Pilot Plant: Volt-seconds Needed for Flexible Operation.

1. Pilot Plant: Volt-seconds Needed for Flexible Operation. John Sheffield, ISSE – University of Tennessee – Knoxville October 25, 2010.

2. Should we allow for Pulsed Operation? • How much should one rely on non-inductive start-up? • Non-inductive start-up in ARIES-AT is estimated to take 1.5 hours. • On the other hand, ITER takes about 3-5 minutes to raise the current and fusion power. • The likelihood is that a Pilot Plant would have an intermediate start-up time e.g. 10 to 20 minutes, and need to provide some of the initial volt-seconds (~ 1.5 LI) inductively. • Is fully non-inductive operation possible with acceptable current drive power? • Rob Goldston has raised the point about German studies. • Issue of operation of a Pilot Plant at lower bN where the bootstrap current and non-inductive current drive do not provide all the current. • This suggests that there need to be volt-seconds available for a partially inductively-driven flat-top—of a few hours?

3. Some Key Points • The Pilot Plant should be able to meet its mission under a range of operating conditions –bN, n, T, Zeff. • Current drive will be needed to ensure long pulse lengths (SS), and also for profile control. • In pulsed operation, shut down/start-up times must be long enough to minimize stresses in the blanket. • Pulse length must be long enough to meet the limit on number of pulses to meet the mission.

4. What pulse length and down-time? • What pulse lengths should be available when bootstrap current and non-inductive current drive cannot provide all the current, e.g. at intermediate bN levels? • A study at Argonne, Ehst, D.A., Jardin, S., and Kessel, C. “Starlite,” ANL/FPP/TM-284, 1995 looked at the consequences of repetitively pulsed operation for a tokamak reactor. They concluded that pulsed operation would be acceptable if there were a massive heat storage system available to maintain acceptable temperature gradients in the blanket during the down time (I recollect that their study assumed that it would be desirable to restrict gradients to ~ 50OC across the blanket. • How many cycles the plant should experience in its lifetime. • For example,15- 30 minutes down might have 5-10 minutes to shut down and reload the transformer and 10-20 minutes to start-up. • Incidentally, if the system has a double null divertor it might be simpler to reverse the current on successive shots. • Pulse length > 2 hours might be a good idea ~ 10 pulses/day, ~ 1000/year.

5. Reference Pilot Plant-1 • I have used a combination of information from Tom Brown and Chuck Kessel. • R = 4 m, a = 1 m, k = 2, q = 3.6. • B = 5.4 T, I = 8.2 MA. • fG = 0.8, bN = 3, PF = 510 MW. • Jon Menard’s recent paper has similar values.

6. V.s required Required V.s is freq = LpI + 0tr RpIdt + Rp I tF • tF = the flat-top time, Lp and Rp are the plasma inductance and resistance respectively. • I is in Amps. 0tr RpIdt ≈ 0.5 LpI (V.s). • Lp ≈m0R[ln(ac/a) + 0.25} (H), where (ac/a) is the ratio of the average poloidal coil radius divided by the average plasma radius. • Rp ≈ {2.6 x 10-4Rgr Zeff ln(Le)}/a2kTe01.5 (Ohms).

8. Questions for Pilot Plant-1 • What is acceptable maximum for solenoid current density and field? • At fG = 0.8, the peak electron temperature is ~ 16 keV, but 31 keV in ARIES-AT. • Lower fG would lead to more inductive current drive at fixed beta—longer pulses. • What range of density and temperature should be explored in meeting mission?

10. Pilot Plant-2 is more flexible than Pilot plant-1 • An example Pilot plant-2 is 10% larger, with the radial space used mainly to provide more V.s. • For the same solenoid field, the current density is lower than in case 1. Higher field and even more V.s. should be possible. • Compared to Pilot Plant-1, it has longer flat top times particularly when non-inductive start-up is limited –greater flexibility to explore a range of plasma conditions at high duty factor.

11. Conclusions • The following areas should be explored: - Maximum permissible current density/field in the solenoid. - Extent one can rely on non-inductive start-up. - Issue of whether non-inductive flat top is possible or too restrictive on operating space. - Operation with less than full non-inductive current drive. - Necessary down time and limit on number of cycles to keep stresses at acceptable level.