Reply To: Barack Obama on Energy

[asymmetric_implosion]

jamesr wrote: I agree, if Be can be avoided that would be great.

However, for a device hoping for good fusion yields you need to be very careful that no high-Z ions can be eroded off and pollute the hottest part of the plasma as they will cause rapid cooling. But more importantly the pinch X-ray flux will be orders of magnitude higher than any ‘low power’ studies done before, and we need to be able to recover the X-rays in the onion. Which means the anode must be as transparent to them as possible, to avoid shadowing a large portion of the collecting area.

Having said that I wasn’t aware Molybdenum had been studied before – do you know the source paper(s)?

General materials survey

R. K. ROUT, A. SHYAM and V. CHITRA “EFFECT OF ELECTRODE MATERIALS ON THE NEUTRON EMISSION FROM A PLASMA FOCUS” Ann. nucl. Energy, Vol. 18, No. 6, pp. 357-358, 1991

Moly in a high rep rate source

R. Petr, A. Bykanov, J. Freshman, D. Reilly, and J. Mangano “Performance summary on high power dense plasma focus x-ray lithography point source producing 70nm line features in AlGaAs microcircuits” Rev. Sci. Instrum. Vol 75 No 8. pp 2551-2559 2004

To start, the paper by Rout et al shows that a very tiny fraction of the high Z atoms make it into the plasma but it doesn’t specify where the atoms are. Petr et al showed long lifetime operate of a Ne based plasma focus for ~5 million shots with moly and very little anode erosion.

R. Verma, P. Lee, S. Lee, S.V. Springham, T.L. Tan, R.S. Rawat and M. Krishnan, “Order of magnitude enhancement in neutron emission with Deuterium-Krypton admixture operation in miniature plasma focus device” App. Phy. Lett. 93 2008

B. L. Bures, M. Krishnan, R. Madden and F. Blobner “Enhancing Neutron Emission From a 500-J Plasma Focus by Altering the Anode Geometry and Gas Composition” IEEE Trans. Plasma Sci. Vol. 38 pp 667-671 2010

To further complicate the matter, high Z may not be a bad thing. People intentionally introduce high Z gases with the fuel gas to improve neutron yield. The high Z gases appear to improve compression of the pinch in small quantities (less than 10% by mass). Measurements with X-ray cameras do not show any contribution from a deuterium pinch. If the pinch was contaminated with high Z impurities they would by highly visible.

The final consideration is the x-ray contribution to the total yield. Even if the x-ray yield increases by orders of magnitude, the production of fusion charged particles will increase by orders of magnitude. The fusion contribution typically trumps the x-ray contribution by nearly 100x. If direct energy conversion is applied to the charged particles, something like 70% can be converted to electricity. For a 100 MJ per shot (fusion gain) system, that means nearly 69 MJ of energy derived from direct charged particle conversion. The x-rays are likely to be converted to electricity by heat then turbine. The x-rays make up ~1 MJ and can be converted at best 40%. Therefore, the x-rays provide ~400 kJ of electricity at best. One could claim some sort of exotic photovoltaic, but the conversion efficiency is not going to exceed 40%. By these arguments, the x-rays need to be shielded and not converted; a tiny fraction of the radiated power that is converted very inefficiently is not significant. Therefore, a refractory metal is the best choice for an anode.