Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: p + B11 reaction
Poster: Pierce Nichols

On , Pierce Nichols wrote:

>What are the practical voltage limits for amatuer built fusors?

In practice, the limits are set by the size of the HV vacuum feedthrough and the current-voltage relation in the fusor. The first is entirely a money issue -- 100kV+ feedthroughs start at nearly a thousand dollars new and are unlikely to appear on the surplus market due to their rarity.

The second is more complicated. In the glow discharge regime that most amateur fusors operate in, the voltage-current relation is set by the physical dimensions of the fusor and the internal pressure. In this case, your peak voltage is set by how much current your power supply can supply. In the glow discharge regime the residual gas in the fusor self-ionizes throughout the volume. Since current is proportional to the density of charge carriers (ions, in this case), and the number of ions is dependent on voltage, you can't really control the relation btwn the two very well, except by twiddling the pressure, and that is a bitch.

However, relief is at hand! By going down in pressure below the glow discharge regime, you can gain complete control over the current. In this regime, gas ionizes only where you tell it to. The Hirsch-Meeks solution is to use a dispenser cathode and a grid near the fusor wall.

>What sort of voltages do people think will be required for engineering breakeven (more power out than in) using say the D-D or D-T reactions in a fusor?

This is not the right question to ask. Each potential fuel combo has a voltage at which its cross-section (which reaction rate is proportional to) peaks. For D-He3, it's about 100 kV; for D-T less than 50 kV. The proper question to ask is how low do the losses need to be and how high does the ion current have to be? The answer to both questions is currently unknown.