Heating and Driving the Current

Abstract

A tokamak power plant needs external authority before, during, and after the burn. Transformer action can initiate current, but a steady-state plant must rely on self-generated bootstrap current and efficient external drive. Neutral beams deliver particles, heat, momentum, and current. Electron-cyclotron, ion-cyclotron, and lower-hybrid systems couple electromagnetic power to selected plasma populations and locations. I develop these actuators as a portfolio rather than as interchangeable megawatts. Their launch efficiency, absorption, spatial precision, availability, protection, and wall-plug demand determine whether the plant can start, shape profiles, suppress instabilities, recover from perturbations, and still export electricity. I reserve diverse actuators because a burning plasma needs control authority more than it needs maximum auxiliary heat.

Startup is a controlled state transition

At room temperature, deuterium and tritium gas is electrically neutral. The plant first establishes magnetic fields and a clean vacuum, introduces a small gas inventory, breaks it down into plasma, raises current, shapes the column, and increases density and temperature. Each stage has different control limits.

The central solenoid changes magnetic flux through the torus and induces loop voltage:

$$V_{\mathrm{loop}}=-\frac{d\Phi}{dt}.$$

The resulting plasma current provides ohmic heating and poloidal field. As temperature rises, plasma resistivity falls strongly, so ohmic heating becomes less effective. External heating takes over.

A transformer-driven current consumes finite flux and naturally supports a pulse. A plant seeking steady operation needs non-inductive current sources and a large bootstrap fraction. The current is not only a total number. Its radial profile determines magnetic shear, stability, confinement, and susceptibility to damaging modes.

Neutral beams carry several forms of authority

A neutral-beam injector creates ions, accelerates them electrostatically, converts them into neutrals, removes unwanted charged particles, and sends the beam through a vacuum duct. Neutral atoms cross the magnetic field, ionize inside the plasma, and transfer energy and momentum through collisions.

Beam penetration must increase with reactor size and density. This drives high particle energy. ITER’s heating neutral beams are supported by a dedicated test facility because negative-ion sources, acceleration grids, high voltage, vacuum, cooling, and beam optics must work together at unprecedented scale.

Wall-plug efficiency is a chain:

$$\eta_{\mathrm{NBI}}=\eta_{\mathrm{supply}}\eta_{\mathrm{source}}\eta_{\mathrm{accel}}\eta_{\mathrm{neutral}}\eta_{\mathrm{transport}}\eta_{\mathrm{absorb}}.$$

If the factors multiply to $0.35$, delivering $50\ \mathrm{MW}$ to the plasma requires about $143\ \mathrm{MW_e}$. Energy recovery from residual ions and more efficient sources can improve the result. The full beamline must also tolerate neutron streaming through its port and protect grids during plasma faults.

Radiofrequency systems select resonances

Electromagnetic waves exchange energy and momentum with plasma particles when frequency and propagation satisfy resonance conditions. Different systems address different populations and functions.

System	Primary coupling	Valuable plant role
Electron cyclotron	Electron gyromotion	Local heating, startup assistance, instability suppression
Ion cyclotron	Ion gyromotion and mode conversion	Bulk ion or electron heating and species control
Lower hybrid	Electron parallel motion	Efficient off-axis current drive in suitable regimes
Neutral beam	Collisional slowing of injected fast ions	Heating, torque, particles, and current drive

Electron-cyclotron systems use high-power microwave sources, transmission lines, windows, mirrors, and steerable launchers. Their localized deposition can suppress magnetic islands if diagnostics locate the island and control aims power accurately. Ion-cyclotron antennas launch lower-frequency waves from close to the plasma edge and must survive strong electric fields and particle exposure.

Each system creates a different failure mode. A vacuum window can leak. An antenna can arc. A transmission line can overheat. Reflected power can damage a source. A steering mechanism can seize after activation. Plant architecture therefore favors modular sources, isolation, instrumented transmission, and remotely replaceable launch components.

Bootstrap current is useful internal feedback

Pressure gradients and trapped-particle effects generate bootstrap current without external electrical input. A high bootstrap fraction reduces current-drive power, but the bootstrap profile follows plasma pressure and transport rather than arbitrary command.

I write the current balance schematically as

$$I_p=I_{\mathrm{inductive}}+I_{\mathrm{bootstrap}}+I_{\mathrm{NBI}}+I_{\mathrm{RF}}.$$

For steady state, the time-averaged inductive term approaches zero. External systems must fill the gap between the desired profile and bootstrap contribution. Too much reliance on an uncertain bootstrap model can leave insufficient control authority. Too little reliance increases recirculating power.

The design variable is therefore not maximum bootstrap fraction. It is the combination of self-driven current, actuator efficiency, profile robustness, and recoverability that minimizes plant penalty.

Alpha heating reduces heat demand, not control demand

As fusion power rises, alpha particles heat the plasma. External heating can fall, but actuators remain necessary to shape profiles, suppress instabilities, control rotation, and enter or leave operating states.

A simplified burn balance is

$$P_{\mathrm{loss}}=P_\alpha+P_{\mathrm{aux}}.$$

If alpha power supplies most losses, small changes in fuel, impurities, confinement, or exhaust can produce large temperature response. Fast reduction of auxiliary power provides one negative control action, but it cannot remove alpha energy already being generated. Fuel and impurity systems must coordinate.

Recirculating power sets an efficiency floor

Suppose a plant generates $700\ \mathrm{MW_e}$ gross and needs $200\ \mathrm{MW_e}$ for heating and current drive plus $100\ \mathrm{MW_e}$ for all other loads. Net output is $400\ \mathrm{MW_e}$. If actuator demand rises by $100\ \mathrm{MW_e}$, net output falls by one quarter.

I track the recirculating fraction

$$f_{\mathrm{recirc}}=\frac{P_{\mathrm{plant,loads}}}{P_{\mathrm{gross,e}}}.$$

This fraction links plasma scenarios directly to economics. An externally driven current that looks modest compared with fusion power may be large compared with net electrical power after conversion losses.

InitiateBreak down and form current

Coordinate solenoid flux, gas, microwave assist, and shaping fields.

RaiseHeat toward the burning state

Combine beams and radiofrequency power with measured absorption.

SustainDrive and shape current

Exploit bootstrap current while preserving independent profile authority.

ProtectSuppress modes and terminate safely

Retain fast, localized actuators throughout the operating envelope.

Availability favors portfolios and graceful degradation

A plant with only one heating technology may be efficient at its design point and unavailable whenever that system is under repair. I reserve enough diversity to start and shut down safely after a single train failure. Routine operation may use the most efficient mix; abnormal operation uses whatever preserves control.

Source modules remain outside high-radiation regions where possible. Transmission and beamline components are segmented by valves and shutters. Calibration loads allow testing without plasma. Spare windows, sources, and power modules are standardized. Protection trips distinguish a local arc from a plasma event so one failed channel does not unnecessarily remove all actuators.

Cold startPrecondition and verify transmission Plasma formInitiate current and capture shape Burn enterRaise heat, density, and bootstrap fraction Burn holdTrade bulk heat for precise control

These systems can only control what the plant can observe. The next report develops the diagnostic and computational layer that estimates plasma state, predicts proximity to limits, and commands the actuator portfolio without granting an opaque model authority over machine protection.