Scans

Since most of the experiments at HASYLAB are performed in a scanning mode, two menus have been implemented that serve this purpose: The general scan menu and the q scan menu. They are explained in this chapter. Chapter 15 gives some examples.

Before we go to the details, let's look at the list of features that are common to both scans:

• Before a scan can be started, parameters have to set, e.g.: Start, stop, delta, etc.

  The scan parameters can be changed in the menu or they can be provided on the command line which calls the scan menu, e.g.: scan/energy/_start=8900.

  When a menu is exited, ONLINE stores the scan parameters in symbols. These symbols provide the defaults for subsequent menu calls. Qualifiers have higher precedence than the symbols.

• A scan is defined by three command files:
  • Before_type_scan is run once after the motors have been moved to their starting positions.

  • During_type_scan specifies what is to be done at each stop.

    Warning: This file must not contain block statements like if, for and witch because in contrast to other .gra files, it is understood as a plain sequence of commands. This command list is read once for a scan and it is executed without an interpretation of the meta verbs. Disk I/O is reduced and the first step of the command interpreter which consists of building up cross refrence tables, etc. is avoided. These measures have been taken in order to reduce the dead time of the measurements. If control statements are definitly needed in the during-file, an extra command file has to be started by an explicit run command.

  • After_type_scan is executed once when the scan is finished. It is used for writing data to disk, producing hard-copies and cleaning up the internal stoarage. An example for After_type_scan.gra can be found in

  Before~, During~ and after_type_scan are symbols that translate to file names. The string type has to be replaced by the scan type: “energy”, “q”, “mot25” etc. If none of these specific types is found, ONLINE searches for before_general_scan, during_general_scan and after_general_scan.

  Almost “everything” can be done with well tailored scan macros. However, many users are not willing to study the details of ONLINE before they start a scan and lukily, many measurements are simple enough to fit into a scheme that has been prepared in order to avoid any unnecessary programming effort: Fast scans. It is therefore recommended to execute scans in the fast scan mode whenever possible. See the section 8.1.11 for explanations of fast scans. Examples for other scan macros can be found in 8.1.10 and 15.12.

• A scan can be interrupted from the before- and during-macro by setting the symbol interrupt_scan to 1 (in fact it is sufficient to create the symbol). The file after_type_scan may use this symbol to decide whether a scan has come to a normal end. Interrupt_scan is set by ONLINE, if the scan has been interrupted for some other reason.

• It is possible to perform scans with a variable step width, qualifiers /var and /np=ival.

• Scans can be executed automatically (/exec) without passing control to the menu driver. The function gra_status() returns the completion status.

• Scans are logged in on_line.log.

• It is possible to use generic scan names consisting of a prefix which is followed by a number, e.g.: test_007. This mode is enabled by defining the symbol generic_scan_name before the scan menu is invoked. It is recommended to do it in /online_dir/exp_ini.exp, e.g.: generic_scan_name = test