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September 25, 2008

Superconducting Radiofrequency Cavities and the Emdrive Project


Operating at temperatures just above absolute zero, superconducting cavities accelerate bunches of electrons and positrons toward the detectors in a proposed international linear collider. Nine smooth cells, polished in all possible ways. Made of the purest niobium. Not a speck of dust or the slightest difference in shape. Superconducting when supercold Photo: Fermilab

Superconducting cavities are a key component of an enhanced version of the controversial Emdrive [electromagnetic drive].


Prototype C-band emdrive (The emdrive has been funded by China and could be a breakthrough in space and terrestrial propulsion or it is a one to two million dollar scientific mistake.) [by the cinderblock in the background it appears to be less one foot tall]

Effect of increased Q for the Emdrive

Q=50,000 (1st gen.) Static thrust=315 mN/kW Specific thrust at 3km/s=200mN/kW

Q=6,800,000 (supercond) Static thrust=42.8 N/kW Specific thrust at ??km/s=??N/kW

Q=5* 10**9 (supercond) Static thrust=31.5 kN/kW Specific thrust at 0.1km/s=8.8N/kW

Q=10**11 (supercond) Static thrust=630 kN/kW Specific thrust at 0.1km/s=??N/kW



SPR ltd is working on a superconducting demo which should be 100 times more powerful than the first version and provide 30 newtons of force instead of 315 milli-newtons. China is also building a large S-band thruster.

Superconducting radiofrequency (SCRF) cavities are also the main technology for a new international linear collider.

The main vehicle for SCRF technology is the cavity, a hollow structure that drives particles to higher energies. For the ILC, we will use roughly 16,000 metre-long nine-cell 1.3 GHz (gigahertz) niobium cavities. So far ILC scientists have achieved the target gradient goal in roughly a dozen cavities. “We have proven that the technology works,” says Cornell University’s Hasan Padamsee. “Now we need to improve the yield.” In some cases, ILC cavities have actually exceeded the
target goal and reached a gradient of 40 MV/m. Consistently reaching a gradient of 35 MV/m, however, in a large number of cavities remains a problem.

The European Commission has accepted to fund the ILC-HiGrade, or “International Linear Collider and High Gradient Superconducting RF-Cavities,” proposal within its Seventh Framework Programme (FP7) with five million Euros over the next four years. Under this contract, at least 24 superconducting cavities will be created to demonstrate the gradient feasibility for the ILC.


This kind of superconductivity cavity would be one thousand to twenty thousand times better than the crude superconducting demo that SPR ltd currently has. A very good nobium cavity could probably be created for $400,000 to 800,000 [converting the Euro price to dollars and increasing the cost for one instead of 24].




A 2002 study of superconducting cavity costs worked out to $100,000 per meter.

In the same 2002 presentation by Pierre Bauer, it appears that the maximum Q value is at least higher than 100 billion for certain kinds of nobium cooled to about 1 degree kelvin.


Possible research for higher Q cavities [2002 list]:
1) cavity manufacturing (“seam-less” techniques such as hydro-forming,…)
2) materials – e.g. replacing bulk Nb with Nb on Cu
3) surface resistance – e.g. understanding the surface chemistry that leads to low surface resistance, exploring materials which produce lower surface resistance,..etc
4) Higher gradients in view of a second stage in the same tunnel – e.g. pushing Nb to the absolute limit, exploring other materials such as Nb3Sn and NbN;



This dynamic test rig moved at 2 cm per second using the first generation emdrive. If the crude superconducting test system can be made to work then it should move the a heavier (ten tons instead of 100kg) dynamic test rig at 2 cm per second [the system loses energy with more speed].

A couple million dollars of equipment and labor at risk over two years to verify what could be a huge multi-trillion dollar breakthrough or a fairly cheap mistake (or something inbetween).

FURTHER READING
Superconducting Radio Frequency at wikipedia

It is commonplace for a 1.3 GHz niobium SRF resonant cavity at 1.8 Kelvin to obtain Q=5×10**10 [50 billion]. Such a very high Q resonator and its narrow bandwidth can then be exploited for a variety of applications. At present, none of the "high Tc" superconducting materials are suited for RF applications. Shortcomings of these materials arise due to their underlying physics as well as their bulk mechanical properties not being amenable to fabricating accelerator cavities.

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