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To: frankw1900 who wrote (2323)	5/19/2001 8:06:49 PM
From: Bilow	Read Replies (2) \| Respond to of 24758

Hi frankw1900; Thanks for the links. They verify my claim that superconductors have a current limitation, rather than a power limitation. Note where the article says that there are AC losses in superconductors. For the case of magnets, the current is DC, so you this isn't much of an issue. The same could be had with the transmission of power long distance with superconductors. That is, it might be more effective to send the power as DC instead of AC. (Edison lost on DC against Tesla's AC back before electronics learned how to efficiently convert between DC voltages and back and forth to AC.) Re: " You gave as examples of working superconducting devices things which are low temperature super conducting technology, are relatively low powered, and use materials which are normally conducting anyway, I think. Are there MRIs which use high temperature (cooled by nitrogen) superconductors?" I wouldn't call 50,000 watts "low powered", LOL, but I guess it depends on your perspective. I'm guessing that the commercial units are low temp superconductors because it's cheaper, and the cooling problem has been "solved": Toshiba Medical Systems in Europe Cryogenless Magnet Opart magnet technology: open and superconducting Commercially available open MRI systems typically use either resistive or permanent magnets, while conventional MRI units are built around superconducting bore-type magnets. Each magnet technology has its own advantages and disadvantages and is listed in the table below. Magnet type Advantages Disadvantages ------------ ------------------------ ----------------------- Resistive 1. Technology well known 1. High power consumption. and understood. 2. High cooling demand. 2. Relatively inexpensive 3. Field homogeneity technology. temperature dependent. Permanent 1. Passive: no cooling, 1. Steep cost/field electricity or cryogens curve. needed. 2. High weight/B0 ratio. 3. Field homogeneity temperature dependent. Supercon 1. Higher field strengths 1. Need for cryogens. can be achieved. 2. Technically more complex. 2. High field homogeneity. 3. Compact magnet design in relation to fieldstrength. When designing the OPART, Toshiba has chosen for the distinct advantages of a superconducting magnet as performance with superconductive MRI systems are perceived as the gold standard. OPART is based on a 0.35T superconducting magnet with a vertically oriented field and is open on all 4 sides. In addition, Toshiba engineers have designed a solution for one of the major disadvantages of superconducting magnets: the need for cryogens: the OPART magnet is fully cryogenless. toshiba-europe.com That low temp superconductors would be the first ones engineered into manufactured (and high power) products is to be expected, they were the first ones developed. By the way, here's a good link for superconducting applications, and, by the way, it verifies that DC is used to transmit power, as I stated above: wws.princeton.edu The physics and MRI engineers went to supercooled magnets where they need really big magnetic fields for solid engineering reasons. My point is that superconduction is used in industry now, where appropriate. If this is like any other technology, it will be appropriate for wider uses as time goes by. Materials science advances steadily. If you'd told someone in 1945 that in their lifetime industry would manufacture chips with a billion interconnected transistors for $100 they'd not have believed you. -- Carl