The Varian/Packard Polarization Field Fast Turn-off Technique
uild a Geomagnetic Observatory ! GELLER Labs "Backyard Science"
Thoughts on a proton precession magnetometer design - a Proton Magnetometer Project. Build an Earth's field magnetometer.
The FDM MAGNETOMETER1 project is a low cost high performance proton magnetometer (a digital magnetometer) kit under development for universities and amateur scientists to be able to accurately measure and monitor changes in the Earth's total magnetic F field and to observe geomagnetic storms. Magnetic storms can cause large excursions in the field and are of concern to interests ranging from electrical power grids, radio communications, and satellite operations, to aurora watchers and amateur radio operators.
1 Filter Diagonalization Method "FDM" (harmonic inversion), see Jan 21 and Jan 23 entries, based on: Vladimir A. Mandelshtam, Howard S. Taylor, Harmonic inversion of time signals and its applications, Journal of Chemical Physics (1997), Volume 107, Issue 17, 1997, Pages 6756-6769
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Monday, May 16, 2011 - K4
On Earth's field NMR (EFNMR) and use of the 90 degree pulse (conventional NMR/MRI) versus the polarization field fast turn-off technique which we (and most convention geophysical scalar proton precession magnetometers) use to enable a precession signal: In reviewing an old classic NMR text on my shelf, A. Abragam, The Principles of Nuclear Magnetism (Oxford Press, 1961), I came across a description of the fast (non-adiabatic) polarization field turn-off method. Perhaps not surprisingly, it is attributed to an early Varian paper, Packard M., Varian R., Free Nuclear Induction in the Earth's magnetic field, Phys.Rev. 93, 941 (1954), an American Physical Society (APS) meeting brief. (Much appreciated if someone could send a pdf of the 1954 Varian paper, otherwise I will eventually find one ). In a discussion of free precession in the Earth's field (Abragam, page 64), Abragam describes a fast turn-off method (the Varian paper) as an "elegant alternative" to the 90 degree pulse method (Abragam, page 65). In the alternative method, the polarization field is reduced somewhat quickly (short compared to 2 or 3 seconds) from 100 Gauss down to about 2 or 3 Gauss. Then, the field is reduced very quickly (non-adiabatically) from the remaining 2 or 3 Gauss polarization field to zero in a time short compared to (1/(gyromagnetic constant times the 2 or 3 Gauss field)). Our "fast discharge" is accomplished by use of the reverse avalanche mode of our FET, and our secondary dump resistor (which takes over when there is no longer enough energy to support the FET reverse avalance mode)with a very fast "non-adiabatic" (L/R) final discharge of the powered coil. The polarization field (Abragam's Hp) is physically oriented at 90 degrees to the Earth's field for optimal free induction decay (FID) signal amplitude.
Afternote: While GoogleTM(ing) around I also noticed there is a new version of Professor Koehler's paper on proton magnetometers. The Varian-Packard method (his "Is" term) is discussed on page 36. I looked at an old notebook, this discussion has been in Professor Koehler's papers for some time.
Tuesday, May 17, 2011
Wednesday, May 18, 2011
Thursday, May 19, 2011
Friday, May 20, 2011
Saturday, May 21, 2011
Overnight: PDF, TXT. The field was bit unsettled around 6 pm local (UTC -4), but is quiet now (evening) PDF. I have been off doing other projects for a week or so and writing a paper on the polarization controller modes. looking forward to a warm sunny day to take a days worth of polarization controller data in the fixed polarization time mode (polarization controller off) to show the ampitude variation in contrast with a day in the PEV servo mode.
QUESTIONS/COMMENTS/notice of typos, etc. send email to joegeller @ gellerlabs dot com
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