E94-010 Polarized 3He Target NMR System

Last updated on October 22nd, 1998.

This page describes how to use safely the NMR system to measure the polarization of the polarized target in the best conditions. It should be read by every target operator before taking any shift. For more detailed information and if you have any question, contact Sebastien Incerti. Click on any subject below or simply read the entire page.

Experimental NMR Setup

The NMR experimental setup consists of four sets of coils surrounding the target cell :

NMR Electronics Setup

This schematic presents you the NMR electronics setup located in the Hall A Counting House. The NMR Rack is located on the right hand side of the Macintosh table, and the Controls rack, on the left side, close to the white board.

Angle Convention

This top view of the target system in the Hall explains you the way to define the angle between the beam line and the holding magnetic field :

The schematic also shows you the direction of the field created by each set of Helmholtz coils when their corresponding Kepco power supply displays a positive current (I>0 on the graph).

The Macintosh Personal Computer

A Macintosh Power PC 8100/80AV located in the counting House allows us to control entirely the polarization measurement procedure, using both the NMR and the EPR technics. The Macintosh communicates with the electronics exclusively through the General Purpose Interface Bus (GPIB), using the LabView v4.1 software. It has a memory of 96 Mb, and uses two hard disks (HD1 and HD2+HD3), one of which is split in two units (HD2 and HD3). The LabView programs used for the NMR and the EPR systems are stored in the partition HD3 and the NMR and EPR Data are saved on HD2. HD1 contains the Macintosh operating system and all the application programs.

Opening the NMR Master panel

Before starting any NMR measurement, you must open the NMR Master panel displayed below. To open the panel, simply click with the Mac mouse on the Apple icon located on the upper left corner of the Mac screen, hold the mouse button down to reveal the 'Apple menu', and choose NMR Master. the NMR Master panel should pop up.

This panel is a self-running Virtual Instrument (Vi), allowing you to choose between ten procedures, each of which is detailed on this page. To start a procedure, simply click on the coresponding circular Go button. You may also quit the Vi by clicking on the Exit button. We describe below in detail each procedure, in the chronological order they should be performed.

The Record NMR Settings panel

Before any NMR measurement, you must fill out the NMR Settings panel and print it. The print out should be kept in the target NMR logbook. Please, do not forget to enter your name in the blue Target Operator window, and the LAST RUN NUMBER in the data taking. Make also sure the target name is the right one. This sheet allows us to keep track of each important setting :

We describe here each setting :

Once you have filled out the form, simply click on the Print button to print it, and stick it in the target logbook. You will be asked to check if the Mac printer has enough paper (in its lower tray). The Vi will close automatically.

The NMR Measurement panel

This panel should be called after running the NMR Settings panel. It performs the AFP spin sweep of the polarized Helium 3 sample through the resonance. Once again, we remind you that the target must be in the pick-up coils position to perform the measurement. The default settings of the Vi are the following :

They cannot be changed. To launch the sweep, simply click on the Go button. Do not forget to write down on the NMR Settings printout the peak-to-peak voltage displayed on the RF Current monitor when the needle of the power amplifier goes to its optimal value. The Vi displays X and Y readouts of the lock-in. The phase should be set such that the signal is maximum in one channel, and minimum in the other one, as shown below. The UP signal is displayed in red, the DOWN one in blue, as a function of the holding field value. The shift between the up and down value reveals a delay in the coil response. Note the display of the lock-in phase. The X and Y data are automatically saved in the folder :

HD2:NMR Data:Helium

and are stamped with the date and time of the measurement. When you are done, print the screen using the key combination [Apple key]+[P]. A menu will pop-up and confirm to print on the Mac printer. After that, you may press the Exit button to close the Vi.

The Extract Polarization panel

This Vi should be used just after the AFP sweep. It doesn't need any particular tuning. Just run it by clicking on the Go button. You will first be asked if you want to create an R file, from previously saved X and Y data. R is simply the square root of the sum of the squares of X and of Y. Just choose No unless you have been notified to. Another window opens to ask you to choose your data file. Simply go to :

HD2:NMR Data:Helium

and choose the file (X or Y) corresponding to your last measurement and having the largest amplitude. The name of your data file will be displayed in the Data File window for confirmation. Then the Vi will start to fit the UP and DOWN signals. The result of the fit should appear after a few minutes. The Vi will give you the value of the polarization, using both calibration methods : the NMR Water calibration and the EPR calibration. The fitting function is the square root of a Lorentzian with a linear background and has five free parameters : height of the signal, full width at middle height, resonance field, background slope and background constant. Also are displayed the reduced chi square and the correlation matrix for the UP and DOWN signal.

Once again, print the display when you get the polarization values, using the [Apple key]+[P] combination. Press the Exit button to quit. You can also press again the Go button if you want to change the range of the fit (Min Field Up, Max Field Down, idem for DOWN) set by default to 25 G and 32 G. You may also specify your own guessed values of the inital fitting parameters in the grey arrays ; if so, do not forget to switch the two yellow buttons to the UP initial coefficients and DOWN initial coefficients positions.

The Read Saved NMR Data panel

This Vi should be used if you simply need to check the content of a saved NMR data file. Simply press the Go button to run the Vi ; a window will pop-up asking you to choose a data file. We remind you that the NMR data files are located in :

HD2:NMR Data:Helium

The name of your data file will be displayed in the Data File window for confirmation. When done, you can print the window using the key combination and quit the Vi by pressing the Exit button.

The Spin up & Lifetime panel

This Vi allows you to fit spin up or spin down. Spin up data are used to predict the saturation value of the target polarization, and spin down data allow us to extract the lifetime of the cell.

First, switch the red button to Spin up to fit spin up data or to Lifetime to fit spin down data. Then, simply enter the Time values (in minutes) in the first left hand side array named 'Time' and the corresponding Amplitude or Polarization values in the 'Amplitude' array. Then press GO to run the Vi. Your data points and the fit will be displayed together, and you will get an estimation of the maximum polarization the target can reach (Spin up window) or the cell lifetime (Lifetime window).

The Spin Up/Down Curve panel

This Vi should be used if you are asked to draw the spin-up/down curve of the target. It performs several AFP sweeps using the NMR Measurement Vi, their total number being specified by the control button 'Total number of sweeps' regularly spaced in time, as specified by the 'Delay between ramps (min)' control. When you press Go, it will display the evolution in time of the height of the R UP and DOWN lock-in signals. We remind you that the target must remain between the pick-up coils if you decide to run that Vi. Once done, you may print it with [Apple]+[P] and quit by pressing Exit.

The spin-up curves are saved in the folder :

HD2:NMR Data:Spin_up

The Holding Field Rotation panel

The field rotation panel should be used to switch from the parallel (perpendicular) to the transverse (parallel) configuration. Refering to the angle convention, 0 deg points downstream and 270 deg points at 90 deg from 0 deg on the electron arm side. Before running the Vi, you need to select the rotation you need to perform :

When you have made your choice, simply press the Go button to launch the Vi. You will be asked to check that the Rotation Function Generator (Wavetek, with the sticker FG R) is not on Standby mode (yellow steady led). If it is on Standby mode, simply press the horizontal arrow keys on the right to go to the square burst shape. Than the Vi starts. The process is slow and can take up to 15 minutes. You will be able to follow the field rotation on the virtual compass. When done, simply click on the Exit button to quit.

WARNING ! : you must absolutely be in the PARALLEL configuration to measure the polarization of the target using the NMR method !!

The Pick-up Coils Q Curve panel

This Vi allows you to check the response of the pick-up coils circuit using a small excitation loop, glued on the side of the RF coils mount. When you run the Vi by pressing the Go button, you will be asked to disconnect the RF coils cable coming out of the RF function generator (Hewlett Packard with the RF Excitation sticker) and to connect the Q curve cable, going to the small loop. Then the Vi will sweep the excitation frequency and the lock-in will read the pick-up coils response. The sweep process is slow and may take as much as 20 minutes. The frequency is displayed in the Frequency window, in kHz. When done, the resonance curve will appear on the screen and will be saved in the directory :

HD2:NMR Data:Q Curves

When done, do not forget to switch the cables back ! You may print the window using the key combination [Apple]+[P] and quit using the Exit button.

The Current Monitoring panel

The Current Monitoring Vi is used to check the values of the current circulating in the Helmholtz coils. For now, only the large coils current can be monitored. An amperemeter is connected to the large coils in the Hall and is read by a GPIB-ENET box. If you run the Vi by pressing the Go button, the GPIB-ENET box will transmit by default 10 values of the current as shown below. You may change the number of values read using the 'Number of measurements' control.

When done, the data will be saved in the directory :

HD2:NMR Data:Current

You may print the window using the key combination [Apple]+[P] and quit using the Exit button.

The Emergency Shut Down panel

The Emergency Show Down Vi should be used only in case of extreme emergency. By pressing the Go button, you will turn off the RF excitation field, and kill the main holding field. It should be use only to turn off the current in the main Helmholz coils and in the RF coils, when intervention on the coils is required. The target operator should be aware that the polarization of the target may be dramatically damaged if this Vi is used. Press the Exit button if you decide not to run that Vi at the very last minute.

Quitting the NMR Master panel

Simply go back to the NMR MAster panel and click on the small red button displayed near the upper left hand corner of the Vi. The Vi will stop and you can close the window by clicking on the small grey square in the very upper left corner of the window.

To Summarize : Basic Procedures


A NMR mesurement can be performed only in the LONGITUDINAL configuration. NMR measurements are usually performed :

To perform a measurement :


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