Fusion methods

Nuclear fusion is achieved by providing atomic particles with enough energy to overcome electrical repulsion forces. Energy levels of 10 -100 KeV are required, where 1 eV corresponds to a temperature 11600 °K (11326,85 °C). They could be achieved by aiming at single particles with high energy beams or lasers or by heating with microwaves a cloud of gas trapped by the magnetic field. The first method, known as inertial fusion, would produce rapid pulsed systems made of many parallel lines. The second method, known as magnetic fusion, is more massive and continuous thus apparently more suited for energy production at industrial level.

The magnetic field lines deviate and confine charged particles of hot ionized gases, known as plasma

Magnetic fusion has been investigated in the world since 1950, inside solenoids made by copper conductors or by flowing the currents directly inside the gas as electrical discharges. High temperatures and nuclear reactions with production of neutrons were achieved only for few micro-seconds because particles escaped at both ends.

Particle drift out of the plasma confined by toroidal field lines.

Circular solenoids, shaped as donuts (i.e. geometrical torus), were not good because the particles drifted up and down against the walls, until the Russians induced a circular current in the gas. The circular magnetic lines become helical cancelling the vertical particle drift: the Tokamak was invented. Since 1970 the majority of fusion research in the world is based on the Tokamak scheme.

The Tokamak includes a toroidal component produced by external coils and a poloidal component produced by the current induced in the plasma: the result is a helical magnetic field.

After the early success and becoming widespread, the Tokamak improved slowly until the 1980, when some nuclear power plants for testing purposes were built after the first world energy crisis of 1973: the Tokamak Fusion Test Reactor (TFTR) in Princeton NJ-USA and the Joint European Torus (JET) in Cuhlam UK-EU. Both systems have a core that is a few thousand cubic meter wide, they are powered by hundreds of MW and set up for extensive D-T operation. Actually few symbolic shots were made with D-T. New difficulties led to joint efforts worldwide in 1991 and tried to achieve fusion with a new larger Tokamak ITER, whose construction began in 2006.

ITER (acronym of International Thermonuclear Experimental Reactor) aims to study thermonuclear plasma in burning conditions, but it is not a reactor because it needs Tritium from outside. This is why a further prototype DEMO is required after ITER, to demonstrate tritium self breeding capacity and electrical energy production.

Why Polomac

The hard work done to improve the Tokamak since the 1970 led to scarce results because the plasma is disturbed by the large toroidal current, involving mutual forces with the other coils and triggering instabilities. In fact, in the advanced Stellarators using magnetic fields are produced only by external coils without any current in the plasma, the energy confinement is better. The issue of the Stellarator are: the design and construction of the warped coils and the tight clearance allowed between the plasma and the coils.

The Polomac, like the Stellarator, does not need any current in the plasma. Particle drifts due to the curvature of the poloidal magnetic field lines are accepted because they aim inside the plasma and compensate each other. Here the drifts do not produce any charge separation.

Particle drift inside the plasma confined by poloidal magnetic field lines

The steady magnetic field is produced by external coils, which are flat and easy to build. The Polomac differs from earlier closed poloidal magnetic tests ( i.e. Levitron, Stator, Spherator, JFT-1, Intrap, LDX ) in the magnetic tunnels, a new feature which could finally bring us to nuclear fusion energy.


The confinement efficiency is the ratio of the energy density of the plasma respect to the energy density of the confinement system. A burning plasma at 10 keV and density 10^20 particles per cubic meter reaches 0.16 MJ per cubic meter, i.e. 0.16 MPa. Remember that energy density corresponds to pressure. The energy density corresponding to the toroidal field of ITER 5.3 T is 11.17 MPa. The energy efficiency of the Tokamak is then 0.16 MPa over 11.17 MPa equal to 1.4%. This parameter is normally known as Beta.

Instead, the Polomac should replicate the efficiency of the theta pinch and operate with Beta up to 0.7-0.8. Already with Beta 0.5 and a magnetic field of 3 T it could trap plasma at 100 keV.

Deuterium to

While operating at high Beta with an increase by 10 of the confinement time respect to the present values of the Tokamaks, i.e. from 5 to 50s, the Polomac can produce energy from the D-D reaction, without provision of external Tritium.

According to the Lawson criterion for burning plasma, the triple product of density, temperature and energy confinement time, for D-D reaction must be almost 200 times larger than in the D-T reaction, which is the current reference in ITER and in DEMO. The increase of temperature from 10 to 100 KeV and of the confinement time from 5 to 50 s along with few higher density can accommodate the Lawson criterion.

Since the cross section of the D-D reaction is about 100 times lower than D-T, the size of the corresponding reactor could be larger, although better confinement and higher operating parameters expected in the Polomac should allow for a power density 1-3 MW/m3, as assumed at present in the Tokamak rector studies. Surely a D-D reactor is simpler, because it does need any breeding blanket to produce Tritium.

The construction of a large Polomac is not demanding as a Tokamak, because there is only one system of flat coils working at 3T. The superconductor is Nb-Ti instead of Nb-Sn, thus coil fabrication is less demanding.