in diameter and weighing 75 tons, has been installed, completing the foundation that will support the approximately 1,000-ton device. CFS has received an $8 million grant following a milestone achievement evaluation by the Department of Energy.

Manufacturing Facilities

CFS has established dedicated facilities for SPARC manufacturing:

Facility	Location	Scale	Function
Headquarters and assembly plant	Devens, MA	~47 acres	Final assembly
Magnet manufacturing	Devens, MA	20,000 m²	Coil winding and testing
Tape quality control	Devens, MA	-	100% inspection

Approximately 550 km of REBCO tape is used for each toroidal field coil, with about 10,000 km needed for all 18 coils.

ARC: Vision for Commercial Reactors

SPARC is an experimental reactor, and ARC is the commercial reactor to be built following its success. ARC stands for “Affordable, Robust, Compact,” aiming for economically competitive fusion power generation.

ARC Design Concept

ARC is not a scale-up of SPARC but a new design optimized for commercial operation:

Parameter	SPARC	ARC	Ratio
Plasma major radius	1.85 m	3.3 m	1.8×
Plasma minor radius	0.57 m	1.1 m	1.9×
On-axis field	12.2 T	9.2 T	0.75×
Fusion power	100 MW	525 MW	5.3×
Electric output	-	~200 MW	-
Q value	> 10	> 20	2×
Pulse length	10 seconds	Steady-state	-
Bootstrap fraction	10%	> 50%	5×

Distinctive design elements of ARC:

1. Liquid blanket: Uses FLiBe (LiF-BeF₂ molten salt) blanket for tritium breeding and neutron shielding
2. Removable vacuum vessel: Designed for the vacuum vessel to be removable for maintenance
3. Compact footprint: Fits on a site comparable to conventional power plants

Tritium Self-Sufficiency

Commercial fusion reactors must produce tritium fuel internally. The lithium-neutron reactions:

$$^6\text{Li} + n \rightarrow \, ^4\text{He} + \text{T} + 4.78 \text{ MeV}$$ $$^7\text{Li} + n \rightarrow \, ^4\text{He} + \text{T} + n' - 2.47 \text{ MeV}$$

The tritium breeding ratio (TBR) is the number of tritium atoms produced per fusion reaction, and TBR > 1 is the condition for self-sufficiency:

$$\text{TBR} = \int \Sigma_\text{Li} \phi \, dV > 1$$

ARC’s FLiBe blanket is designed to achieve TBR > 1.1 through a combination of ⁶Li enrichment and neutron multiplier (beryllium).

Commercial Contracts

CFS has already concluded major contracts for power sales from the ARC commercial reactor:

Customer	Time	Content	Significance
Google	July 2025	200 MW power purchase	First direct corporate contract
Eni	September 2025	~$1 billion supply	Strategic partner
Dominion Energy	2025	Site provision	Grid connection

Construction Plan

Parameter	Value
Power output	~400 MW (electric)
Households served	~150,000
Site	Chesterfield County, Virginia
Target operation start	Early 2030s

The site selected for ARC construction in Chesterfield County, Virginia, is located near a retired coal-fired power plant. It can utilize existing transmission infrastructure and has symbolic significance for energy transition.

Detailed Comparison with ITER

SPARC and ITER contribute to fusion science advancement through different approaches.

Differences in Design Philosophy

Aspect	SPARC	ITER
Basic strategy	High-field, compact	Large-scale, proven approach
Magnet technology	HTS (REBCO)	LTS (Nb₃Sn)
Development structure	Private company	International cooperation (7 parties)
Schedule	Rapid (8 years)	Long-term (30+ years)
Funding	Private investment (~$3 billion)	Government budget (~$25 billion)
Risk tolerance	High	Low
Decision-making	Rapid, centralized	Consensus-based

Physical Parameter Comparison

Parameter	SPARC	ITER	Ratio
Plasma major radius $R_0$	1.85 m	6.2 m	0.30
Plasma minor radius $a$	0.57 m	2.0 m	0.29
Plasma volume	~12 m³	~840 m³	0.014
On-axis field $B_0$	12.2 T	5.3 T	2.3
Plasma current $I_p$	8.7 MA	15 MA	0.58
Fusion power	100 MW	500 MW	0.2
Power density	8.3 MW/m³	0.6 MW/m³	14
Pulse length	10 seconds	400 seconds	0.025

SPARC is 1.4% of ITER’s plasma volume yet achieves 20% of the power. This reflects its 14 times higher fusion power density.

Confinement Parameters

To compare confinement performance of both devices, we use normalized quantities.

Beta normalized value $\beta_N$:

$$\beta_N = \frac{\beta_T}{I_p/(aB_T)}$$

Parameter	SPARC	ITER	Notes
$\beta_N$	1.0	1.8	SPARC is conservative
$H_{98}$	1.0	1.0	Standard H-mode
$q_{95}$	3.1	3.0	Nearly equivalent
$f_{GW}$	0.37	0.85	SPARC has large margin

SPARC plans to operate at a conservative $\beta_N = 1.0$, providing operational margin. The lower Greenwald fraction is because the high magnetic field provides margin against the density limit.

Technology Maturity

ITER is based on established LTS technology, and its technical risk has been considered relatively low. However, over 30 years of development time and budget overruns have highlighted the challenges of large international projects.

SPARC depends on the new technology of HTS, but the major technical risks were reduced by the 2021 magnet test success. Rapid decision-making as a private company and a culture that does not fear failure are accelerating development.

Complementary Roles

SPARC and ITER are not competitors but complementary:

• ITER: Demonstrates integrated technologies needed for commercial reactors, including long-duration D-T burning plasma maintenance, blanket technology, and remote handling
• SPARC: Demonstrates the effectiveness of the high-field compact approach and shows an alternative path to commercialization

The combination of results from both projects makes the realization of fusion power more certain.

Comparison with Other Private Fusion Companies

SPARC/CFS is a leader in private fusion development, but many other companies have entered the field.

Comparison of Major Companies

Company	Approach	Funding	Target Timing	Headquarters
CFS (SPARC)	Tokamak (HTS)	~$3 billion	2026 first plasma	USA
TAE Technologies	FRC	~$1.2 billion	2030s	USA
Helion Energy	FRC	~$600 million	2028 generation	USA
General Fusion	Magnetized target	~$300 million	2027 demonstration	Canada
Tokamak Energy	Spherical tokamak (HTS)	~$250 million	2030s	UK
Kyoto Fusioneering	Reactor engineering	~$130 million	-	Japan

CFS Advantages

1. Technical maturity: 20 T magnet demonstrated
2. Financial strength: Largest funding in the industry
3. Academic foundation: Close collaboration with MIT
4. Commercial contracts: Major contracts with Google, Eni
5. Clear roadmap: Two-stage SPARC → ARC

Industry-Wide Trends

Investment in private fusion has surged since the 2020s:

Year	Cumulative Investment	Major Events
2020	~$3 billion	Investment continues despite COVID-19
2021	~$5 billion	CFS raises $1.8 billion
2022	~$6 billion	Helion-Microsoft contract
2023	~$7 billion	UK government announces STEP
2024	~$8 billion	NRC begins regulatory framework consideration
2025	~$9 billion	CFS concludes commercial contracts

Economic Analysis

SPARC Development Costs

Item	Amount	Notes
R&D	~$500 million	Magnet technology, etc.
Facility construction	~$1 billion	Buildings, infrastructure
Device manufacturing	~$1.5 billion	Magnets, vacuum vessel, etc.
Operational preparation	~$500 million	Diagnostics, control systems
Total	~$3.5 billion	Nearly matches raised funds

This is approximately 1/7 of ITER’s budget (~$25 billion).

ARC Power Generation Cost Projections

CFS aims for competitive power costs from ARC:

Item	Projection	Comparison
Capital cost	$4-6 billion	Nuclear: $6-12 billion
LCOE	$50-100/MWh	Natural gas: $40-80/MWh
Capacity factor	80-90%	Nuclear: 90%
Operating life	40+ years	Nuclear: 60 years

LCOE (Levelized Cost of Electricity) Breakdown

$$\text{LCOE} = \frac{\text{Capital cost} + \text{Operating cost}}{\text{Total generation}}$$

Cost advantages of fusion power:

• Extremely low fuel cost (deuterium from seawater)
• Low disposal costs due to no long-lived radioactive waste
• No carbon emissions, unaffected by carbon taxes

Key People and Organizations

Founders and Management

Bob Mumgaard (CEO)

• PhD from MIT PSFC
• Expert in plasma physics and fusion engineering
• Co-founded CFS in 2018

Dennis Whyte (Co-founder)

• Former PSFC Director (2015-2022)
• Leader of Alcator C-Mod
• Advocate of high-field compact tokamaks

Brandon Sorbom (Co-founder)

• PhD from MIT
• Major contributor to ARC design
• Leads HTS magnet development

Major Investors

Investor	Cumulative Investment	Notes
Breakthrough Energy	Hundreds of millions	Founded by Bill Gates
Tiger Global	Hundreds of millions	Major tech investor
Eni	Hundreds of millions	Italian oil major
Emerson Collective	Undisclosed	Laurene Powell Jobs
Google	Undisclosed	Also power purchase agreement
Temasek	Undisclosed	Singapore sovereign fund

Academic Partners

• MIT PSFC: Basic research, talent supply
• Princeton Plasma Physics Laboratory: Simulations
• Oak Ridge National Laboratory: Materials research
• Multiple universities: Diagnostics development

Technical Challenges and Future Outlook

Several technical challenges remain for SPARC’s realization.

Mass Production of HTS Magnets

The 2021 magnet test success was an important milestone, but manufacturing 18 toroidal field coils while maintaining quality remains a challenge.

Key challenges:

• Quality consistency of REBCO tape
• Precision of coil winding
• Reliability of joints
• Optimization of quench protection system

CFS is addressing these challenges through automation of manufacturing processes and enhanced quality control.

Implementation of D-T Operation

SPARC plans to use deuterium-tritium (D-T) fuel, which requires radiation management and tritium handling facilities.

Tritium has a half-life of 12.3 years and is a radioactive isotope requiring special permits for handling. CFS is building a safe tritium management system under NRC and DOE regulations.

Burning Plasma Physics

In the Q > 2 regime, self-heating by alpha particles significantly affects plasma characteristics. Alpha particles are generated with 3.5 MeV of energy and transfer energy as they slow down in the plasma.

The slowing-down time of alpha particles is:

$$\tau_s \approx 0.6 \frac{T_e^{3/2}}{n_e}$$

The relationship between this time scale and confinement time determines alpha particle heating efficiency. Additionally, the alpha particle distribution function may drive MHD instabilities, and its control is an important topic in burning plasma physics.

SPARC will be the first private device to experimentally study these physics.

Future Developments

Knowledge gained from SPARC will be directly reflected in the ARC commercial reactor design. Additionally, the high-field compact tokamak concept can be extended to other applications:

1. Space propulsion: Compact fusion reactors are attractive for spacecraft propulsion systems
2. Distributed power generation: Decentralized energy supply through small fusion reactors
3. Hydrogen production: Hydrogen production using fusion heat
4. Industrial heat supply: Heat supply for high-temperature processes

CFS’s first goal is fusion power generation, but its technology holds potential as the foundation for future energy and industrial infrastructure.

Social Impact

SPARC’s success has social impact beyond technical achievement:

1. Restoration of confidence in fusion: Overturning the pessimism that “fusion is always 30 years away”
2. Investment activation: Increased investment in other fusion startups
3. Talent development: Influx of young researchers into the fusion field
4. International competition: Securing U.S. technological leadership
5. Energy security: Breaking dependence on fossil fuels
6. Climate change response: Realization of zero-carbon power source

Contribution to Energy Transition

If fusion power is realized, it can provide revolutionary solutions to energy and climate change problems. The advantages of fusion are:

1. Abundant fuel: Deuterium from seawater, lithium from Earth’s crust
2. High energy density: Millions of times that of fossil fuels
3. Zero greenhouse gas emissions: No CO₂ emissions during operation
4. Safety: No risk of runaway reactions or fission products
5. Waste: Does not produce long-lived radioactive waste
6. Site flexibility: No fuel transport needed, can be built anywhere

CFS’s mission is “to solve Earth’s energy problem with fusion,” and SPARC and ARC are concrete steps toward that goal.

Conclusion

SPARC is a project with the potential to significantly shorten the path to fusion power through innovation in high-temperature superconducting magnet technology. MIT’s 50 years of research accumulation combined with CFS’s agility as a private company is driving development at unprecedented speed.

The 20 T magnet test success in 2021 demonstrated the technical feasibility of the high-field compact tokamak. When the ongoing assembly is completed and first plasma is achieved in 2026, fusion energy development will enter a new phase.

SPARC’s success symbolizes a paradigm shift in fusion development beyond just technical achievement. The complementary progress of government-led large international projects and rapid development by private companies is turning hope for solving humanity’s energy problem into reality.

The high-field compact tokamak approach, leveraging $B^4$ scaling to overturn conventional wisdom, is transforming fusion energy from a distant dream into “an achievable future.” SPARC stands at the forefront of this effort.

Related Topics

• ITER - International Thermonuclear Experimental Reactor
• JT-60SA - Japan-EU Joint Superconducting Tokamak
• Tokamak - Magnetic Confinement Approach
• Lawson Criterion - Physical Conditions for Fusion Ignition
• Plasma Heating - Principles of Plasma Heating Methods
• Private Fusion Ventures - Global Private Companies

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