2007AGS Diagnostics: Extracting Information about the Loss Plane from observed Loss Monitor response - the usual logic. Ahrens 2May07



The following comments actually apply usefully to the usual AGS machine. A situation where optic distortions ( e.g. from the snakes) invalidate the conclusions is yet to be clearly seen.


The AGS ring loss monitor system (displayed via AGSLossMonitor in StartUp) is made up of long detectors, each two AGS magnets long. The monitors are physically contiguous, even slightly overlapping, around the AGS ring. In the program loss, data is displayed labeled as: A2, A4, A6, …,A20, B2,…,L20. The A2 monitor actually spans both the A1 and A2 magnets, starting upstream of the A1 magnet and ending at the downstream end of the A2 magnet. Similarly the L20 monitor stretches from the upstream end of the L19 magnet and ends midway through the L20 straight section. A signal seen in the F12 loss monitor originates somewhere between the upstream end of F11 and the downstream end of F12.


The optics in the AGS results in beam envelopes that are periodically large - the modulation of the beta functions. For the basic AGS the horizontal beta function has maxima ~ every 4th straight section: #1, #5, #9, #13, and #17, and smallest two magnet further on. Similarly the vertical beam size is largest in #3, #7, #11, #15, and #19.

Not that it will help you, but the #15's held the ferrite jump quads once to get max vertical tune shifts - 15's are vertical beta maxes. H11 is where we seem to scrape vertically at injection - a vertical beta max. If you can remember one, then you can work out the others.

If losses show up in a #14 loss monitor the cause is probably a horizontal scrape (in the # 13 ss where the horizontal beam size is large). If beam loss shows up in a #12 loss monitor, the cause is probably a vertical scrape (at the #11 beta max point).


Historically vertical losses show up in #15's and #11's (hence lm 16 and 12). Horizontal losses show up in #13's and #17's (hence in lm 14 and 18.


How can you figure out which loss monitor has the loss - when looking at the usual graphics of the loss monitor display? Two answers. In the loss monitor application, in the top menu bar: Data, and then List Data and a listing of the losses and monitor names occur. So then the high channel can be located unambiguously. However just looking at the graphic display, the channel identification can be figured out. There are ten monitors per superperiod, and the "dot" falling at "zero" in the superperiod is really the #2 (i.e. stretching from the start of #1 to the end of #2) monitor.


As an example - not a particularly good example I must say -, go to the Gold elog for 8April07, and find the 9April entry at 17:05. Here I was being annoyed by losses in the G superperiod. Look at the last plot. The highest point in the G superperiod is the 9th dot starting from the one at the G0 line, hence it is lm18, (G17 - G18) magnets. Probably a horizontal loss since #17 is hori betamax (and vert beta min).


What about the gain setting for the loss monitors, should one ask for high gain or low gain? I think either gives about equally satisfactory results.


The low gain setting has a base line issue. Best information requires taking a data acquisition with no beam in the machine, and using this as a baseline: (data in menu bar, save data, and then select this data for reference - button on the main page, and then set to subtract reference - also on main page). The reference is different for each sample time (really end of integration window) so a new reference is required if you change the times. So don't take a reference until you have the times you want. The gain setting may be only a few hundred.

The high gain setting is noisy. Set the scale to be a large number - 20000 maybe to get acceptable pictures. Taking a background doesn't help.