Historical research guide · 1052 · 1998
Toroidal Alfvén Eigenmodes in TFTR Deuterium-Tritium Plasmas
R. Nazikian · G.Y. Fu · Z. Chang · S.H. Batha · H. Berk · R.V. Budny · Y. Chen · C.Z. Cheng · D.S. Darrow · N.N. Gorelenkov · F.M. Levinton · S.S. Medley · M.P. Petrov · M.H. Redi · E. Ruskov · D.A. Spong · R.B. White · S.J. Zweben
In one minute
The Tokamak Fusion Test Reactor at Princeton burned deuterium and tritium, and this laboratory report records what the fusion-born alpha particles did to the plasma that made them. An alpha comes out at about 3.5 million electronvolts, which puts its speed close to the Alfvén speed of the plasma, so it can ride a wave called the toroidal Alfvén eigenmode and pump energy into it. Nazikian and seventeen colleagues report that this happens, at six different mode numbers, but only inside a narrow window — the hundred to three hundred milliseconds after the heating beams are switched off, when the beam ions that damp the wave have slowed down and the alphas have not. Then they did something new: they looked inside. A microwave reflectometer measured the wave’s shape in the plasma core and found it about a thousand times stronger there than the magnetic coils at the wall implied, sitting exactly where the magnetic shear is weakest. Two features of that shape disagree with the theory.
Connection
Chapter 12 is about getting energy out of nuclei, and this is the machine that did it with deuterium and tritium at reactor conditions — with the fusion products themselves talking back to the field that confines them. Chapter 9 takes the structural lesson: a self-organised wave inside a plasma can be a thousand times stronger in the core than anything a sensor at the boundary reads, so a quiet instrument at the wall is not evidence of a quiet interior.
From the source
Toroidal Alfvén eigenmodes driven purely by fusion alpha particles, with toroidal mode numbers from one to six, were observed in deuterium-tritium plasmas on TFTR. They appear only in a transient window one hundred to three hundred milliseconds after the deuterium-tritium neutral beams are turned off, because the beam ions that damp the mode slow down in eighty to one hundred milliseconds while the 3.5 MeV alphas take three to five hundred.Abstract, and Section II B with Figure 1
The instability needs a raised central safety factor and weak central magnetic shear, not simply a large alpha population. Standard high-performance supershots with a central safety factor below one showed no mode activity even at the record fusion power of 10.7 megawatts and a central alpha beta of about 0.3 percent, while plasmas prepared with a central safety factor of about 1.5 to 1.9 produced modes at alpha beta values as low as 0.01 percent.Section II B and Section III, with Figures 2, 3 and 6
The modes are identified as Alfvén eigenmodes by frequency, mode number and timing together: they sit inside the toroidicity-induced gap in the shear Alfvén spectrum at about 150 to 250 kilohertz, and each burst chirps upward in frequency as the plasma density falls after the beams end and the Alfvén velocity rises.Section II A and II B, with Figures 3 and 4
The first internal measurement of the radial structure of an alpha-driven mode was made with the X-mode core reflectometer. It confirms that the modes are localised to the plasma core where the magnetic shear is weak, and that the peak internal amplitude of about one part in ten thousand in density corresponds to a magnetic perturbation of about one part in one hundred thousand — while the coils at the plasma edge read only about one part in one hundred million.Section IV, Figures 8 to 13, and the Summary
Two features of the measured structure depart from theory. The lowest mode numbers come in about thirty percent below the predicted Alfvén eigenmode frequency, which plasma rotation and diamagnetic effects are too small to explain; and the n equals 2 mode is strongly anti-ballooning, largest on the high-field side of the magnetic axis, where current theory predicts only outward-ballooning modes should be unstable.Section III with Figure 5, and Section IV with Figures 9 and 11
The n equals 4 mode changes shape during a single burst — roughly symmetric across the magnetic axis early on, then transforming into an outward-ballooning structure that couples more strongly to the plasma edge before dissipating — even though its peak internal fluctuation level barely changes while the magnetic signal at the wall nearly doubles.Section IV, Figures 12 and 13
The way in
https://doi.org/10.2172/4519How to cite it
R. Nazikian, G.Y. Fu, Z. Chang, S.H. Batha, H. Berk, R.V. Budny, Y. Chen, C.Z. Cheng, D.S. Darrow, N.N. Gorelenkov, F.M. Levinton, S.S. Medley, M.P. Petrov, M.H. Redi, E. Ruskov, D.A. Spong, R.B. White, S.J. Zweben (1998) Toroidal Alfvén Eigenmodes in TFTR Deuterium-Tritium Plasmas doi:10.2172/4519