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Pulsed Plant Operation

If the plasma current is partially or entirely driven by electromagnetic induction, it is necessary to operate the plant in a pulsed manner as the current swing in the central solenoid cannot be continued indefinitely. PROCESS can perform a number of calculations relevant to a pulsed power plant, as detailed below.

Switch lpulse determines whether the power plant is assumed to be based on steady-state (lpulse = 0) or pulsed (lpulse = 1) operation. The current ramp calculations apply in both cases, as even a steady-state reactor has to be started up.

Start-up power requirements

The auxiliary power reaching the plasma can be constrained to be more than the minimum allowable value auxmin by turning on constraint equation no. 40 with iteration variable no. 64 (fauxmn). The value of auxmin is set in the input file.

The auxiliary power required during the start-up and ramp-up phase is not calculated. (The code contains a routine based on a POPCON analysis of access via the Cordey Pass - the path in plasma density-temperature space which minimises the power requirement - but this is very CPU-intensive, so it is not called at present.)

Plasma current ramp-up time

When the plasma current is ramped up too quickly an unstable current distribution can develop, leading to kink and tearing instabilities. PROCESS allows an upper limit to be set for the rate of change of plasma current. In the default case, the ramp-up time is given by setting the rate of change equal to the maximum proposed in 1, or it can be set by the user. The physics of this constraint is likely to depend on whether the ramp-up is purely inductive or includes current drive, but this is not taken ito account.

In the steady-state scenario (lpulse = 0), the plasma current ramp-up time t_current_ramp_up is determined as follows.

  • If tohsin = 0, the rate of change of plasma current is 0.5 MA/s. The PF coil ramp time t_precharge and shutdown time t_ramp_down are (arbitrarily) set equal to t_current_ramp_up.
  • If tohsin \neq 0, the plasma current ramp-up time t_current_ramp_up = tohsin, and the PF coil ramp and shutdown times are input parameters.

In the pulsed scenario, (lpulse = 1), the plasma current ramp-up time t_current_ramp_up is an input, and it can be set as an iteration variable (65). The ramp-up and shutdown time in the pulsed case are set equal to t_current_ramp_up. To ensure that the plasma current ramp rate during start-up is prevented from being too high, as governed by the requirement to maintain plasma stability by ensuring that the induced current has time to diffuse into the body of the plasma, constraint equation no. 41 should be turned on with iteration variable no. 66 ft_current_ramp_up and input t_current_ramp_up_min, the minimum plasma current ramp-up time.

Burn time

The length of the burn time is calculated from the surplus volt-seconds available from the Central Solenoid and the other PF coils during the plasma burn phase, after the flux required during the plasma start-up is taken into account. A minimum burn time (t_burn_min) can be enforced via constraint equation no. 13 and iteration variable no 21 (ft_burn).

Currents over time

Over the course of a pulse, the timings are detailed as:

  • Precharge (t_precharge) - the CS current ramps from zero to maximum value. The other PF coils also ramp from zero to their required values.
  • Current ramp-up (t_current_ramp_up) - The plasma current ramps up to approx full value. Auxiliary heating is possibly on.
  • Fusion ramp (t_fusion_ramp) - The plasma temperature and density rise to the full values. The CS and other PF coil currents all change steadily. Auxiliary heating is on.
  • Burn time (t_burn) - Flat-top duration. The plasma is approximately steady. Fusion power and electricity are produced. The CS and other PF coil currents all change steadily in a pulsed reactor, but are constant for a "steady-state" reactor. Auxiliary heating is on.
  • Ramp-down (t_ramp_down) - The plasma current, density and temperature all ramp down to zero simultaneously. As a starter for ten we could assume that the CS and PF coil currents also ramp to zero at the same time. Auxiliary heating is possibly on.
  • Between pulse (t_between_pulse) - CS and PF coil currents are zero - a few minutes may be required to permit vacuum pumping. May be much longer for an experimental device such as DEMO.
  • Pulse repitition (t_pulse_repitition) - Sum of all the above times.

A plot showing schematically these timings over a pulse can be found in Figure 2.

current-vs-time-plot

Figure 1: Plot showing schematically the current waveforms for the plasma, a typical PF coil, and the central solenoid. Note that the currents in some of the PF coils may be the opposite sign to that shown, and the central solenoid current may remain positive during the Ip ramp-up period, although it will pass through zero during the burn phase.

Thermal storage and back-up generation

During every cycle there is a period when no fusion power is produced. If the net electricity output from the plant must be maintained this is achieved using thermal storage or back-up generation. Three models are available, determined by the value of switch istore. If istore = 1 (the default), option 1 of Ref1 is assumed, which utilises the thermal storage inherent in the machine's steam cycle equipment. This should be used is the machine down time is less than 100 seconds. If istore = 2 option 2 of Ref2 is assumed, which uses the same method as before, but augments it with an additional boiler. This may be used for machine down times of up to 300 seconds. Finally, if istore = 3, a large stainless steel block acts as the thermal storage medium. These options are obsolete.

  1. Pulsed Fusion Reactor Study, AEA Fusion Report AEA FUS 205 (1992) ↩↩

  2. ITER poloidal field system, IAEA, Vienna, 1990, 203–30, 56–60. ↩