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Subsections

The Harp GUI

Overview

The Harp GUI controls the harp device installed before the tagger magnet and has two main functions :

1.: Putting the radiator into the beam : The incident electron beam on the radiator will create the secondary photon beam used by CLAS.
2.: Scanning the beam profile : Two wires at 90^o relative to each other are mounted on the device. When a scan is performed, these two wires cross the beam and the monitoring of the counting rate in the beam-line detectors allows a reconstruction of the beam profile.

How to launch it

Launching the GUI is very easy once the GUI_launch GUI (see section 4.2) is up. Simply click on the top left popup menu and select 'harp'. A window (Figure 4.2) should shortly pop-up on the screen.

**Figure 4.2:** The Harp GUI window
$\begin{figure} \epsfxsize=15cm \centerline{ \epsfbox {harp_gui.eps} }\end{figure}$

How to use it

What to look at ?

Basically nothing. But there are some important thing to be sure about.

Be sure that the harp is outside of the beam before bringing the beam in or before establishing or releasing vacuum. Remember that before performing a harp scan you have to turn the high voltage for the drift chambers off . The amount of particles generated will make them trip off otherwise. To know if the Harp is really outside of the beam, look at the grey area just above the Position feed-back. It should display ``retracted Position''. If not, hit the ``retract'' button (on the main button bar), the string should appear shortly.

Photon beam run

The current target position should be surrounded by a blue box. Remember that the actual position of the harp can only be estimated by step counts from the retracted switch reference. In doubt, click again the button, the harp will move back to the switch, re-calibrate itself and come back.

What to do ?

Perform a harp scan each time the beam conditions have changed and periodically about one time per day. Remember that before performing a harp scan you have to turn the high voltage for the drift chambers off .

Many buttons and some displays

The GUI display 4.2 is basically composed of 3 distinct sections. One for the control of the harp position, one for the display, and one for the scans.

Table 4.3: Targets nature and position
Target Position (mm/steps) Target radiation lengths

A 70.48/55500 100*10^-6

B 86.80/68350 50*10^-6

C 100.77/79350 5*10^-6D

D 115.23/90731 blank

**Table 4.3:** Targets nature and position
Target	Position (mm/steps)	Target radiation lengths
A	70.48/55500	100*10^-6
B	86.80/68350	50*10^-6
C	100.77/79350	5*10^-6D
D	115.23/90731	blank

Table 4.4: The Harp Position Control Panel
[1] 2l|The Harp Position Control Panel

Retract The retract button puts the Harp outside of the beam. When clicked, the string ``Retracted Position'' should appear in the message box.

A B C D These buttons put the labeled target into the beam.

[1.a] A lighted target tells that the harp is on that position. A flashing target indicates that the system is in the process of reaching that position.

[1.b] The ``<'' and ``>'' buttons move the Harp 0.5 mm respectively out and into the beam.

**Table 4.4:** The Harp Position Control Panel
[1]	2l\|The Harp Position Control Panel
	Retract	The retract button puts the Harp outside of the beam. When clicked, the string ``Retracted Position'' should appear in the message box.
	A B C D	These buttons put the labeled target into the beam.
	[1.a]	A lighted target tells that the harp is on that position. A flashing target indicates that the system is in the process of reaching that position.
	[1.b]	The ``<'' and ``>'' buttons move the Harp 0.5 mm respectively out and into the beam.

Table 4.5: The Harp Display Panel
[2] 2l|The Display Panel

[2.a] The Switches message box displays either :

Nothing
The Harp is not on a limit switch.
Retracted Position
The Harp is outside of the beam.
Fully Inserted Position
The harp is completely into the beam. It corresponds to the ``D'' target position.

[2.b] The position of the Harp in millimeters (See Table 4.3) . Warning, going into the beam negative position values.

2 [2.c] This button is to be clicked when the system seems not to respond to the commands. It exits as nicely as possible every loop the SNL program could be locked in.

[2.d] This popup-menu will allow you to open the help window or the ``expert'' GUI window (the last one is not documented).

**Table 4.5:** The Harp Display Panel
[2]	2l\|The Display Panel
	[2.a]	The Switches message box displays either : Nothing The Harp is not on a limit switch. Retracted Position The Harp is outside of the beam. Fully Inserted Position The harp is completely into the beam. It corresponds to the ``D'' target position.
	[2.b]	The position of the Harp in millimeters (See Table 4.3) . Warning, going into the beam negative position values.
2	[2.c]	This button is to be clicked when the system seems not to respond to the commands. It exits as nicely as possible every loop the SNL program could be locked in.
	[2.d]	This popup-menu will allow you to open the help window or the ``expert'' GUI window (the last one is not documented).

Table 4.6: The Harp Scan Window
[3] 2l|The Scan Panel

It contains every controls to perform a beam scan. It is described in the chapter 5.

**Table 4.6:** The Harp Scan Window
[3]	2l\|The Scan Panel
		It contains every controls to perform a beam scan. It is described in the chapter 5.

Table 4.7: The Harp Warning Message Area
[4] 2l|The Warning Message Panel

It will display ``WARNING : It's very likely that the motor is NOT moving !!'' if the Harp switch does not fire when it should. It means that we are probably not where we should be, letting us inferring that the motor is not responding to commands.

**Table 4.7:** The Harp Warning Message Area
[4]	2l\|The Warning Message Panel
		It will display ``WARNING : It's very likely that the motor is NOT moving !!'' if the Harp switch does not fire when it should. It means that we are probably not where we should be, letting us inferring that the motor is not responding to commands.

Hardware and Software

The harp GUI uses the records and hardware detailed in 3.2.3, it runs in the classc3 IOC (see Figure 2.9).

The harp itself is a piece of aluminum about 10 inches long, (Figure 4.3), that supports the radiators and the wires for the scan. This object is moved in and out of the beam by a stepper motor under control of the Harp GUI.

**Figure 4.3:** Harp : targets and wires
$\begin{figure} \centerline{ \epsfbox {harp.eps} }\end{figure}$

The software for the scans is detailed in chapter 5, the routine that moves the Harp is simple. Anyhow, it provides a mechanism to calibrate the harp when it hits the ``fully retracted'' switch, and a fancy flashing icon when the harp is moving.

File informations

The application is under CVS monitoring and is known as harp. To check it out, type cvs checkout harp. Then make it by typing make in the main directory (EPICS compilation variables and flags must be set, of course).

There are several files of some interest :

harp/db/harp.db: is the database file. Consist mainly of the declaration of the Motor Record, plus some simple records to manage the display and the scans.
harp/medm/harp.adl: is the main GUI, written with medm.
harp/medm/harp_expert.adl: : A second GUI with almost every field of the motor record displayed. It can be used to perform sophisticated actions that the standard GUI cannot do, or as a tool to debug the software.
harp/src/harp_move.st: : the SNL routines for standard operation management.
harp/src/harp_scans.st: : the SNL routines for scans, see chapter 5.

Next: The Scaler GUI Up: Beamline EPICS applications Previous: The GUI_launch GUI

Garp patois@cebaf.gov