Tokamak with poloidal field coil arrangement for double null merging ignition, method therefor and nuclear fusion reactor with the same

1. A tokamak comprising:

a toroidal containment vessel configured to contain a plasma;

a plasma initiation system comprising upper and lower poloidal field, PF, coil sets, each PF coil set comprising:

at least one inner PF coil located outside the containment vessel;

an outer PF coil located inside the containment vessel;

wherein the inner PF coil is located radially inward of the outer PF coil;

shielding located between the outer PF coil and a location of the plasma during operation of the tokamak and configured to protect the outer PF coil from heat emitted by the plasma;

the inner and outer PF coils being configured so as to form a PF null within the containment vessel between the inner and outer PF coils, such that the upper and lower PF coil pairs are operable to initiate a plasma in the containment vessel via double null merging.

2. A tokamak according to claim 1 , wherein the shielding is further configured to shield the outer PF coil from neutrons emitted by the plasma tokamak.

3. A tokamak according to claim 2 , wherein the outer PF coil is formed from high temperature superconductor, HTS.

4. A tokamak according to claim 1 , wherein the outer PF coil is formed from a material which is not superconducting during operation of the outer PF coil.

5. A tokamak according to claim 4 , wherein the material comprises any of:

copper;

aluminium;

beryllium;

a metal;

a metal alloy.

6. A tokamak according to claim 1 , wherein the outer PF coil is located within a shielding blanket of the containment vessel, and wherein said shielding located between the outer PF coil and the location of the plasma is a part of said shielding blanket.

7. A tokamak according to claim 1 , wherein the shielding is composed of several modules such that each module extends around only a portion of the toroidal containment vessel.

8. A tokamak according to claim 1 , wherein the tokamak is a spherical tokamak.

9. A tokamak according to claim 1 , wherein the outer PF coil is configured to magnetically couple better to a plasma located at the PF null than to the containment vessel, such that when power is applied to the PF coil, a proportion of the power which results in creation and induction of currents in the plasma, is greater than a proportion of the power which results in creation and induction of currents in the containment vessel.

10. A nuclear fusion reactor comprising a tokamak according to claim 1 .

11. A method of initiating plasma within a tokamak, wherein the tokamak comprises a toroidal containment vessel configured to contain a plasma, a plasma initiation system comprising upper and lower poloidal field, PF, coil sets, each PF coil set comprising at least one inner PF coil located outside of the containment vessel and an outer PF coil located inside the containment vessel, the method comprising:

creating upper and lower PF nulls using the respective upper and lower PF coil sets by passing respective constant currents through each coil of each set;

reducing the current in each PF coil in order to generate a loop voltage at the PF nulls and create a plasma ring at each null;

the plasma rings merging to form a single plasma.

12. A method according to claim 11 , and comprising reversing a polarity of the current in each PF coil in order to aid the merging of the plasma rings.