How to set magnetic fields in the GPD spectrometer

The main magnetic field of 0-0.66T can be powered by two different power supplies: Only *positive values in Gauss* are allowed for both power supplies.

The respective power supplies can be set from DELTAT by

  • Exp. Magnets... GOSSEN... or
  • Exp. Magnets... HELM...
Examples for command lines in an AUTO RUN Sequence are (to set a field of 150G and wait 30s):
  • SET Magnet   HELM       150   30
  • SET Magnet   GOSSEN   150   30
For details click here.
NOTE:
  • A 16-bit DAC is used to set the 6.6kG-Helmholtz power supply, i.e. the resolution is about 0.1G. An ADC with only 12-bit resolution
(1.6G resolution) is used for read back. This causes a discrepancy between the values displayed in 'Demand' and 'Measured' in 'Deltat/Exp. Magnets/'). The value set by the power supply always corresponds to the 'Demand' value within the DAC-resolution of 0.1G. This value will also be stored in the header of the data file.
  • Like all other magnet power supplies HELMH2 (650A/250V) is located on the second floor above the Studios but it is placed at the end
of the front row above Studio E.
  • Setting HELM to zero causes the Helmholtz power supply to switch
off physically. You can also see the settings of the beam line power supply on the beam line PC in the windows for TCP/IP COMBI CONTROL or TCP/IP SETTINGS. Note: HELMH2 is the corresponding name in the device list on the beam line PC for the new GPD, HELMH was used for the old instrument.
  • The GOSSEN can be switched off only manually. The GOSSEN power supply
is located in the bottom part of rack No. 2.
  • There is no auxiliary field available, i.e. the coils have to be rotated for calibration
measurements when working in LF-mode.

The Helmholtz coils can be rotated by 90o in order to allow TF and LF measurements.

Latch lock (rotation locked (red), free (green)).
Latch lock (rotation locked (red), free (green)).
INSTRUCTIONS:
  • Make sure that the vacuum chamber used for the horizontal cryostats (CCR and Chemistry Cryostat) is not inserted.
  • Make sure that UP/DOWN detectors are not inserted.
  • Locate the latch lock on the spectrometer table on the beam entrance side and unlock it by rotating the latch lock bolt by 180o. In this way the lock bolt is lifted out of the hole in the supporting table of the coils which become now free for rotation into the desired orientation.
  • Watch the high-current cables and the cooling water tubes during rotation.
  • Use the latch lock to fix the coil orientation.
  • Insert the UP/DOWN detectors and the horizontal vacuum chamber if necessary.
Setting Helmholtz fields in transverse geometry causes down(up)ward deflection of the µ+(µ-)-beam depending on the muon momentum and field strength. This is compensated mainly by moving down(up) the collimator in order to direct the beam to the center of the coils. To get back the full beam intensity the whole spectrometer is moved up(down) and the S-detector is optimized as well.



The GPD-stepper-motors are controlled from DELTAT using Modify Devices... smngpd.... Please read the operating instructions carefully.


Calculator of settings for µ-Momenta 60...110MeV/c and TF < 6.5kG:


µ+


µ-		 µ-momentum :


magnetic field:


MeV/c


Gauss

GPD_VERTICAL
mm



COLLIMATOR
mm



DETECTOR
mm

LS
+0...+110mm



LS
-31...+40.5mm



LS
-26...+30mm

Note the ranges of the axis given by the limit switches ("LS" in the last column)!
Please refer to the PDF-table for a list of spectrometer settings at given magnetic fields and muon momenta as well as for the model functions and parameters. A little program for intermediate values is also available on the desktop of the experiment console (PC6011).


Please note: The motor drives are limitted as follows: 
					GPD_VERTICAL   0 ... 110mm
					COLLIMATOR   -31 ... +40.5mm
					DETECTOR     -26 ... +30mm


Intensity adjustment by slit FS81: Please leave the vertical slit drives allways at 200 (FS81-O = FS81-U = 200) and adjust intensity by FS81-L and FS81-R only. Please note: Uncontrolled movement of any motor-driven parts of the spectrometer can cause serious damage to the spectrometer and its components! Please check carefully by visual inspection inside the area if all parts you intend to move under remote control from the Studio can move freely. Please, switch off all motor drives not in use on the manual control board in the area.
A zero-field probe is available for the adjustment of the compensation coils. The field to be compensated in µE1 is significantly larger than 0.05 Gauss. The main contributions are:
  • The average background in the area is of the order of 0.5 Gauss.
  • The remaining field originating from
some ferromagnetic components of the spectrometer after application of a 5 kG Helmholtz field is also of the order of 0.5 Gauss. Note that this contribution is not constant but depends on the previous field settings.
  • The beamline magnets cause a field of 0.1 to 0.2 Gauss in the center of the
spectrometer depending on the beamline setting.
  • The passage of the crane causes a change of approx. 0.025 Gauss and may
be larger if the crane is used with large components.

-- Main.AlexAmato - 12 May 2014