Brief Info Update.

Equipment:
     We have two sets of Beam Cavity Monitors for G0:
     Each set has three resonant cavities, One cavity for X position, one cavity for Y,
     and one cavity for Current.

     The salient point and attraction for these cavity style monitors is they should
     be much more precise than the common, stripline-BPMs.

     Each set of cavities has an signal amplifier in the hall which feeds upstairs
     to V/F converters then into scalars.


     One set of these cavities, (C,X,Y) and a LINEAR amplifier is located ON the Gzero girder
     just before the target.
     
     The other set of cavities (C,X,Y) and a LOGARITHMIC amplifier is located in the alcove near
     the fast raster.

     Each cavity, as a standing wave device plus antenna, is 'tuned' to 1499MHz resonance.
     subtle changes in length then, make for changes in performance; sensitivity, noise, etc.
     Thus each of the cavity sets need to be thermally stable and to this end each set is heated within
     a thermal enclosure.  This heating helps against long-term calibration drifts.
     In the past These heaters have turned them selves off during power glitches.  
     Both are now with power-supplies which auto-recycle and power-ON after a reset.  


     With the exception of the amplifier, these cavity sets are identical.
     We are currently testing and calibrating the performance of the different type amplifiers.
     In theory, these resonant cavity monitors should yield beam in formation with much more precision
     than the current stripline BPMs (standard, ubiquitous BPMs)

     In addition, we have cavity 'test-signal' generators, located in the hall, which can be controlled
     from upstairs.  These send about 1 Watt of  1499MHz microwave test signal INTO the respective cavity
     set.  This signal is divided by the 3 cavities, and different attenuations may be selected
     to give a variation of test-signal strength.  Normal status of these generators is OFF.
     


Operation:
	Both types of amplifiers are designed to give 1volt DC if there is no signal from the
	Cavities: I.e:  When no beam, the V/Fs should see 1 volt.  This then, creates a nominal
	pedestal.  

	These cavities  operate by picking up the signal strength of a standing wave built up 
	by beam passage through the cavity.

	The current cavities are designed such that the 1/2 standing wave max amplitude is near the
	pick up antenna.  This makes the current cavity sensitive to current and insensitive 
	to position.

	The position cavities are designed to measure the amplitude of a full standing wave.
	The full-wave design makes the amplitude of the standing wave sensitive to beam position.
	However - The strength of this standing wave is also sensitive to beam current.
	The position signal must therefor be normalized to beam current.


Analysis to date:  
	 There is good pedestal data.

	 Good data from late October was used for calibrating both Current cavities, Linear amp
	 and Log amp up to 20uA. 
	 The cavities have been nicely calibrated against standard BCMs and MCC current measurements.
	 (See attached slide)

	 Work in progress to calibrate and understand the position sensitivities and noise.

	 Noise response so far has not been good:
	 Approximately: 80ppm for the Log Amp set (at about 10uA)
		       400ppm for the Linear Amp set (at about 10uA).
		       (this data is also convoluted with the noise from the standard BCMs against
		       which they are compared:  Work in progress)
		       		       


	 To this End:
	 data from testing of Coil modulation gave reasonable data with steady 
	 beam and a multitude of beam positions.
	 Ideal for analyzing the position cavity response (after normalization, of course)

	 However... 
	 First step was to double check the response of the Current Cavities to the BCMs,
	 And if still nominal, proceed to normalization of the position cavities by the
	 current cavity signal.

	 Without necessarily calibrating the Cavities to the BCMs, one would at least expect a 
	 correlation between Current Cavity signal and BCM[1or2]. This was not the case.
         Cavity data from these runs are pathological in that none of the signals correlate to 
	 *any thing*. This does, as you might next guess, interfere with meaningful results.


	 Subsequent investigations by Ed Strong (builder of the amplifiers) and myself showed that
	 the amplifiers and test-signal generators did seem to be performing as expected at 
	 least as far as the V/Fs.

	 Latest data shows that Current Cavity with the Log amp (alcove) seems to be working
	 properly.  Calibration check and pedestal check are being done now. 
	 Data from the Girder Current Cavity, with the Linear amp, shows values indicating
	 a railed V/F.

	 We are currently looking at when and why this data changed.
	 

Questions?  Send them to me, Thanks.

Gary

-- 
---
---------
Gary Rutledge
_______________________________  ________________________________
 Department of Physics         \/       Jefferson Lab, VA
 University of Manitoba        /\     office:  (757) 269-7346  
 rutledge@jlab.org             \/     pager:   (757) 584-7346
_______________________________/\ ______________________________
`[Life] isn't safe; It's wondrous... But it's not for the timid.'
                                        -Q, Star Trek, TNG.

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