Startup, Ramp-Down, and Steady-State Operation

Turning on the power in a large tokamak is not an easy task. The vacuum system, the cryogenic system, discharge-cleaning of the walls, the magnetic field system, the tokamak current drive, and the various plasma heating systems, and various auxiliary systems have to be started up in sequence. Operators have learned by experience how to do this in large tokamaks. The plasma has to be maintained stably while it is being heated and while the current is being increased in synchro­nism with the toroidal magnetic field. Each power supply has to be ramped up at a certain time at a certain rate. Turning the discharge off also requires careful ramp — down of each system. Only after a good routine has been found can automatic controls take over.

All present tokamaks run in pulses, not continuously. Even if the pulses last for minutes or an hour, they will not uncover problems that will arise with truly steady — state operation. In the 1980s, a machine called the ELMO Bumpy Torus was run at the Oak Ridge National Laboratory. Though the magnetic configuration never caught on, the machine was run in steady-state and revealed problems that are not seen in pulsed machines. The Tore Supra tokamak in Cadarache, France, near the ITER site, has been gathering information on long-pulse operation for 20 years [20]. It is a large tokamak with high magnetic field, large current, and powerful heating. The first wall is water-cooled boronized carbon. In a deuterium plasma, the retention of deuterium by the carbon was found to be significant. This is one reason for rejecting carbon as a wall material. Damage to the ICRH antennas was noted. Electrical faults in the magnet system were found to limit the length of discharges. It was found that turning the lower-hybrid power on slowly greatly alleviated this problem. Water leaks were found to occur 1.7 times per year. The frequency of disruptions was also recorded. These were found to be caused mainly by the flaking of carbon off the walls after many days of operation. Pulses lasting 1 or 2 seconds were possible with transformer-driven currents, but with the addition of lower — hybrid current drive, 6-min pulses with 3 MW of lower-hybrid heating (LHH) were achieved in 2007. At the 2-MW level, 150 consecutive 2-min discharges could be routinely produced [21]. These are the types of problems that will be encountered when ITER is operated in continuous mode.

Maintaining the Current Profile

Advanced tokamaks utilize reversed shear and internal transport barriers for enhanced plasma confinement. These require precise shaping of the safety factor q (see Chap. 8), which determines how the twist of the magnetic field lines changes across the radius. The shape of the q(r) curve controls the stability and loss rate of the plasma. Since the twist is determined by the poloidal field created by the plasma current, this current has to be shaped in a particular way. Some of the current is naturally produced by the bootstrap effect (Chap. 9); the rest has to be driven by lower-hybrid and electron cyclotron current drive. The blue curve in Fig. 9.29 shows an example of a q(r) curve which stays above q=2 and gives reverse shear. The red curve shows the auxiliary current needed to produce this q(r). Only precise control of the localized heating can produce this current profile. As the plasma starts up, the currents will be changing, and the power supplies will have to be programmed to keep the current in a stable shape.