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OTF Degaussing v.2

v.2 updated 05-Dec-2005

Summary

This report summarizes measurements on the green quadrupole labeled OTF. The aim of these measurements was to find a degaussing procedure, with a remnant field strength /Integrated gradient of 10 Gauss or less.
Below I describe the recommended procedure, and considerations made in its development. The recommended procedure is a series of ramps with decreasing currents alternating in sign.
An important finding is that turning off the power supply to the magnet produces a current spike of 2.5 A; this spike re-magnetizes the magnet to produce a remnant field of >39 Gauss. Therefore it is important to let the power supply stay on after degaussing.

Recommended Procedure

The recommended procedure is to cycle the magnet 8 times, with the current reduced by a factor 0.8 from the preceding cycle, starting with 10 A (that is, ramp, in sequence, to 10 A, -8 A [cycle 1], 6.4A, -5.12A[cycle 2], 4.096 A, -3.2768 A [cycle 3]........) and then ramping to zero current. We used a linear ramp, with 10A/sec; and 2 second wait after ramp; 8 cycles were accomplished in less than 75 seconds.

Cycling Results

Using the above procedure, however varying the number of cycles, shows the remaining integrated field as in this figure: RemStrength-r102.jpg
One sees that 8 cycles are optimal, for a remaining field better than 2 Gauss, and increasing the number of cycles leads to a slight increase to less than 6 Gauss. After 8 cycles, the last current cycled to was -0.351844 A. (Remember that one cycle is two ramps, one going to + X A, then to - 0.8*X A) These data are also shown in the following table (from strdat.102): 

After  Cycle     :    1    2    3    4    5    6   7    8    9   10   11   12
------------------------------------------------------------------------------
Rem Field [Gauss]: 106.4 79.4 53.2 33.8 11.6 14.4 7.07 1.54 1.9 3.68 4.63 5.10

After  Cycle         13    14   15   16    17   18   19    20     21   22   23
-------------------------------------------------------------------------------
Rem Field [Gauss]   5.38  5.49 5.57  5.6  5.62 5.65  5.65  5.64  5.66 5.53 5.87 

After  Cycle         24     25    26     27   28     29
---------------------------------------------------------------------------------
Rem Field [Gauss]    5.62  5.64  5.63  5.64  5.64   5.61
The remaining current in the magnet during these measurements was less than 0.5 mA (except for the first four, there it was 0.94, 0.7, 0.55 and 0.5 mA) Each field measurement is an average of 4, and the error derived from these 4 is typically 0.03 Gauss or less (Run 104).
I considered changing the wait-after-ramp time from 2 seconds to 10 seconds (run 104) and found practically identical results (strdat.104) (with value 1.58 Gauss after cycle 8, and 5.46 above 20 cycles).
Changing the ramp-rate from 10 A/s to 20 A/s (run 103) leads to a lower remaining field during the first few cycles ( and 0.39 Gauss after cycle 6!), but to a slightly larger field, 6.57 Gauss after 20 cycles.


During the field measurements, a remaining current of typically less than a mA is still present. To investigate the effect of such currents, I took some measurements at small currents, (Run 99, 100; also 20,44), and estimated a slope of the field vs. current line of about than 0.4 Gauss/mA. .jpg .jpg
The latter figure indicates that a remnant field of about 6.3 Gauss will be canceled by a negative current of 23 mA.

Current Spike

When measuring the remaining field at the end of a cycle, the power supply was normally turned on. (The power supply used is a Kepco Bipolar Power Supply/Amplifier) As a concern had been raised about the remaining current at the mA level, another measurement was also taken with the power supply turned off
Surprisingly, the new measurement gave a field of about 37 Gauss (at a current measured at 0.03 mA; when PS was on, it was 1.5 mA; in typical measurements though it was less than 1 mA). This effect could be reproduced (Runs 17, 18, strdat.r18).
Looking at the transducer output with an Oscilloscope, it was found, that turning off the power supply, created a current spike of 2.5 A, which apparently re-magnetized the magnet. (A figure of the current spike is at here as pdf xls
After ramping the magnet to +-3.125, +-2.5A, the remaining field is also 39 Gauss, and increases to 59 Gauss, after the PS is turned off. (Run 21, 22)
So I conclude that the current spike produced by turning off the power supply re-magnetizes the magnet to about 39 Gauss.

Time Structure of Degauss Cycle

We used a linear ramp, with 10A/sec and 2 sec wait after ramp

From timestamp-data below, it is about 2*4=8 sec, including the 2 second wait after ramp for each cycle (of going to + x A and then -0.8*x A)
So 8 cycles take about 1 minute and 12 seconds; add a few second for the ramp to 0.
(From logfile.102) 
17:15:51  Magnet current ramping to      10.000000 A...
17:15:56  Magnet current ramping to      -8.000000 A... end of cycle 1 17:16:01
17:16:01  Magnet current ramping to       6.400000 A...
17:16:06  Magnet current ramping to      -5.120000 A...  2  
17:16:11  Magnet current ramping to       4.096000 A...
17:16:15  Magnet current ramping to      -3.276800 A...  3
17:16:20  Magnet current ramping to       2.621440 A...
17:16:24  Magnet current ramping to      -2.097152 A...  4
17:16:28  Magnet current ramping to       1.677722 A...
17:16:33  Magnet current ramping to      -1.342177 A...  5
17:16:37  Magnet current ramping to       1.073742 A...
17:16:42  Magnet current ramping to      -0.858993 A...  6
17:16:46  Magnet current ramping to       0.687195 A...
17:16:50  Magnet current ramping to      -0.549756 A...  7
17:16:54  Magnet current ramping to       0.439805 A...
17:16:58  Magnet current ramping to      -0.351844 A...  end of cycle 8:  17:17:02
17:17:02  Magnet current ramping to       0.281475 A...
17:17:06  Magnet current ramping to      -0.225180 A...  9
17:17:10  Magnet current ramping to       0.180144 A...
17:17:14  Magnet current ramping to      -0.144115 A... 10
17:17:18  Magnet current ramping to       0.115292 A...
17:17:22  Magnet current ramping to      -0.092234 A... 11
17:17:26  Magnet current ramping to       0.073787 A...
17:17:30  Magnet current ramping to      -0.059030 A... 12
17:17:34  Magnet current ramping to       0.047224 A...
17:17:38  Magnet current ramping to      -0.037779 A... 13
17:17:42  Magnet current ramping to       0.030223 A...
17:17:46  Magnet current ramping to      -0.024179 A... 14
17:17:50  Magnet current ramping to       0.019343 A...
17:17:55  Magnet current ramping to      -0.015474 A... 15
17:17:59  Magnet current ramping to       0.012379 A...
17:18:03  Magnet current ramping to      -0.009904 A... 16 
17:18:07  Magnet current ramping to       0.007923 A...
17:18:11  Magnet current ramping to      -0.006338 A... 17
17:18:15  Magnet current ramping to       0.005071 A...
17:18:19  Magnet current ramping to      -0.004056 A... 18
17:18:23  Magnet current ramping to       0.003245 A...
17:18:27  Magnet current ramping to      -0.002596 A... 19
17:18:31  Magnet current ramping to       0.002077 A...
17:18:35  Magnet current ramping to      -0.001662 A... 20
17:18:39  Magnet current ramping to       0.001329 A...
17:18:43  Magnet current ramping to      -0.001063 A... 21
17:18:47  Magnet current ramping to       0.000851 A...
17:18:51  Magnet current ramping to      -0.000681 A... 22
17:18:55  Magnet current ramping to       0.000544 A...
17:18:59  Magnet current ramping to      -0.000436 A... 23
17:19:04  Magnet current ramping to       0.000348 A...
17:19:08  Magnet current ramping to      -0.000279 A... 24
17:19:12  Magnet current ramping to       0.000223 A...
17:19:16  Magnet current ramping to      -0.000178 A... 25
17:19:28  Magnet current ramping to       0.000000 A...

Raw Data

The raw data for this study are in the "Magdata OTF directory" (in files with extension .ruX or rXX or .XXX where X, XX or XXX refers to a Run Number)
A previous preliminary draft version of 28-Nov-2005 of this report, using a different procedure, is here (txt)


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Last updated 05-Dec-2005 by Achim W. Weidemann.