# Tokamaks

SULI Talk By Battagalia

Tokamak: - external coils produce toroidal field - a tokamak must maintain the toroidial current at all times - this current can be induced by the central solenoid - can’t be used indefinitely to sustain current $I_p \alpha V_\phi = - \frac{d\phi}{dt}$ What this equations shows is that to have a current induced in the plasma, we need to have voltage which is created by the magnetic flux of the solenoid as it changes over time. - tokamaks have a boot strap current that accounts for ~60% of required current - Disruption: losing plasma current and thus losing confinement time - Plasmas are created in a vacuum and anything that isn’t deuterium and tritium that is in the plasma is an impurity. - external poloidial magnetic fields shape the plasma

#### How can we drive current in tokamaks?

Two Possible Methods: RF, NBI or Helicity Injection

Neutral Beam Injection (NBI): - neutral particles are created at high velocities - since they’re neutral, they can penetrate the magnetic field cage - when they’re inside the plasma, they can lose their electron and become charged - they transfer their energy through collisions with other particles and thus heat the plasma; they can fuel the plasma, drive current and induce plasma rotation - NSTX-U uses NBI

#### Tokamak Design Considerations:

• there is a limit to plasma current in a tokamak
• you want to increase this because confinement time increases
• if you increase plasma current, q decreases because magnetic fields make fewer toroidial lines per poloidial line

Safety Factor q - q is the ratio of the toroidial fields to the poloidial fields - it describes how stable a plasma is and whether instabilities will occur

Plasma Pressure - there is a limit to the plasma pressure $\beta = \frac{plasma pressure}{magnetic pressure}$ $\beta = \frac{2 \mu_0 <p>}{B^2}$

Confinement Time - plasma turbulence is what leaks the most energy from the core - radiation also leaks energy; this can be used for diagnostics - in toroidial systems, collisions between particles on banana orbits set the ‘energy diffusion step size’