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

Days before (safety check, tests)
Before first beam (calculation of settings, online analysis)
Primary beam on production target
Primary beam through FRS
Energy change
Fragments through FRS
Identification
Usual problems
 

This page was written by Dr. Helmut Weick, 08th April 2016, contact h.weick(at)gsi.de,   Impressum


Days before

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.

Tests

The FRS will be 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 once some current to them.


Operating console in Messhütte.


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 (on LINUX /u/weick/JavaAtima). For the calculation with fragments and optics LISE or MOCADI can be used. LISE++ runs on KSPC014 in the FRS console. picture: screenshot of LISE++.

Online Analysis

start online analysis program GO4

picture: screenshot of running Go4

 

Test MWPCs

Generate a pulse with the pulse generator and distribute it to all MWPCs anodes. With a scope 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.

 

Close cave

Close the doors of the caves, call HKR (tel. 2222) 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.

 


Primary beam on production target

Switch on magnets

Switch on power supplies in the hall by turning the two keys. Otherwise, hands off from the power supplies (dangerous high current !).

 

Prepare beam request

Then the display of the "Pulszentrale" in the FRS console should show also the number off the machine, still with a red LED as there is no beam yet. Once you really request beam the green LED shines.

To activate the switch beam on/off the lemo connector at the back of the display inside the FRS console has to be plugged at the right position corresponding to the virtual machine shown in front.

 

Start control programs

On the touch screen of terminal TCL1SF1 select the beam beamline:

Start the programs SDIBHS, MGSKAL, green means they are running, click again to stop them.
MGSKAL and IBHS will appear on screen of L connected to TCL2SF2. 
IBHS is used for saving / loading of magnet settings.
MGSKAL scales a series of FRS magnet values by a common factor.

In the folder [Service] you can find the buttons [Print ..] for printout of a screenshot of the left, middle, right or upper left screen (connected to TCL2SF1 or TCL2SF2).
By default the graphics files end up in the folder /common/usr/production/operating/screenshots/FS as png, gif and ps.
Pressing [Paper] directly afterwards redirects the output also to the paper printer P01.

SD is the main control program appearing on the screen of TCL2SF1, to see more details activate “Piktogrammzeile”.
The number of the selected virtual accelerator machine and the beam particle/energy should be written on top.



Some magnet power supplies have to be changed according to the beam line selected. Do this with the SD program 

Finally, in SD program call “Sonderoptionen” -> “Aktivieren der Magnete für aktuellen Beschleuniger”

Hall probe read out (HALL82 )

Currently the program hall82 has to be started separately in a nodal terminal window.

Open vacuum valves

The valves are controlled from the SD program in the line "Gateventil". There are two modes [F] for "Fahren" (drive) and [M] for measuring. You can switch between the two by clicking on the buttons at the beginning of the line, red means active.

Check vacuum:
Make sure you are in mode [M], left mouse click on a certain valve.
The pressure measured in the section upstream of this valve will be displayed in the blue box in the top of the screen, e.g..
   TS4VV1    (P1)    8.98*10-7 mb    8.98*10-5 Pa
     TS4VV1    (P2)    1.10*10-6 mb    1.10*10-4 Pa
There are always two measurements (P1, P2) for safety.
Some pressures are measured directly inside the pump. Those values are usually to good and don't really represent the pressure in the beam pipe. This is indicated by the extra line  Druck abgeleitet aus Pumpenstrom P1, P2  .

Typical values are 10-9 to 10-8 mbar in front of target vacuum window and 10-7 to 10-6 mbar behind in the FRS. In case the pressure rises above 5*10-6 mb in one section the valves around this section will close automatically. An independent pressure measurement and display is at the back of the electronics racks.

Open valves:
go to mode [F], left mouse click on valve, the red cross indicating a closed valve should change to a green circle, then for safety back to mode [M].

 

Close slits for safety

With SD program close the slits behind the target (TS2DS3 V,H), in front of S1 (TS3DS2H) and insert the beam plug (TS3SV2).
The slits are driven by step motors and can be found in the line “Schlitze”. Left click on point, a dialog box will open, click on I on all sides until the green bars cover the whole area.

Prepare Beam Diagnostics

Insert current grids at the target called CG01, CG02 or TS1DG5 and TS2DG2, respectively. With SD step motor control move them to position 0.0 (not I ), see picture. They are very slow.

Apply voltage to them with the help of terminals in electronics room. Usual are 80V.

Start current grid display program. In the SD program the box on top of the corresponding slit motors. A program called “Profilgitter” will appear on the neighboring screen.

Start program DI from terminal TCL1SF1, will appear on the screen M. Choose correct virtual accelerator by clicking [+] many times then click on [A]. Choose the detector from the menu (e.g. SEETRAM is named TS1DI4S), press [Start input] and [automatic]. If there is beam you will see a histogram of counts per time.<

 

Set Synchrotron (SIS)

Ask operators to set SIS machine to demanded energy.
phone 2222, a virtual accelerator machine should have been prepared before and name the energy.
load optics setting for part from SIS to target.

 

Set magnets for ion optics

Load optics setting for part from SIS to target. (More details about the FRS ion optics).

a) from theory
in SD program choose “Sonderoptionen” -> “Theoriewerte setzen” -> select filename of optics setting (SISTSHFS$_SISTA2006B_TA1_LO.SET, SISTAHFS$_SISTA2006B_TA1_HI.SET or SISTSHFS$_SISTA2006B_TA2.SET), view file with “Anzeigen” and apply it with “Setzen”. Make a printout of the magnet status to spot errors compared with “Anzeigen”.More details on these settings on the MOCADI download page

b) from old setting
look in log book, but remember the target geometry was changed in Oct2006. Older settings are not good anymore.
Use program IBHS to search for data sets. In the filter mask you can type the keword also of th eold data sets still save with SRMAG. 

 

Check Magnet Values

Make print out of magnets status, click MagStat in SD program and wait until the status appears on the printer.
Do the magnets have roughly correct values? I.e. are the dipoles and quadrupoles scaled by a factor corresponding to the new Brho divided by the old Brho (for R122_03, 12C6+, E = 200 MeV/u, Brho = 4.2852 Tm). If not try again to set the values!

 

Request beam

Request beam by pushing the green button for beam to HFS (S4), for the caves put 50 Ohm to some cable, for ESR simply start machine #14.

The scope on top in the console should then show a SEETRAM signal of the spill as well as the DI program.

 

Adjust beam position

Look at beam position on the two current grids CG01, CG02 (TS1DG5, TS2DG2). To adjust scale use mode "halbauto". Be patient the program updates only slowly.

The beam should be as narrow and centered as in the example shown above. Sorry, the picture is only black and white. You can use the program MIRKO to center the beam. On TCL1SF1 select MIRKO. MIRKO should appear on screen M. With MIRKO you can change all magnet settings of the FRS and even more beamlines, so be careful! Better use only the one option described here.
From a field of many numbers select your virtual machine number, then you should see an ion optics plot of the FRS showing beam envelopes. On the left of this window there is the option [gerade legen]. This opens another window where you can enter the actual beam position "ist" on TS1DG5 and TS2DG2. "Soll" should be zero. This can be done for x and y direction. To cause changes click [Korrektur berechnen + setzen], then watch the current grids whether further corrections are necessary.

Adjust the intensity

The Seetram (secondary electron transmission monitor) is used to to measure the intensity of intensive beams. There is a default calibration factor in the DI program. But you can also predict your own value with a program called (SEETRAM).
You might have to adjust the Seetram sensitivity, this is done in the DI program by choosing a value from 1 to 7, which corresponds to values of 100uA - 100pA. On a screen at the back of the FRS electronics you can monitor the SEETRAM setting using a camera in the target area. Different lights indicate the SEETRAM sensitivity.

Scheme of lower left module, SEETRAM set to sensitivity 10-9A. Camera may be broken due to strong radiation in target area.

Watch the number of particles per spill shown by the DI program measured with the SEETRAM. This should be roughly correct (10% error). For better values you need to calibrate the SEETRAM.
Compare the number of particles on SEETRAM with those in SIS (ask operators for beam transformator value), for longer runs it has to be at least 70% !

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

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 (TS2DI1_S at position 0.0mm + TS2DI1_P in) and switch on HV, check signal height and CFD threshold, then intensity on scaler, ask for fine adjustment of intensity.


Primary beam through FRS

Load optics setting for FRS

a) from theory
In SD program choose “Sonderoptionen” -> “Theoriewerte setzen” -> select filename of optics setting (e.g. SISTSHFS$_RUN81B.SET), view file with “Anzeigen” and apply it with “Setzen”. Make a printout of the magnet status to spot errors compared with “Anzeigen”. More details on these settings on the MOCADI download page.

>b)from old saved setting
Use IBHS to load an old setting documented in an old log book (list of good reference settings).
The safe way is:

  1. Use kyword or  filter options to find the file.
  2. Choose section to load, when unsure about magnet names check on magnet plan of FRS over your head.
  3. restore data set with old set parameters (default). Check box "info" to display old set parameters, they should agree to the old printout.
  4. Check values for example by making a magnet status printout. Now you should have exactly the same values as in the old experiment.

Scale FRS to correct Brho

The loaded setting usually still is for a different Brho of the beam and the FRS magnets have to be adjusted. Even if the factor is 1.0 do the scaling because of hysteresis.

  1. Calculate the energy and Brho of the beam after the target and the factor missing to the loaded setting.
  2. In MGSKAL select part of FRS to scale. “Gruppe auswählen” e.g. “TA-S4”.
  3. Scale selected part by factor “Magnetwerte skalieren” -> “Skalieren mit Faktor” -> Enter calculated factor -> confirm with “y”.
  4. "Rampenprozedur fahren, (k)eine, (d)ipole, (a)lle ?", choose "d"
    Wait about 2 min until the ramping procedure is finished.

 

Beam to S1

Open slits and remove beam plug (TS2SV3).
Apply HV to MWPC detectors. Use small terminals in electronics room that control the power supply crate 2 and 3 in the rack FRSMHE6 ???.


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

Centering:
To center beam at S1 scale FRS parts (TA-S4). The factor F is calculated using the dispersion coefficient (x,δ)TA-S1.
F = 1 - x / (x,δ)TA-S1    (the minus comes from different coordinate systems used on detectors and in optics calculation).
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 in IBHS.
Choose box: “neuen Save ablegen” (bottom left)
--> add a comment, it should start with short consecutive keyword (e.g. S417_05) followed by more, for example the fragment beam type or real Brho.
Print a magnet status, glue it into the log book and write the keyword next to it along with the Brhos and beam type for this setting.

 

Calibrate target and degraders

Insert a target, click on target pictogram and choose from list (TS1ET5)

and center 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 SRMAG.

 


Energy Change

In principle a change of SIS energy just requires a phone call to HKR and asking the operators to change the energy of SIS in the control program SIS-Modi. The new SIS energy will appear in the header row of the FRS SD program. In this case the beamline from SIS to FRS target will be scaled automatically to the new Brho. Any change behind the production target should be done by people of the FRS experiment.
However, there exist also numbers for set nuclei, charge state and energy for the sections TA-S2 or S2-S4. In case these parameters are changed (only possible in HKR) also the magnets after target will be changed. Usually we deal with fragments and do not have a clear energy and it changes not only with the accelerator setting but also with targets and degraders. Therefore, in practise it is the easiest to set these init parameters to one value at the beginning of the beam time and not to change them anymore afterwards. Then we can avoid wrong automatic changes of magnets.
The init numbers for all sections can be seen on the printout of the magnet status. In case you want to load an old data set saved with SRMAG or IBHS at a time when init parameters were different, you must check the option "info" in IBHS, the old values from the printout should be used and be default,  then restore the magnet values. Otherwise you will not get the same values as in the file saved.

Examples of magnet status printouts before and after SIS energy change.

 


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. Knowing the Brho of the previous setting one can calculate the necessary scaling factor F.
 

Scale FRS

in MGSKAL
select virt. accelrator number (does usually not change during one experiment), 
select group of magnets (section of the FRS),
type scaling factor,
confirm -> Ramping procedure, yes, at least for factors larger than 1.002.
Wait about 2 min until the ramping procedure is finished.

 

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.

<
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 the SD program:
    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: