How to set up the FRS - From SIS extraction of primary beam to isotope identification

Control Console
Control programs
Before first beam (calculation of settings, online analysis)
Prepare detectors
Primary beam on production target
Primary beam through FRS
Energy change
Fragments through FRS
Identification
Usual problems

This page was last modified by Helmut Weick, 14th Nov 2019, contact h.weick (at) gsi.de,   Imprint (Impressum), Privacy Policy (Datenschutzerklärung)





Control Console

Console
The operation programs run on three linux computers on the console: TCL1034, TCL1035 and TCL1051, each one feeds three screens. They are inside the accelerator network (acc.gsi.de) and can connect only to GSI computers not to the outside (e.g. web pages).

The FRS is controlled and monitored from the console in the FRS Messhütte. Test detectors offline and check computers and programs on the console. Test magnets by applying some current to them. Below: Operating console in Messhütte.

Right click in empty background, choose "GSI Applications" in small menue and start the "CSOAP launcher". All other programs (apps) can be started from the launcher panel.
For access to the general asl filesystem it is possible to start the launcher (and from this other apps) directly from the asl74x machines using the scheidenb account (>launcher).
 


Prepare pattern
A running scheme of the accelerators up to the target and beam dump is called pattern.
First such a pattern must be created and scheduled with the app "Scheduling".
Detailed instruction on how to create a pattern. Of course it will work only if the corresponding ion source and UNILAC machine is available.

Scheduling shows all available patterns and in a panel below all scheduled patterns.
Right click on one box and chose "schedule"from context menue.
After a short waiting time it should appear in the lower panel as scheduled.

To really send the data to LSA do not forget to click on supply .

To modify the patterns select [edit] in the scheduling app. You can change the SIS18 exit energy, extraction time, and even the type of ion injection and the SIS18 injection energy. But the latter two you better do not touch.
[view] opens a window to indicate schematically the chain of accelerator and beamline devices used in this pattern.
In the example below the cycle time of the pattern is 3.4s. Even though the entered SIS energy is 1000.0 MeV/u the beam to the target shows only 999.94 MeV/u, because so far the displayed energy is taken from the end of the FRS and the energy loss in the non-removable Ti foil window is already included.


Scheduling is not enough the pattern must also be activated in the "BSS control" app (beam scheduling system).
The [I/O] button on the left activates or stops a pattern, and the[R] or [NR] button makes a beam request.beam. Be careful right now all users have rights to manipulate any other pattern as well. Ask in HKR whether it is ok to activate a pattern before.

The App "What's running" gives a graphical representation of the different running patterns.
In the example below you can see that HFS just got one spill.

A useful feature is the "Screenshot" app. You can select of which screen and the resulting pictures are available on a GSI internal webserver (https://clipboard.acc.gsi.de/dav/screenshot-app/screenshots).




Control Programs

Device_Control

To move drives and check status of devices The standard tool is the app "device_control".
Select the beamline (context)
   0.) click link for pattern
   1.) select the pattern (use filter e.g. *HFS*)
   2.) select the context (same name with .C1),
   3.) confirm with [ok]



Devices in green are ok while red means not functioning properly. In the example the selecton of devices was first reduced to display only quadrupoles (quad simbols in dark grey), then the the TS2QT11 was marked and by clicking [status] the detailed status of GTS2QT11 was openend. It is simply turned off while other properties like cooling water, temperature are ok (green). Clicking [On] would turn it on. [Reset] clears interlocks after failures. [Set/Act. magnets] displays values of magnets.

Tiny differences in symbols indicate the status with respect to the beam: out of beam = , half way in =, in beam = .
For devices with a set and and readout value you can monitor the relative deviation graphically. Select the "device type", for example only dipoles, and choose the scale in percent.
To move a drive click on the symbol. A pop-up window will appear in which you can type the wanted position.

Detailed symbols for drives:
valve closed: ,   valve open
drive in: ,   drive or slit out:,   slit, partially open:

Symbols for magnets:



Move drives
In the table click on the stepper motor device. In the pop-up window enter the wanted position and press [Drive].
Then the blue dashed line will display the goal value and the black line the moving drive position.
 Arrow button can alternatively be used to make input or to select the end positions.



Show magnet values
Deviations of set values from actual values are displayed graphically in device_control. You can choose the scale in % on the right.
By clicking on [Set/Act. magnets] you can get a full list 


Improved display apps - DRIVESTAT
For a better overview on the whole FRS  DRIVESTAT is used to display of all drive inserts.
The list of all devices is grouped in color according to focal planes.
  0 = out of beam, 1 = in beam, 2 = moving
This status is also stored periodically or on demand also for printing the status on paper.


Improved display apps - FMGSTAT
For a better overview on all FRS magnets FMGSTAT is used.
It displays the set value from data base (DB) in BL or B'L, the set current on the device, the measured current, and for dipoles the Hall probes readout converted to B.
The DC read out even works without a running pattern, for example to monitor the precycling of magnets, select "Update DC Value".

The Hall probes in the dipole magnets are used for determination of the Brho values used in the fragment identification. From B and the calibrated effective radius follows Brho. The B values are displayed by the FMGSTAT program.



Load and save magnet data sets
A detailed description on how to load and save optics data sets and magnet settings is given in the separate FRS ion optics manual.

Beamline settings
The app "ParamModi" is used to adjust FRS magnets. For the FRS basic setup only the option "ParamModi_resident" with the two folders "beamline matter" and "extraction line" are important. Also here first the right pattern, context, and transfer have to be selected.

The folder "Extraction line" contains the magnet values for the selected transfer. The integrated field values are given normalized to set Brho (KL, K'L or K''L), which means they will not change for different Brho settings. They are so-called "top level values".
For dipoles the default deflection angle is fixed, but a small additional HKICK (in mrad) can be added.

The folder "FRS operation" contains the Brho settings for each zone of FRS.
You can set the Brho in the overwrite Brho fields. The magnet values will be scaled accordingly. Afterwards [Send to hardware] in case of an error (e.g. out of range) the input will be refused.

When used for first time activate the input field, and in the input mask first right click in the upper left corner to create an input line. Only then you can enter the value.

Right click in corner to add input row   , then enter number  .



The folder "SFRS Benchmark" contains all matter in the beamline.
You can select the matter in the beamline piece by piece (set [x]), also including drive positions to calculate the energy loss and from this the Brho value.
Also changes in the type of nuclide or ion charge state can be entered by marking the production flag [x], under development.


More details on the set values can be checked in the "Trim" folder of ParamModi. After selection of the pattern, context, transfer, parameter group, type, and name (red circles), it can display normalized strengths (KL), BL and currents I for each magnet. Similar can be done also for beam properties (Brho, atomic mass, Z, q, m/q - with ion mass, and energy).

All these values can be trimmed in the value or by an offset. A nice feature but also dangerous as these offsets will remain forever in this pattern and are not easy to find again, even with a history of trims. Click the [Trim] button and enter values (green circles). There are two values for start and end of a ramp, but for FRS both should be the same. Afterwards press [Apply]. A change in KL will also change the BL and current (KL - > BL -> I), but no calculation in the opposite direction is done.


FMGSKAL

FMGSKAL runs a special sequence of settings outside of a pattern.When the Brho of a FRS section changes the magnet values must be scaled accordingly. But one cannot just change the current directly because of the hysteresis of the iron. Instead a ramping procedure (called pre-cycling) must be performed to reach the BL or B'L in the same way as during the initial magnet calibration. It means first go to maximum current, wait, go to zero current, back to maximum, back to zero, and wait for input from other programs (e.g. parammodi) for new set value. This way the new set value is always approached from below.
In FMGSKAL you can select for which FRS sections you want to do this and whether for all magnets or only for dipoles. The sequence is started by clicking [Precycle].
When going up this is in principle not needed as one stays in the order of approaching values only from below. Also for very small scaling steps it is not needed.

Procedure:

  1. stop pattern (stop request and set FRS interlock)
  2. set new Brho in param_modi
  3. precycle (monitor progress in MGSTAT, ends at zero current)
  4. remove interlock and request beam, pattern will start and set magnets to new set value


Another special ramping sequence is foreseen for Hall probe calibration (HP Cal).


To send data to hardware an active pattern is required, to avoid beam coming in a not ready setting and extra FRS interlock can be used.
This is created by a switch on a box on the console, "BEAM ON" = no interlock, "BEAM OFF" = interlock.



Switch FRS branches

The FRS has three possible destinations for beam. Depending on this some power supplies must be connected to different magnets.

To change the connection:
  1. Stop patterns.
  2. Select chain with magnets to decouple, switch relevant magnets off, wait 20s.
  3. Select chain with magnets to couple to, switch relevant magnets on.
  4. Check connection
  5. Turn on pattern for new chain selected
Sometimes the dipole hangs a bit, then try again. An alternative is to do it by hand on power supplies.

The green "switch enabled" LED should be off and the two red LEDs should be on in the correct row.
Left: connected to HFSMU1, right: in the middle of switching.

Power supplies used for different magnets must be switched  correctly:
The green "switch enabled" LED should be off and the two red LEDs should be on in the correct row.
Left: ok for HFSMU1, right: in the middle of switching.

 
    


Open vacuum valves

The valves are controlled from the device control program in the line "Drive" with device type "Gate valve".
Click on the gate valve and in the pop-up window you can see the actual status as not available for selection.
Be careful, a green icon does not mean valve open, it just means valve is ok. You must check in the pop-up window.


For more detailed analysis you can check the [status] of the valve in the bottom row.
In the example below both pressure readings are too high and one cannot open the valve due to an interlock.


To display pressure values, switch the mode from "Drive" to "Measure".
Then click on the gate valve and update the readout.

 

  


An independent check of the vacuum conditions is with the FRS pressure gauges displayed on the back row of the electronics room.



RunSheet Manager

An electronic version of the run sheets is available. In this entries on drive position and magnets can be filled automatically.
Select "Import Data" -> "Default" and "Import" to read the last written FMGSTAT and DRIVESTAT status file.
The corresponding fields in the file sheet will be filled.



Before first Beam

Calculation of settings

Prepare a data set in ATIMA, MOCADI or LISE with layers of matter (targets, detectors, windows and degraders) and optics mode.
For the primary beam ATIMA alone is enough (WWW C-Atima). For calculation with optics and fragments LISE++ or MOCADI can be used. LISE++ runs on SFSPC001 in the FRS console. picture: screenshot of LISE++.


Safety check

“Sicherheitsabnahme” safety check done by safety department of GSI. Before the participants need to have a special FRS safety instruction and confirm this by signing the safety book. The experiment has to be set up, dangerous parts removed, HV clearly labeled and more.

Online Analysis

start online analysis program GO4

picture: screenshot of running Go4

 


Prepare Detectors

Apply HV to current grids, MWPCs and TPC detectors.
We have three CAEN HV crates. Most high voltages are supplied by a CAEN SY4527.
It must be switched on (left side) and the switch on the right be set to "local",

It is controlled by a web interface. URL=140.181.81.21, user="hvpsu_user", for password ask expert.
From the web page you can start the "Channels Controller" Java application to set the voltages.

The Java application is a big table in which you can click and type.


A few voltages for scintillators (+3.5 kV) come from an old CAEN crate which is controlled by a VT320 terminal.


In addition we have a mobile CAEN crate with controls directly on a built in PC (password ask expert)



Test MWPCs

Generate a pulse with the pulse generator and distribute it to all MWPCs anodes. With an oscilloscope you can check the cathode signals. In the analysis raw spectra you will see sharp peaks.
The MWPCs in vacuum can also be tested with an electron source mounted near the out position of these detectors. You will obtain a broad spectrum centered in the upper left corner of the detector. Inserting the detector in the beam line will move them away from the source. Details on MWPCs here.

High voltages for the anodes are set in the CAEN HV-crate with a Java app for input run from a web browser (e.g. on SFSPC002), see far below.

The MWPC signals (anodes and cathodes) go to 3 MESYTEC discriminator modules.
They can be controlled from a GUI by typing the alias "mwpc_cfd" on lxi logged in as profi.
All incoming polarities will be negative, usually gain =1 is enough. A typical threshold is 25mV.
You can load and save settings, at program start the last saved settings will be loaded.


 


Close cave

Close the doors of the caves, call HKR (tel. 2222) or radiation protection (PSA 12-5291-xxxx) and ask to switch to controlled access mode.

Radiation safety person will come and check the areas called NE4 and NE7.
Do this before 22:00h otherwise they have to come from at home as "Rufbereitschaft" !
Wait until you hear the horn and the red light at the gate is on.

Switch on magnets

Switch on power supplies in the hall by turning the two keys. Otherwise, hands off the power supplies (dangerous high current !). You can ask magnet group or Karl-Heinz Behr / Philipp Schwarz to do this.




Primary beam on production target

Close slits for safety

With the device control program close the slits in front of S1 (GTS3DS2H) and insert the beam plug (GTS3SV2).
The slits are driven by step motors and can be found in the line "Drive", device type "Stepper motor" of DeviceControl.
Left click on point, a dialog box will open, click on [<|] or [>|] and "Drive" for both sides until the slits are closed.

Load optics setting for FRS

a) from data base reference
Each beamline has at least one reference data set, which is chosen for a beamline (chain) when the accelerator pattern is created. Changing this mode means a modification of the pattern (see above).
Otherwise a theory data set can be loaded into ParamModi.< Some theory data sets are stored on the MOCADI download page.

b) from saved setting
As the FRS has many modes ParamModi will be used to load and store the many different settings. Use "File -> Export" or "Import" function of ParamModi. The data sets also contain a Brho definition for each section (zone).
Some theory data sets are stored on the MOCADI download page. They can be used after download to the >/home/rifr/scheidenb/lnx/parammodi directory.


c) from old saved setting
Old data sets still saved in the IBHS format can be converted to the new ParamModi format. However, in old times the data sets lacked the information on the actual Brho setting. Here the old logbook with the printout of keyword, assumed Brho values together with the handwritten real Brho values are the only help. IBHS save files are on asl741.acc.gsi.de, to find files use "grep " on the long list.

The old IBHS data sets can be converted to the new ParamModi format with the script
/home/rifr/scheidenb/lnx/ibhs/ibhs2parammodi.pl on aslxxx.

Similar is possible from older files saved with SRMAG. The corresponding script is called srmag2parammodi.pl .
All old saved files were copied to >/home/rifr/scheidenb/lnx/ibhs/srmag . A good method is to first load the old data set with the same Brho values as used at that time, then you can nicely compare all the single magnet values for deviations. 

Check Magnet Values

Make print out of magnets status (FMGSTAT) and wait until the status appears on the printer.
Do the magnets have roughly correct values? I.e. the first two magnets should deflect by 7.5°, then the BL should be 7.5° *pi/180° * Brho. If it is not close to this, try again to set the values.

You can also run a GICOSYBACK or Import into MIRKO to check whether the beam really looks proper.

Request Beam

Ask operators to activate the accelerator pattern with the beam you want and request the beam in the BSS app (phone 2222).

Beam position on target

Apply voltage to the profile grid detectors CG01, CG02, .., CG82 with the help of the HV terminals in electronics room. Usual are 80V. This is no amplification it is just to collect slow electrons.
Insert current grids at the target called CG01, CG02 or GTS1DG5 and GTS2DG2, respectively. Use "device_control" for the step motor control move them to position 0.0 (not inside end). They move very slowly. All other profile grids have faster pressure drives with only in or out positions.

Start app profile grid

Click on [1.#]  and select the pattern, the chain, the profile frid and where to display the result on the screen. You can select up to three profile grids at once.

              

You can move the detector in beam also from the profile grid app (buttons IN, OUT). For stepper motors be sure that the position really is 0.0mm.
In the app only 0% (=OUT), and 100% (=IN) are displayed.

Be sure you see up to recent measurements by checking the last update time.
[auto] gain adjustement should work, otherwise select [man] and click arrow buttons. Be patient update is slow. Updates will come only with a running pattern. In test mode you can check the amplifiers for each wire. There is also an option for automatic scale adjustment but this takes a few iterations.



mm

Intensity / spill structure

Start app "Lassie-spill", it can display time dependent values of beam intensity or synchrotron ramps. Choose a detector from the long list of "available devices", for example the FRS SEETRAM is named TS1DI4S. It will appear in the shorter list of "selected devices". Marking a detector shows more information and allows you to set the range of the current digitizer (100pA ..100mA). They wires can stand up to 10^9 Uranium ions (tested).
For correct data supply chose the "Process ID", you can find it for your beam within a pattern with the help of the snoop tool. Click on [Add] to add the graph to the shown histograms of intensity against time. Next to the graph the total intensity per spill is displayed. For calibration a fit to old experiment data is used (T. Brohm program).
When you mark the name of the detector (here GST1DI4S = Seetram) you can make more settings, like for example choose the sensitivity range.


The Seetram (secondary electron transmission monitor) is used to measure the intensity of intensive beams. There is a default calibration factor formula in the DI program which in only good to 10-20%. But you can also enter your own factor after a more careful calibration, see (here).
The Seetram sensitivity, has levels from 1 to 7, which correspond to currents of 100uA - 100pA. They can be selected in the Lassie-spill program.

Watch the number of particles per spill shown in Lassie program measured with the SEETRAM and compare the number of particles on SEETRAM with those in SIS. The SIS current transformer can also be displayed in Lassie. For longer runs it has to be at least 70% !

If necessary attenuate / increase the beam intensity (ask operators).


Adjust beam position

In the app "profile grids" look at beam position on the two current grids CG01, CG02 (GTS1DG5, GTS2DG2)
The beam should be as narrow and centered as in the examples from the old program shown below.

 

For centering a small extra script can be used which recalcultes the optics and based on the readout of CGs.

Usage:

1.)    Read beam position on current grids

2.)    Export magnet setting from ParamModi to directory ~/center-beam

3.)    >center-beam.pl  <x-CG01>  <x-CG02>  <y-CG01>  <y-CG02> 

4.)    Import new steerer settings into ParamModi (file center-beam.txt) and [set]

5.)    Check new beam position

Example screenshot below, link to pdf file with full documentation.



The similar function in MIRKO ("gerade_legen") is only available on WINDOWS computers. After a transfer of the parammodi export file (command >copy-status) MIRKO can also read the values and from the input of old an dnew position compute a new parammodi import file. With another copy-status it will be copied back to asl, from where you can read it with parammodi.

Or there is an EXCEL sheet running on SFSPC001 (gerade-legen_x+x_2019.xlsx) in which you enter the positions and old HKICK/VKICK values of dipole or steerer correctors to get the new dipole/steerer values. This program works with fixed coefficients for magnification and dispersions pecalculated for one of the 5 main focusing modes.






Primary beam through FRS

Adjust the intensity

To work with primary beam on particle detectors the beam has to be strongly attenuated, until no or almost no counts on SEETRAM. Then insert the scintillator SC01 (GTS2DI1_S at position 0.0mm + GTS2DI1_P in) and switch on HV, check signal height and CFD threshold, then intensity on scaler, ask for fine adjustment of intensity.


Scale FRS to correct Brho

An old loaded setting usually still is for a different Brho of the beam and the FRS magnets have to be adjusted. For this the Brho values have to be redefined in ParamModi, and all the magnets in the corresponding section will be scaled accordingly.
However, due to the hysteresis of the magnet iron only scaling is not good enough, and instead a complete cycle as used in the initial calibration of the magnets should be used. This precycling is executed by the program FMGSKAL.

The safe way is:
 1.) Remove beam request (in BSS), insert Beam Plug (DeviceControl)
 2.) Switch FRS interlock box to "No beam"
 3.) Precyle magnets with FMGSKAL, this ends with zero current.
 4.) Set new magnet values with ParamModi, check new data base values in FMGSTAT
 5.) Request beam again and check actual values (current and Hall probes) in FMGSTAT
 6.) Remove beam plug, switch FRS interlock box to "beam on"


Beam to S1

Open slits and remove beam plug (GTS3SV2). Look at MW11 at S1 in GO4 online analysis program and adjust the sum conditions. Measure beam position (x), x can be off because of a mismatch in Brho.

Centering:
To center beam with position x at S1 add a HKICK to the theory dipole angle (30°). The angle can be calculated using the dispersion coefficient (x,δ)TA-S1.
HKICK = xS1 / (x,δ)TA-S1  * 30°/180° *pi *1000 mrad
Here x is with orientation like in the FRS Go4, opposite to normal ion-optics.
The fact that the beam is centered along with the measured B-field defines the effective radius of the first dipole.

 

Beam to S2, S3, S4

Do the same at S2, now scale only S1-S4 (or S6,S8), use dispersion coefficient (x,δ)S1-S2.
Again the same for S3, S4, if there is not removable matter at S4 try to calculate the Brho afterwards as precise as possible.

 

Save magnet setting

This is already something and should be saved with ParamModi.
Choose File -> Export -> and save the file in ~/parammodi/.. . By default the name is composed of the start and end (SIS18_RING .. HFS) and the date and time). It contains all the normalized magnet values and the Brho settings.
Run the script "comment-status" to add a comment and keyword to the latest saved file.
Print a magnet status, glue it into the logbook and write the keyword next to it along with the Brhos and beam type for this setting.

 

Calibrate target and degraders

Insert a target, check the number of the target according on the list on the FRS web page for TS1ET5 or TS2ET2 and enter the number in parammodi to move the drive to the right position. Alternative, use device control and enter position of each drive in mm.
Then center the beam again at S1, this defines the effective target thickness. 

You can calculate backwards in ATIMA to see what the real or effective target thickness is.

Insert degrader at S1 or S2 and again center the beam in the following of final focal plane. This defines the effective degrader thickness. With variable degraders it is better and easier to adjust the thickness until the beam is centered.

Save the magnet setting with export in paramodi.


Energy Change

In principle a change of SIS can be done in the pattern editor, which takes less than a minute. But a phone call to HKR is not a bad idea as often it will require SIS retuning.
For the section SIS-TA the new Brho will be calculated directly by ParamModi and with thw new Brho also the B-fields of  magnets will be adjusted.
For the following sections Brho is set by the overwrite value. Before you enter the new Brho stop the pattern (request off, FRS interlock on).
Next precycle the dipole magnets with FMGSKAL, afterwards start the pattern again. This will then bring the magnets to the values for new Brho.


Fragment beam through FRS

Calculate fragment Brhos

use LISE or MOCADI to determine the best Brho of the FRS for the fragment setting (highest transmission, least contaminants). After matter at S1, S2 or S3 the Brho will change.

Scale FRS

stop the beam request and enter the new Brho values for each zone into ParamModi, for scaling use FMGSCAL to do the proper precycling. One such ramping cycle takes 2 minutes. Afterwards check Hall probes and request beam again.

Adjust intensity

you will have to increase the intensity now, ask operators, watch SEETRAM.

Look for fragments on detectors

watch MWPCs and scintillators (S2, S4) to see some kind of beam. You can look at the raw signals with an oscilloscope or with the online analysis program Go4.

 


Identification

The basic equation for identification is:
Brho = m / q * c0 *beta * gamma
We want to measure Brho, charge = Z and the velocity (beta = v/c, gamma = Lorentz factor) via time-of-flight. Then we can calculate the mass.

 

Time-of-flight calibration

First the Time-of-flight (TOF) needs to be calibrated. For this you take primary beams of 3 well known energies. Either change the SIS energy or use well calibrated targets or degraders and calculate the exit energy with ATIMA. Get the beam centered and measure the TOF from the scintillator at S2 (Sc21, TS3ESA) to the scintillator at S4 (Sc41), (Some experiments measure from  S3 to S4 or S2 to S8). The scintillators have photo tubes on both sides. First measure the time differences (dTll = Sc21_left - Sc41_left) and (dTrr = Sc21_right - Sc41_right). The total TOF is the average of dTll and dTrr. Plot TOF as a function of the velocity (beta=v/c). Of course it should be linear but there can be quite some offset due to the dT in the cables. Fit a line and use the coefficients in your analysis program. The slope can be also obtained from a calibration of the TAC with a time calibrator.

In Go4 plot the histograms SCI(2)_TofLL and SCI(2)_TofRR for each energy. The TOF (dTll or dTrr) will be shown as raw data in TAC channels. The real TOF you get from the distance between the scintillators and the known velocity. Fit a straight line into a plot TOF (dTll, dTrr) versus TAC cannels (ch). Don't be surprised about the negative slope, for the TAC shorter TOF means more channels since start and stop are swapped.
   dTll = tof_all - tof_bll * ch ,  dTrr = tof_arr - tof_brr * ch .
Enter the path length (id_path) and change the calibration coefficients tof_bll, tof_brr (edit setup.C and run ".x setup.C").
Note, the offset (id_tofoff) exists only once, it is the average of the offsets for left and right (tof_all, tof_arr). Path length is in units of pico seconds, which means path length in meters *104 / 2.99792458.
The formulas used in Go4 are:
   sci_tof2 [ps] = (tof_bll * dTll + tof_brr * dTrr) /2
   beta = id_path [ps] / (id_tofoff [ps] - sci_tof2 [ps])
   gamma = 1 / sqrt(1-beta^2)
The calibrated sci_tof2 is in the histogram SCI(2)_Tof2. But this is still not the real TOF because it still has the wrong sign and an offset.

figure: Velocity distribution for a setting on many fragments after calibration from Run120, F0016. 22Ne primary beam at 292 MeV/u to produce 18Ne and other fragments. For identification see below.

 

Calibrate Brho

One has to know the Brho which corresponds to a centered beam. Either you started with a well known Brho of a centered primary beam and remember by which scaling factor you have changed this setting, or you noted the B-field value of the Hall probes with a centered primary beam. Then you can read the actual B-field and calculate Brho from the ratio.
This is only valid for a centered beam. The Brho for an off center ion you deduce from the measured particle position together with the dispersion coefficient (D) and magnification (M) for the optics setting used. The coefficients and the centered Brho should be entered in your analysis program.
The formula used is:

Brho = Brho_cent. * [1 - (xS4 - MS2-S4  xS2) / DS2-S4],

x at S4 (XS4) can be measured with the MWPCs or TPCs, at S2 the rate can be too high for the gas detectors. In this case one can take the position information from Sc21, however, with less accuracy. But the MWPCs/TPCs can be used for calibration of the Sc21 position. The optics coefficients usually come from the theory values calculated with GICOSY. These coefficients are independent of the absolute Brho, as they are sensitive only to relative deviations.

In Go4 DS2-S4 is named dispersion[1] in the file setup.C, MS2-S4 is magnification[1]. The centered Brho is calculated from the B-fields (frs->bfield[]) as shown by the Hall probes and the effective radius of the dipoles (rho0[1]). The latter can be calibrated with a well centered primary beam of known Brho.
  Brho = B * rho_eff.
Go4 uses only one value, namely the average of the radii of both dipoles in the second half of the FRS. rho0[1] in meters and the B-field values in Tesla have to be entered again in setup.C.

The flags x2_select and x4_select (hidden in the code) switch between positions measured by the MWPCs in the focal plane (=1) or by the scintillators (=0). MWPCs are calibrated with the coefficients mw->x_factor[i] and mw->x_offset[i] which can also be found in setup.C. To derive the values in the focal plane also the distances inside the FRS are used (dist_MW21, dist_MW22, dist_MW41, dist_MW42). Scintillators use a 6th order polynomial to get mm values from the channels (x_a[0-6][i] in setup.C). Calibration can be done comparing the MWPC spectra with SCI21_X or SCI41_X and looking at the two dimensional histograms SCI21_TxMWx or SCI41_TxMWx.

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figure: Uncalibrated position from Sci21 vs. the position measured by the MWPCs. Setting on fragments to fill the whole momentum acceptance and to obtain a broad distribution at S2, from RUN120 F0016.

 

Calibrate the MUSIC

The charge is determined from the energy deposition (dE) in an ionization chamber (MUSIC).
First you measure a setting with a target but the FRS set on primary beam. At this moment you still have no calibration and you look simply at the energy deposition as an output of a QDC. The biggest peak in the MUSIC spectrum corresponds to the primary beam. Smaller ones will appear on the left, may be also one peak on the right for proton pick up. You can count downwards and assign the channels an atomic number. dE depends roughly on Z2.

Next dE depends on the velocity of the ions. Though this dependence is well known from theory (ATIMA) one can also calibrate it as one needs the 3 different energies for TOF anyways. Plot dE as a function of beta, fit it with a polynomial and use the coefficients in your analysis program.<

As dE in the MUSIC is a function of the position (x) where you enter the MUSIC you can still improve the resolution. Make a broad beam by switching off the preceding quadrupoles or scan the beam with the dipole over the whole MUSIC aperture. The position can be measured with the MWPCs. In a plot dE as a function of x you will notice the reduced energy deposition at the sides of the MUSIC. You can fit this curve and again use the values in the analysis program.

In Go4 the coefficients for the 6th order polynomial for position correction (pos_a1[0-6]) are used as follows:
    dEc = dE * pos_a1[0] / ( pos_a1[0] + pos_a1[1]*x  + pos_a1[2]*x+ .. + pos_a1[6]*x6 )
The position information (x) is taken from the MWPCs at S4. The two histograms MUSIC1_dEx, MUSIC1_dExc show the energy deposition as a function of x-position in the MUSIC without and with correction, respectively.

The velocity correction is derived as a fourth order polynomial from beta. From this the atomic number is calculated as 
    v_cor = vela[0] + vela[1]*beta  + vela[4]*beta+ vela[4]*beta3  + vela[4]*beta4
v_cor also includes the nomalization of the square root (dividing by the dEc for the primary beam).
    Z = primary_z * sqrt( dEc / v_cor ) + offset_z .
This means pure Z2 dependence is assumed, an assumption which is not so good for high Z. Only close to the Z of the primary beam (primary_z) used in the calibration it is safe. offset_z can be used to match the integer number Z better. All coefficients are set in setup.C.


figure: Go4 example from Run120 file F0016, setting on 18Ne.

 

Identification plot

The best separation you get in a two-dimensional plot of  TOF vs. energy deposition in the MUSIC. Calibrated it becomes A/Q vs. Z. Here different fragments show up as separated blobs like in the examples below.

In Go4 such a plot you can see as the histogram ID_Z_AoQ. Many conditions, ID_Z_AoQ(0-4), can be put onto it to gate other histograms.


figure: identification plot for a setting on 18Ne from Run120, file F0016. Still needs a small shift in A/Q to the right.

 

Path length correction for ToF

For better resolution the path length (L) needs a correction depending on the different Brho or different angle in horizontal plane, because passing the dipole magnets more on the innner or outer side causes a significant path length deviation and thereby a wrong velocity measurement (in the standard FRS mode dL/L ~ 3E-3). At least in normal optics modes corrections for shifted position are not important, also not deviations in vertical plane as due to plane symmetry they can contribute in higher order. This note explains why the whole correction can be done with one simple additional term to be added to the determined A/q.

 


Usual problems

1.) A drive (step motor) does not want to move by control from software.
    possible help:

 

2.) A magnet fails, i.e. it turns red in the "Piktogrammzeile" and the icon starts blinking.

 

3.) Suddenly no beam any more:


 

4.) Windows on console do not react anymore

Close app and restart it, also helps to update values sometimes.
Close all windows on this terminal (TCL1035, TCL1034 or TCL051) and open them again.
In the worst case reboot with hardware reset, should start again with operator ("op") logged in.


5.) Reset on power supplies needed:

With a special key an expert (K.-H. Behr or magnet group) can also reset interlocks and switch magnets directly on the power supplies.
You can check the status and error indicators.
No interlock LEDs should be on. The left column are old interlock messages which dissapear after a reset and the right columns are current interlocks.
In the example the FRS interlock (V5) indicates that the power-supply is not connected correctly to a magnet. As a result it is off (last row).