Alcator C-Mod

Alcator C-Mod

Type	Tokamak
Operation date	1991–
Size (Major radius/Minor Radius	0.68 m/0.22 m
Plasma volume	1 m3
Magnetic field	3–8 T (toroidal)
Plasma current	0.4–2.0 MA (typical)
Location	Massachusetts Institute of Technology, Cambridge, USA

Alcator C-Mod is a tokamak, a magnetically confined nuclear fusion device, at the MIT Plasma Science and Fusion Center. It is the tokamak with the highest magnetic field and highest plasma pressure in the world. It is one of the major fusion research facilities in the United States, together with DIII-D at General Atomics and NSTX at Princeton Plasma Physics Laboratory.

Operating since 1991, it is the third of the Alcator (Alto Campo Torus, High Field Torus) tokamak series, following Alcator A (1975-1982) and Alcator C (1982-1988). This reactor represents the largest fusion reactor operated by any university and is an integral part of the larger Plasma Science and Fusion Center (PSFC).

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.

History

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).

Characteristics

Heating and current drive

Alcator C-Mod uses ion cyclotron range frequencies (ICRF) heating as its primary auxiliary heating source. The source frequency is 80 MHz and the standard minority heating scenarios are: D(H) for 4.4-6.9 T and D(3He) for high field operation (7.3- 8.0 T) [1]. Note that a minority species (Hydrogen or He3) is indicated, and ICRH scenarios use a two component plasma.

Absorption efficiency varies with the minority concentration. It is also possible to transition between minority and mode conversion (MC) heating by varying the minority species concentration. The relative H fraction $\eta_H = n_H/(n_H+n_D)$ concentration can be scanned from roughly 2-30% via gas puffing and measured using passive charge exchange [1]. The relative He3 fraction $\eta_{He3} = n_{He3}/n_e$ concentration can also be scanned from roughly 2-30% via gas puffing. Phase Contrast Imaging (PCI) can be used to measure the mode converted ion Bernstein waves directly in the plasma.

Minority Heating

This is the most common scenario used at C-Mod. The ICRF heating system operates at 80 MHz in D(H), D(3He) or 3He(H) plasmas. This frequency corresponds to on-axis minority fundamental cyclotron resonance of protons at 5.3 T and absorbing fast waves by hydrogen minority species in a deuterium plasma can be very efficient (typical single pass absorption in C-Mod is 80-90% for minority concentrations of 5-10%) [3]. Minority heating at 80 MHz and 7.9 T in a deuterium majority plasma is achieved using the He3 minority resonance (on-axis), but single pass absorption with He3 minority ions in deuterium tends to be much lower than for protons [3] (e.g. the minority heating scenario at 5.3-5.4 T).

Mode Conversion Heating

Mode conversion of a fast magnetosonic wave to an ion Bernstein wave (IBW) in the ion cyclotron range of frequencies (ICRF) can be used to heat electrons. Electron heating via mode converted IBW is done at C-Mod using the ICRF in two-ion component D(3He) plasmas [1]

Lower Hybrid Current Drive

Lower hybrid current drive (LHCD) is used to supplement the current driven by the Ohmic transformer. The LHCD system is capable of delivering 1.0+ MW of microwave power (planned upgrade to 2+ MW with addition of 2nd antenna in 2013) to the plasma at 4.6 GHz. Non-inductive operation for up to 0.5 s pulses at 500 kA has been achieved. Lower hybrid waves are launched preferentially in the direction opposite the plasma current (i.e. in the direction electrons travel) and deposit energy on electrons moving at approximately 3 times the thermal velocity via Landau damping.

The future

Alcator C-Mod has been slated to be shut down in 2013 by the President's Fiscal Year 2013 budget request. Graduate students at the project have set up a website at FusionFuture.org where concerned parties can email Congress asking them to continue funding the project.

References

[1] Wukitch et al. EPS 1998

[2] Bonoli et al. Phys. Plasmas, Vol. 7, No. 5, May 2000

[3] Porkolab et al. p. 79, cP485, Radio Frequency Power in Plasmas, edited by S. Bemabei and F. Paoletti (1999)

External links

• Alcator C-Mod homepage
• Information about FY2013 budget and Alcator C-Mod shutdown