ToriX: the little tokamak that could

In fusion, bigger is always better: the larger the plasma volume, the more efficient its insulation and the longer the system's energy confinement time. Confinement is one of the key parameters for achieving fusion reactions. Early tokamaks, like the 1958 Soviet T1, provided plasmas in the range of 0.4 cubic metres. In 1982, plasma volume in the American TFTR, was 30 m³. It is about 100 m³ in JET and in the Japanese JT-60, and will be close to 900 m³ in ITER. Future industrial reactors will operate with plasmas three to four times larger.

While large tokamaks dominate fusion research, there is still a lot to be learned from small, even tiny machines. ToriX, with a plasma volume of 0.03 m³ (30 litres), is one of them. It was built some 15 years ago at the French École Polytechnique, one of the most prestigious scientific institutions in the country, and has been used ever since to train students in various diagnostic methods.

"We use ToriX as support, both for thesis work and for students' science projects," says Dominique Grésillon, a physicist and professor who recently led a small group of Polytechnique second-year students—or "Xs," as they are nicknamed—on a visit of the Tore Supra installations. "Some of the diagnosis methods which have been developed here are now implemented on Tore Supra. There's a place for our "baby tokamak" in support of the much bigger installations."

Taking in the size, volume, and complexity of the Tore Supra installation was a defining experience for the students. "But what impressed us most," says Alexandre Mougel, one of the visiting Polytechniciens, "is how well the people we met understood the issues we're facing with our little ToriX. We felt much honoured as they took our students' project very seriously and treated us with great consideration."

Not all the students who take part in this project will become fusion physicists. But over the past 15 years, ToriX has contributed to the training of several specialists now working at Tore Supra, teaching plasma physics, or simply spreading the word on fusion energy.