In a tokamak, the plasma current is important in two ways: It confines and shapes the plasma by generating the poloidal magnetic field and contributes to the increase of the plasma temperature by ohmic heating. Electromagnetic coils induce the current in the plasma. They operate with an increasing current pulse and hence cannot yield a continuous plasma current as needed for the steady state operation of a fusion reactor. Noninductive methods such as launching radio frequency waves into the plasma serve to generate or drive a steady current. One of them, Lower Hybrid Current Drive (LHCD), use waves, the frequency of which is located between that of ion and electron resonance frequencies. The LHCD electromagnetic wave is generated by klystrons – tubes, which produce high power waves with frequencies in the GHz range. Waveguides transmit it to the LHCD antenna (also called launcher), which launches it into the plasma.

TORE SUPRA CIMES project close to commissioning state

At Tore Supra, one of the aims of the CIMES project is to test LHCD solutions that are recommended for the second phase of ITER. CIMES is currently the largest upgrade of an LHCD system. During its design and installation, CEA scientists and engineers acquired unique knowledge and experience in producing, testing, and assembling the components of such a system.

To fulfil ITER requirements, CIMES will inject high power wave into the plasma during long pulses. This causes heat loads that are too high for the metal bulk of the launcher to suffice as a heat sink. CIMES employs a fully watercooled Passive-Active Multi-junction (PAM) LHCD launcher for the first time. Eight new steady-state high power klystrons, each producing a continuous electromagnetic wave with 3.7 GHz frequency and 700 kW power, are connected to the PAM launcher. Another existing launcher is also equipped with

a set of eight new klystrons. When fully
commissioned, the new LHCD system is
capable of injecting up to 7-8 MW into
the plasma.

The klystrons

The first series of nine klystrons (eight plus one spare) has been delivered to Cadarache. CEA scientists have developed a unique test bed which enables  the klystrons to be tested under the conditions expected at the machine – i.e. 10 successive 720kW pulses of 1000 seconds duration every 3000 seconds. Eight klystrons have already successfully passed the final acceptance tests. Manufacture of the second series is underway and the last klystron is expected to be on site in March 2010.

The first half transmitter refurbished with eight “first generation” klystrons will feed the new PAM launcher during its 2009 commissioning campaign. The second half transmitter, which will feed the already existing launcher, is currently being equipped with the first eight new tubes. The full transmitter, feeding both launchers with 16 new klystrons, will be ready for the second phase of 2010.

PAM launcher

The PAM Launcher being prepared for installation on Tore Supra. One can see alternating copper and iron plates with waveguide openings in between.

Given the required precision and the complexity of the interleaved RF waveguides and water channels, welding the PAM launcher plates together turned out to be an extremely difficult task. CEA solved it by proposing a new, innovative procedure to the manufacturer. Welding the upper and lower half of the launcher together, each weighing four tons, with a precision of one tenth of a millimetre posed the last industrial challenge. In April, the task was successfully completed by means of electron beam welding.

Recently, the launcher received its diagnostics (Langmuir and HF probes, thermocouples) and underwent the final vacuum validation test. It is now being prepared for implementation on Tore Supra. Mounting the LHCD launchers on Tore Supra is the last delicate phase of the machine transformation. It has been realised with a very good positioning accuracy for both ports hosting the existing fully active launcher (July) and the new PAM launcher (September).

CIMES is now in its completion phase on Tore Supra and the first tests on plasma are predicted to take place in autumn of 2009. All teams interested in participating in the commissioning of the new LHCD system of Tore Supra are welcome to join in the experimental campaign starting mid October 2009.

Alain Becoulet, CEA, and chairman of EFDA Heating and Current Drive Topical Group