E94-010 HARP Scan E94-010 HARP Scan

HARP Scan

by Mark Jones and Arun Saha
Last updated on 5/29/2002

NOTE

Never run harps with more than 5 microA CW
This is an invasive measurement

Data taking STEPS

On the HAC computer from Hall A Main Menu click on blue menu near ARC Scanners and select Scanner Control Screen. This wiindow will pop up:
Harp 5 (1HA1H03A ) is next to bpmA and harp 6 (1HA1H03B) is next to bpmB on the beam line
Harp 5 and 6 are SEM devices and do not need PM tube HV
The above is after a scan of the harp for an energy measurement. The following describes how to set it up for just using harps 5 and 6.
Run with conditions (set these conditions or the analysis code has trouble discriminating signal from noise):

Mode: CW
Current: 5.0 microA
Arc Tuning: ACHRO
Command file name: data56 ( The example above is for arc energy measurement where they used harps 1,2,3 and 4)
Make sure to hit the return button in the entry space for the value to register

Don't worry about the Running command . This is just feed back on the commands that have been carried out after START button is pushed.
When everything is ready, click on the START button
For command data56, the harp #5 will first do a forward scan and at the end a red profile with 3 peaks will show up. Then harp #6 will do a forward scan. Then harp#5 will proceed to do a return scan and a green profile will show up. Both the profiles should overlap each other. Then harp #6 will do a return scan. You can repeat the scans as many times as you like, just save the good ones for later analysis. The harp consist of three wires : the first into the beam is a vertical wire ( this will give the horizontal width of the beam directly), the next two are at 45 degrees to the vertical ( this will give a combination and x and y width. If it is a circular beam of radius A then width of the next two peaks will be A*sqrt(2) .
A good scan will have the signal strength of the peaks between 5 and 10 volts. The gain of the signal can be adjusted by selected an amplification from the blue box just below the plot. Typically an amplification factor of 4 will work, but it is trial and error to find the right factor.. If signal is saturated a red spot will appear next to the plot and this flags a bad run.
Once you have conditions for a good run. If you want to calibrate espace parameters: Start of CODA so that we have a data on tape with the BPM information. Then start a HARP run. Once you can a good run click on the Save button and stop CODA run.

Analysis

On the NCD xterm hiresxt20 using the Accerlerator Main Screen:
One click on the blue menu with the ! in it next to "Hall A" and select "Hall A Scanners and Arc Intergral Analysis" and a window pops up
Click on Run arc-scan and the window pops up:
First it only ask for the run number. Type in the Run number and hit return. Then after a few seconds , A the line for plot option appears. Click on button next to "last".
The following plot will appear.
The top plot is for harp 6 and the bottom plot is for harp 5. The x axis is microns. The harp consist of three wires : the first into the beam is a vertical wire ( this will give the horizontal width of the beam directly), the next two are at 45 degrees to the vertical ( this will give a combination and x and y width. If it is a circular beam of radius A then width of the next two peaks will be A*sqrt(2) . The peak at the far right is for the vertical wire , so its width is directly the horizontal beam width. The two peaks ( countin from the left ) are from the wires at 45 degrees and the width is a convolution and the x and y width of the beam. If the beam is very small in y so y width << x width then the widths of all the peak will be the same. If the beam is very small in x so y width >> x width the width will be the y width of the beam. In general the analysis program assumes the beam spot is an ellipse and extracts the tilt angle and the size of the major and minor axis and the y width of the beam is extracted from these numbers.
For the quality of the data one should zoom in on the picture and check that the back and forth scans lie on top of each other. If the beam moved during the scan then there might be two peaks: the first for when wires went forward and then second when they were retracted. Another possible problem is that fast feedback is off and the peaks look choppy.
Click on exit button in HIGZ window. Then a window pops up with the results.
These results are not corrected for the offset of the harp position relative to beam line. Click on the OK button and then the quit button in the ARC-SCAN window. If one to see details of the analysis click on the DATA1 and DATA2 buttons that appear. Results for DATA1 (for this run the proper info of beam current, tune was not set when taking data)
The results for DATA2:
One interest in looking at DATA2 window is if the raster is on then one can get the baseline width of the raster. For raster: Wire #3 is the vertical wire and so the average peak base width give the baseline width of the raster in the x direction. For wires #1 and #2 the average peak base width should agree and it is the x width + the y width of the raster.
To get the corrected position click on the Correct Arc-Scan button in the Beam Energy Measurement window, then the window
enter the run number and hit return and then the window shows the survey harp offset positions :
Select the latest survey ( in this case #24).
Then the results are shown
The run number is automatically updated to the next run. Quit by closing the window.