355. Introduction into thermonuclear reactors

Posted by Dmitry

April 15, 2009

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After a brief layman review of the theory of thermonuclear fusion let me get more practical and discuss a bit how thermonuclear reactors are supposed to work.

Basically, we want the energy release of the thermonuclear reactor to be larger than the energy we pump into the reactor. Depending on a particular scenario of the energy pumping, we will distinguish two types of thermonuclear reactors.

Type A reactors are such that the energy is pumped in the initial moment of time just to start the reaction, which then becomes self-sustained. On one hand, the energy of the plasma gets lost due to its finite heat conductivity and radiation – the temperature of the plasma naturally wants to drop. On the other hand, the temperature of the plasma may be supported by the energy released in thermonuclear reaction. For example, if the fuel is d-t, the reaction can become self-sustained due to $\alpha$ – particles – products of the reaction or, more accurately, due to the Coulomb interaction with particles of plasma.

We can write the following criterion for the reaction to become self-sustained:

$n_e\tau_E>\frac{T}{\langle\sigma{}v\rangle{}E_\alpha-{\rm brak.rad.}}$ .

Here n_e is the electron density in the plasma, – its temperature, $\tau_E$ – a characteristic time scale at which the energy of the plasma remains constant, $\langle\sigma{}v\rangle$ – thermonuclear reaction rate averaged over Maxwell distribution, $E_\alpha$ – energy of released $\alpha$ particles (about 3.5 MeV for d-t reaction), ${\rm brak.rad.}$ – energy loss due to the braking radiation.

Two examples of Type A reactor are a Tokamak or a stellator.

If released energy of the products of thermonuclear reaction is not enough to keep the temperature of plasma sufficiently high, we will denote such reactors Type B. Apparently, to sustain the nuclear reaction in the Type B reactor, we need to constantly pump energy into the plasma. It is still fine as long as the energy release of the thermonuclear reaction is larger than the energy cost to support the nuclear reaction.

Reactors can be also classified according to how we are going to confine the plasma. Usually, stability of the plasma is achieved by using external magnetic field, but there are also attempts to build reactors with so called inertial confinement mechanism (for example, HiPER).

HiPER scheme

In the latter case, the energy sufficient for the start of thermonuclear reaction is injected into the reactor by a short ( $10^{-8}-10^{-7}$ sec) laser impulse (or by ion/particle beam).

Miniupdate: I was corrected – for HiPER, $10^{-8}-10^{-7}$ sec is the length of pulses for lasers managing compression, the ignition energy is delivered by $10^{-11}$ sec pulses.

A reactor with inertial confinement will work in the regime of short impulses, unlike a reactor with magnetic confinement – the latter can work in a continuous regime.

Entered Apprentice, Nuclear physics

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