Posted on: 3rd October 2005
In a magnetic confinement fusion reactor the plasma self-heating is provided by “alpha particles” – charged fusion products. JET unambiguously observed alpha particle heating in the deuterium-tritium experiments of September 1997.
“The Joint European Torus (JET) tokamak was designed with sufficient plasma current that alpha particles, at their birth energy of 3.5 MeV, have orbital excursions away from their mean magnetic flux surfaces which are at most 10% of the plasma minor radius.”
“The TFTR team was the first to observe alpha heating in an MCF plasma. The alpha power was 3% of the total heating power absorbed by the plasma, so the electron heating due to alphas was only twice the error arising from pulse to pulse variation. With a fusion power gain 3-4 times that of TFTR, JET was in a better position to observe alpha heating.”
“Alpha particle heating has been unambiguously observed in JET DT plasmas. A scan of DT mixture was used successfully to separate the effects of alpha heating and potential isotopic dependence of energy confinement. A change in central electron temperature of 1.3 +/- 0.23 keV is ascribed to 1.3 MW of alpha heating. (…) With a plasma energy confinement time of 1.2s, the alpha heating produced an increase of plasma energy content of more than 1 in 9 MJ. Alpha heating was observed, in this study, to be as effective as hydrogen minority ICRH. This is a strong indication that there are no unpleasant surprises with respect to alpha heating and that
there are no anomalous effects on trapping or slowing down. Furthermore, it is higly encouraging that the peaked alpha heating profile shows up in the heating rate and the energy confinement time.”
From “Observation of Alpha Heating in JET DT Plasmas” by P.R. Thomas et al, published in Physical Review Letters Vol 80, No 25 (1998) page 5548.
So far, only two tokamaks have been capable of handling Tritium, and thus experimenting with Deuterium-Tritium (D-T) fusion – by far the most efficient nuclear fusion process: TFTR in the US (now decommissioned) and JET in the EU. In 1991, JET was the first tokamak ever to run plasma discharges with Tritium (on November 9th, discharge #26147), when D-T fusion was confirmed by observing 14.1 MeV neutrons. In 1994, TFTR ran with the optimal 50% D, 50% T fuel mixture and was the first to observe plasma heating from fusion alpha particles. In 1997, JET set the current world record of released fusion power – on October 31st, discharge #42976, 16.1 million Watts (by comparison, a family house central heating needs a few thousands Watts of power) – and energy, on November 5th, discharge #42982, 21.7 million Joules (enough to hoist one hundred tons by twenty-two metres).
Below we recall some of the D-T plasma discharges from the December 1997 issue of the “JET News” newsletter . Notice that the fusion gain factor Q in the above quote is the ratio of fusion power produced to the net external power for plasma heating.
“The first successful high power tritium beam injection into the plasma took place on the evening of 20 September. Following some further high-voltage conditioning with tritium the record-breaking JET DT discharge (#42676) was obtained on the evening of 22 September producing 12.9 MW of fusion power. (…) Also a world record fusion energy (21 MJ) has been produced in a 3 second duration pulse. (…) The afternoon following the press conference (31st October 1997) brought our best high power results so far. Shot number 42976 reached a fusion power of 16.1 MW and Q rose to 0.65. An accessible overview of technology and physics involved in the DT experiments can be found in J. Wesson’s book “The Science of JET“, chapter 14 (starting page 153). For detailed scientific information, we also recommend the following articles on the JET record fusion performances: “JET Deuterium-Tritium Results and their Implications“, “Deuterium-Tritium Operation in Magnetic Confinement Experiments: Results and Underlying Physics” and “Overview of ITER Physics Deuterium-Tritium Experiments in JET“.