# Alcator C-Mod

### Alcator C-Mod

 Type Tokamak 1991–present 0.68 m/0.22 m 1 m3 3–8 T (toroidal) 0.4–2.0 MA (typical) Massachusetts Institute of Technology, Cambridge, USA

Alcator C-Mod is a tokamak, a magnetically confined nuclear fusion device, at the Massachusetts Institute of Technology (MIT) Plasma Science and Fusion Center (PSFC). It is the tokamak with the highest magnetic field and highest plasma pressure in the world, which is especially notable as it does not use superconducting magnets. It is one of the major fusion research facilities in the United States, together with DIII-D at General Atomics and NSTX-U at Princeton Plasma Physics Laboratory.

Operating since 1991, it is the third of the Alcator (Alto Campo Toro, High Field Torus) tokamak series, following Alcator A (1973–1979) and Alcator C (1978–1987). This represents the largest fusion reactor operated by any university and is an integral part of the larger Plasma Science and Fusion Center.

The Alcator C-Mod tokamak experiment at the MIT Plasma Science and Fusion Center. Overview showing the device itself (under concrete shielding) and diagnostics in surrounding bay.

## Contents

• History 1
• Alcator A 1.1
• Alcator B and C 1.2
• Unfunded ideas and the C-Mod proposal 1.3
• Characteristics 2
• Heating and current drive 2.1
• Minority heating 2.1.1
• Mode conversion heating 2.1.2
• Lower hybrid current drive 2.1.3
• Future 3
• References 4
• Sources 4.1
• Footnotes 4.2

## History

### Alcator A

In the late 1960s, magnetic-confinement fusion research at MIT was carried out on small-scale "table-top" experiments at the Research Laboratory for Electronics and the Francis Bitter Magnet Laboratory. At this time, the tokamak was being developed in the Soviet Union (though this was unknown in the United States), and the stellarator was being developed at the Princeton Plasma Physics Laboratory (PPPL).

Scientist Bruno Coppi was working at the Institute for Advanced Study at Princeton University and was interested in (among other things) the basic plasma physics problem of the plasma resistivity at high values of the streaming parameter, as well as the behavior of magnetically confined plasmas at very high field strengths (≥ 10 T). In 1968, Coppi attended the third IAEA International Conference on Plasma Physics and Controlled Nuclear Fusion Research at Novosibirsk. At this conference, Soviet scientists announced that they had achieved electron temperatures of over 1000 eV in a tokamak device (T-3).

This same year, Coppi was named a full professor in the MIT Department of Physics. He immediately collaborated with engineers at the Francis Bitter Magnet Laboratory, led by Bruce Montgomery, to design a compact (0.54 m major radius), high-field (10 T on axis) tokamak which he titled Alcator. The name is an acronym of the Italian Alto Campo Toro, which means "high-field torus". With the later construction of Alcator C and then Alcator C-Mod, the original Alcator was retroactively renamed to Alcator A. Alcator was approved by the Atomic Energy Commission (AEC) in 1970 and was first operated in 1972. Performance problems (poor-quality vacuum and arcing in toroidal field magnets) led to the rebuilding of the machine in 1973–1974 with a new vacuum vessel, with very successful scientific results beginning in 1974. Alcator A was powered by the Bitter Laboratory's 32 MW DC motor-generators and was the first tokamak in the world to use an air-core transformer for Ohmic current drive and heating.

### Alcator B and C

The success of Alcator A led to the conceptual design, beginning in 1975, of a larger machine called Alcator B. However, the motor-generators used for Alcator A would not be powerful enough to drive the larger Alcator B machine, necessitating the purchase and installation of new power supplies, a cost which the Energy Research and Development Administration (ERDA) was unwilling to fund. The ERDA was however enthusiastic about building another Alcator, and a solution was found: a 225 MVA alternator was donated to MIT by Con Ed from a plant on the East River in New York City. The conceptual design was changed to accommodate the different power supply, and the project was renamed to Alcator C.

Alcator C was officially authorized in 1976. This same year, the Plasma Fusion Center (today, called the Plasma Science and Fusion Center) was spun off from the Francis Bitter Magnet Laboratory. Construction of Alcator C proceeded rapidly and initial low-power shakedown tests were conducted at the end of 1977. The alternator arrived from Con Ed in early 1978 (transportation being somewhat complicated by the blizzard of 1978) and was incorporated into the system in the summer of 1978. Scientific operations began in September of that year.

Alcator C was a larger machine (R0 = 0.64 m) and operated at a higher field (B0 ≤ 13 T) than Alcator A. With the addition of 4 MW of lower hybrid heating in 1982, electron temperatures over 3.0 keV were reached. While Alcator C did not originally have the energy confinement time expected, due to the onset of ion temperature gradient turbulence at high values of \eta = \text{d} \ln T / \text{d} \ln n, pellet fueling was used to produce peaked density profiles and values of the nτ product of over 0.8 × 1020 s·m−3 were achieved in 1983.

### Unfunded ideas and the C-Mod proposal

There were several ideas for new devices and upgrades at the PSFC that were never funded. From 1978 to 1980, a design activity was carried out for Alcator D, a larger version of Alcator C that would allow for more heating power, and possibly even deuterium–tritium (D–T) operation. This design was never formally proposed to the Department of Energy (DOE), but continued to evolve under the direction of Bruno Coppi, eventually becoming the Italian–Russian IGNITOR device currently planned for construction at TRINITY near Troitsk, Russia.

In 1982, an even more ambitious device called Alcator DCT was conceived. This machine would have superconducting coils producing 7 T on axis. 4 MW of lower hybrid current drive would drive a steady-state plasma with 1.4 MA plasma current. As this design was similar to the French Tore Supra, a joint French–American workshop was held in Cadarache in 1983 to compare the two designs and exchange ideas. Alcator DCT was formally proposed to the DOE in late 1983, but was not funded.

## References

### Sources

• "An Alcator Chronicle, or: What Happened to Alcator B?" R. Parker, presentation at IAP 2011. Available online at MIT PSFC library
• Bonoli et al. Phys. Plasmas, Vol. 7, No. 5, May 2000

### Footnotes

1. ^ a b c Wukitch et al. EPS 1998
2. ^ a b Porkolab et al. p. 79, cP485, Radio Frequency Power in Plasmas, edited by S. Bemabei and F. Paoletti (1999)
3. ^ Fusion Budget 2015
4. ^ Information about FY2013 budget and Alcator C-Mod shutdown