Van de Graaff operating manual

NAME
Description of the Van de Graaff accelerator and its components
- Inside the tank.
- The beam transport system
Starting up the Van de Graaff
Bringing the beam to the RBS setup
The RBS setup, hardware.
The OS-9 computer
Hardware connected to the OS-9 computer.
Structure of the data taking program control.
Starting up program control
Bringing samples in or out.
- Removing a sample
- Inserting a sample
Performing a simple RBS measurement.
Performing a channeling-RBS measurement.
- Procedure TRIANGLE
- A scan through a string.
Analysis of results

NAME

VAN DE GRAAFF - ACCELERATOR AT NVSF.

release 0.22, April 16, 1999

An introduction to the use of the Van de Graaff accelerator.

Description of the Van de Graaff accelerator and its components

Inside the tank.

The tank is filled with a mixture of N2, CO2 and SF6 for electrical insulating purposes. The main components for producing high voltage are:

The COLUMN is a structure of metal rings supported by insulators. The rings of the COLUMN are connected to a chain of resistors to define their electric potentials. The column current is proportional to the terminal voltage.
The rotating BELT transports positive electric charge to the TERMINAL.
The high voltage TERMINAL is surrounded by a polished metal hemisphere.
The CORONA TRIODE is a structure of needles pointing to the TERMINAL. The distance must be controlled such that the corona current is 40-50 μA. Under normal operating conditions, the corona current is controlled by a small steering voltage, derived from the stabilisation slits SPLEETSYSTEEM2, resulting in a stabilised terminal voltage V and thus a stable beam energy.

Current equilibrium condition:
```
   Belt charge current =  column current
                        + corona current
                        + beam current
```
The GENERATING VOLT METER (GVM) measures the terminal voltage.

The main components for making a beam:

The ION SOURCE is loaded with dilute gas (hydrogen or helium), the pressure of which is controlled by the valve GAS. A plasma is created in the source by means of a powerful RF field. Positive ions are extracted from the source by the ANODE. They are concentrated by the FOCUS, and enter the ACCELERATOR TUBE.
The ACCELERATION TUBE contains a homogeneous accelerating field.

All functions in the terminal are controlled by means of insulating perspex rods that are rotated by motors on earth potential.

The beam transport system

See the schematic drawing.

The main components are:

Bending magnets: Analysing magnet AM and switching magnet SWM.
Quadrupoles: (electric or magnetic): Lens for focussing the beam. Large astigmatism: different focal lengths for horizontal and vertical focusing. Steer the beam always through the centre of the lens to prevent undesired steering effects by the quadrupole.
Beam steerers: (electric or magnetic): Bending the beam over a small angle.
Slit (spleetsysteem): Horizontal and vertical limitation of the beam. The position indication is magnified by a factor 10. The beam current on the slits is measured. Display on the main control desk: sum of the currents = SOM, difference of the currents = VERSCHIL.
Valve: For closing the vacuum systems.
Faraday cup (BUNDELSTOP): Can be inserted into the beam for accurately measuring the beam current.
Beam profile monitor (BPM) (spleetsysteem): Small wire moving horizontally and vertically through the beam to measure position and width of the beam. Display on oscilloscope on main control desk.

Starting up the Van de Graaff

Find a checklist of previous settings.
Turn main key.
Make sure no people are present near the machine during the startup. Walk through the hall to the Van de Graaff.

Near the machine, reset safety relays (red button 'Reset' below Q-POLE1), at the same time check vacuum system (below Q-POLE1: <2×10^-6 mbar).

Check slit settings, while walking back to the control desk.
Select gas. Set the switch GAS SELECTOR to position 1 for hydrogen, 3 or 4 for helium4. Don't use up helium3 by mistake, it's expensive.

Turn gas potmeter to value from checklist, or lower. The valve will only open after the drive motor is started.
Set Voltage stabilisation control to MANUAL.
Take care that the terminal voltage goes positive as soon as possible:

After starting the machine, before the belt charge supply becomes active, there may be a period that the terminal is charged to a negative voltage, due to 'selfcharge'. For safety reasons (Bremstrahlung), this period should be kept as short as possible.

Press belt drive START.

After starting the belt drive motor give a little belt charge (BELT CHARGE LIMIT potmeter about 0.10 .. 0.20). If the reading of the column meter does not go positive within a few (say 10) seconds, turn off the machine and get help.
Regularly check the vacuum. Normal values during operation are 2×10^-6 to 5×10^-6.
If necessary, condition the machine. Follow the procedure Conditioning the machine.
Adjust terminal voltage with BELT CHARGE LIMIT potmeter, and corona point position (small switch in the compartment behind transparent door). Set corona point position such that the corona current is 40-50 μA.
Adjust focus, anode (start values from old check list). Select beamline. Set VOLTAGE STABILISATION CONTROL to GVM

If you are lucky the ion source will ignite within a few minutes. This is visible on the Beam Profile Monitor oscilloscope display, panel 3. If the source does not start automatically, fiddling with the anode and focus potmeters, or just plain patience, may help.

Conditioning the machine.
The bending magnets
Controlling the bending magnets and the NMR frequency
How to (re)start the magnet control PC
What to do if the frequency gets lost.
Turning off the Van de Graaff accelerator.
Bringing the beam to the RBS setup
Starting up program control
Bringing samples in or out.
Back to the INDEX

Conditioning the machine.

This process is necessary after the machine has been open, and also when one wants to obtain a higher voltage then was used in the previous period. It involves the gradual increase of the high voltage, waiting after each change until the system gets used to the new condition. Conditioning is accompanied by small 'gentle' discharges, that are evident by an irregular corona current, and an increase in the tube pressure. Take your time to do the conditioning. Conditioning periods of the order of one or two days may be necesary to bring the voltage up to 2 MV.

Failure to properly condition the machine results in an instable behaviour, and, moreover, heavy tube sparks that cause damage and shorten the life of the accelerator tube.

The bending magnets

The first bending magnet, or analyzing magnet, is used to define the ion beam energy. Remember that

    I<B> = I<p> . I<q> / I<R>

where p = linear momentum, q = charge, and R = radius of curvature.

The field is measured very accurately by means of an NMR (Nuclear Magnetic Resonance) device. The NMR device contains a marginal oscillator running at an adjustable frequency F. Part of the oscillator is a coil wrapped around a small volume of some material containing lots of protons, usually water. This probe is mounted inside a set of coils (Helmholtz coils) operated at a much lower frequency, say f, and placed in the magnetic field to be measured. Thus inside the probe, the field of the magnet is modulated, due to the Helmholtz coils.

When the frequency F is close to the resonance frequency F = k×B of the protons, the increased energy dissipation in the coil of the oscillator results in a decreased amplitude of the high-frequency signal.

The oscilloscope at the control desk is horizontally steered by the current through the Helmholz coils (the modulation of the magnetic field at the coil) and vertically by the amplitude of the high-frequency oscillation. At the field value corresponding to the exact resonance condition, F = k×B, a dip becomes visible.

Controlling the bending magnets and the NMR frequency

is done by program 'magnet' running on sytem 'OS91'.

This program is used to set the NMR frequency and the current delivered by various power supplies. It is actually an interface to a different program that resides in a Personal Computer (PC) close to the power supplies, and it 'talks' with this PC via an RS232 link. Therefore, the PC should be up and running the program VDG.BAS; see: How to (re)start the magnet control PC

(Re)start OS91 if needed (reset switch on OS-9 computer in accelerator hall near the RBS chamber).
Log in on the OS91 computer with username 'operator' and password 'vdg'.

This is usually done from the terminal at the Van de Graaff control desk.

If necessary change the TERM environment variable (not needed if the terminal at the control desk is used); see Terminal setting.
Start program 'magnet' by the command 'magnet'. This should produce some output like:
```
    device /t2 opened, fd= 4
    v1 = 16650
    v2 = 8262
    f = 284450
    frequency  locked in
    dip not locked in
    measure:off
    sum check enabled
    magnet>
```
If the previous user left 'magnet' up, you may check if everything is working by the command 'inq', that actually provides the output above.

To modify parameters the following commands are available:

    v1 <new value>
    v2 <new value>
    f <new value>
    d1 <return> <char> <char> ...
    d2 <return> <char> <char> ...
    df <return> <char> <char> ...
    <char> is a special character with possibilities:
      + : small increment
      - : small decrement
      > : large increment
      < : large decrement
      = : same as +
      _ : same as -
      <return>: quit command

In commands f and df the program waits until the frequency is actually locked in. This may take a few seconds (up to 30).

The file settings.pro contains prerecorded settings for various beam ions and energies. These may be accessed by command 'run'. Examples:

    run alfa1000 : settings for 1000 keV He+ ions
    run p2000    : settings for 2000 keV protons

As an alternative, there are commands:

    run energy : asks for an energy value
    run freq   : asks for a frequency value

To modify these settings command 'update' may be used. This command starts up the editor 'vi'.
Another useful command is 'run help' to get a survey of the 'run' options. For a full list of commands, type 'help *'. For help on a particular command type 'help <command>'.
To terminate a session:
```
    run down
    stop
    logout
```

Some built-in variables:

    Ebeam   beam energy
    FF      nmr frequency (desired value)
    v1val   digital value for magnet1 current
    v2val   digital value for magnet2 current

For a complete list, give the command

    list

See also What to do if the frequency gets lost.

Next:

Bringing the beam to the RBS setup
Starting up program control
Bringing samples in or out.
Back to the INDEX

How to (re)start the magnet control PC

At the time of writing, this is a Tulip PC located near the switching magnet. The program resides on a floppy disk in the 'A' drive.

At the prompt 'A>' type

    QB VDG.BAS

Once the program is loaded it may be started by pressing:

    F5 (function key 5)

When the program is running, local commands like:

    v:1=<value>

may be given, but if possible, remote operation is preferred. To switch the PC to the online state, type:

:O+

After this, commands from program 'magnet' may be accepted.

What to do if the frequency gets lost.

Sometimes, after a spark or for no good reason at all, the NMR oscillator goes on the blink and shows a frequency of some 59 MHz. Giving command 'f FF' in program 'magnet' does not result in any change. What to do?

Give the command 'f FF+100', and then a while later 'f FF'. Admittedly a sick situation, which makes the 'dip stabilizer' a device of limited use.

Turning off the Van de Graaff accelerator.

Switch MANUAL/GVM/SLIT to MANUAL (Panel 2)
Lower BELTCHARGE LIMIT to 005
When terminal voltage < 100, switch the belt drive motor off
Switch key to OFF position
Close valve BUNDELSTOP2
On OS91 terminal, run 'down'

Bringing the beam to the RBS setup

With BUNDELSTOP1 (retractable Faraday cup) in position IN, optimize the beam on Faraday cup 1. Watch also the beam profile monitor BPM1, and asjust focus, anode, Q-POLE1, and beam steerer X1 X2 Y1 Y2.
Open BUNDELSTOP1 (Retractable Faraday cup).
Go into SLIT stabilization, adjust AC gain and slit gain.

Check that beam ON/OFF system (Panel 5) is not trying to switch the beam off. Set 3-position switch labelled COMP/ON/OFF (below display COUNTRATE) to position ON.

Optimize beam on BUNDELSTOP2 with focus, anode, Q-POLE1 and X,Y-steerers. Normally, the current on DIAFRAGMA1 should be less then 10% of the current on BUNDELSTOP2.
Open BUNDELSTOP2.

Optimize beam on BUNDELSTOP4 with Q-POLE2 and further steering magnets. Ideally all the beam on BUNDELSTOP4 (a few nA) will go the sample when this stop is removed.

Q-POLE3 is generally not used, so the particle trajectories are straight lines over a large distance, from the switching magnet to the target, and the shadow of SPLEETSYSTEEM4 is sharp.

The RBS setup, hardware.

The 2-axis goniometer rotates the sample around a vertical axis (motor 1), and around a horizontal axis (motor 2) that is rotated in the horizontal plane by motor1. The normal to the sample is tilted by 10 degrees, such that the polar angle theta and azimuthal angle phi of the normal, with respect to the incoming beam, are (motor1 - 10) and motor2, respectively.

The linear position of the target can be controlled in two perpendicular directions, X and Y.

Three semi-conductor silicon detectors are mounted on the detector caroussel and the angle with respect to the beam can be controlled manually.

The electronics of the detectors generally does not need any adjustment, the bias voltage being permanently present.

The vibrating string device causes RBS scattering towards an extra detector. The pulses from this vibrating string detector are sent to a pulse height discriminator. The logical output pulses are used to trigger the test pulser. Pulses from the test pulser are fed to the preamps of the RBS detectors, and thus generate a pulser peak in each of the spectra. This is the standard way of measuring the beam dose with dead-time correction. The same pulses are also sent to the CLOCK input of the ADC multiplexor

The size of the beam spot is defined by the slits SPLEETSYSTEEM4A and SPLEETSYSTEEM4B.

The OS-9 computer

An OS-9 system is used for data acquisition and control. The program responsible for this is 'control'. Other relevant programs are 'dmidma' (started up by control) and 'magnet' (run from the operator console).

OS-9 is a 'Unix-like' operating system. Each command corresponds to a filename (like in Unix), and most of these files are in directory /dd/cmds. Usually a short help is given after you type a command followed by '-?'. Example: 'dir -?' generates the following output:

    Syntax: dir [<opts>] {<dir names> [<opts>]}
    Function: display directory contents
    Options:
      -a        show all files
      -d        show directories with a slash
      -e        extended dir listing
      -n        treat dirs like files
      -r        recursive dir listings
      -r=<num>  recursive dir listing to depth <num>
      -s        unsorted dir listing
      -u        unformatted listing
      -x        directory is execution dir
      -z        get list of dir names from standard input
      -z=<path> get list of dir names from <path>

Note: The options above also pertain to the command 'dir' of program control on OS-9.

Documentation of OS-9 is present in the Van de Graaff control room.

Terminal setting.

The environment variable TERM should correspond to the type of terminal or terminal window used. Unlike Unix, the number of lines and rows can not be changed and should correspond to the terminal definition selected by TERM. This is important if you use the OS-9 version of vi !!!!

Some useful options are:

    hpterm (24 lines, 80 columns)
    hp46   (hpterm with 46 lines and 80 columns, default for user login)
    vt100  (vt100 or xterm, 24 lines, 80 columns)
    vt46   (vt100 or xterm, 46 lines, 80 columns)

For example, to change to vt100, type:

    setenv TERM vt100

If you make a habit of this, put the above in your $HOME/.login file.

Unix workstation to drive the OS-9 computer

A unix workstation is used as a user interface to the OS-9 computer. The tasks of this workstation are:

Text terminal for the OS-9, via unix program telnet.
Display of spectra, and other graphical output.
Optionally, Graphical User Interface (GUI).

For the tasks b) and c) daemons should be running. These are started up by the command 'svc' on the unix workstation. These daemons start up further Unix programs as needed, i.e. 'grint' (GRaphical INTerpreter) and 'showmenu' (GUI).

See: grint_command and the showmenu page.

OS-9 only accessible from nvsf5 and from nvsf10

The OS-9 systems are protected from direct Internet access. Only workstations nvsf5 and nvsf10 can directly access the systems OS90 (LEIS) and OS91 (RBS). One should therefore first login to one of these before connecting to an OS-9 system. Access protocols include telnet (but not remote login), ftp, nfs, and rpc.

Basic OS-9 commands. Differences and similarities with Unix.

See the OS-9 operating system manual.

Hardware connected to the OS-9 computer.

The ADC multiplexor (VME)
The ISMC (Intelligent Stepping Motor Controller) (RS232)
XY movement (digital IO, and ISMC)
Test pulser (digital IO)
Beam on/off control (digital IO)
RS232 bus
PC controlling the NMR and magnet power supplies (RS232 bus)
The operator terminal (RS232 bus)

Structure of the data taking program control.

The syntax of the command language used in programs CONTROL and MAGNET is described in CIO command language. The operator can control the measurement and manipulate data with two kinds of commands:

Direct commands (examples: MTR go, XYPOS).
For a description of specific Control commands, see: CONTROL.
For a description of general purpose commands, see: CIO online help
CIO Programs (example: triang).
For a survey of 'standard' CIO programs, see: CIO programs

Programs are started with 'lgo' (example: 'lgo windows'). If windows has already been loaded or performed, 'lgo' can be omitted and the command 'windows' will start this program.

The user can also make his own CIO programs, in its simplest form a text file made with a standard editor, containing a list of valid commands. The filename should consist of 8 or less alphanumeric characters with suffix '.pro'. The part in front of '.pro' may then be used in the lgo command. For details, see: CIO command language.

Starting up program control.

Reset the OS-9 computer if necessary.

On workstation nvsf5, type command 'telnet os91' (best results from an hpterm window with geometry exactly 80x46).

Login as 'peter balings'.

Start program 'control' by the command 'co' and follow instructions.

    Motor positions    : normally default values will do, but check,
                         especially after maintenance.
    Pulser amplitude   : 'standard' value is 200.
    File name          : e.g. name.date.spc
    Number of detectors: 3
    Number of windows  : 0 (may be modified later by 'lgo windows')

The two motors control polar angles of the target with respect to the beam.
```
    theta = m1 - 10 degrees
    phi   = m2
```
Command to put target perpendicular to the beam:
```
    mtr go 10 0
```
Put target at desired XY position with respect to the beam, e.g.:
```
    xyp 15 20  (= X-coordinate 1.5 mm, Y-coordinate 2.0 mm)
```
Note: Answer all questions with actual values, not just <return> (with the exception of the motor positions). If you are not sure about a filename, give filename 'tmp'. If you are not sure about windows, type nr of windows '0'. These things can be corrected later.
Again on nvsf5 (in a different window) start display demon by the command 'svc'.
Open another os91 window as above, but now start program 'control' by the command 'cocon'.

Use 'control' to move motors, control the measurement, etcetera. Use 'cocon' to inspect the results.

Bringing samples in or out.

Study the layout of the vacuum control panel near the RBS chamber and get familiar with the following components:

IKR1 : vacuum in the RBS chamber
IKR2 : vacuum in the high-vacuum lock-in chamber (INSLUISKAMER)
TC1 : vacuum in the roughing lock-in chamber
KLEP1: valve between roughing lock-in chamber and roughing line
KLEP4: valve between RBS chamber and the beam line
manually operated valves involved:
- VAT klep between high-vacuum lock-in chamber and RBS chamber
- valve between roughing lock-in chamber and high vacuum lock-in chamber
- air inlet valve of roughing lock-in chamber

Removing a sample.

Type command 'insluis' (brings goniometer to the proper position).

Note: It may be necessary to type 'lgo insluis' instead.
Close KLEP4. Remove covers from windows, as needed. Switch internal lamp on.
Check vacuum in high-vacuum lock-in chamber. This should be less than 10×10^-6 mbar.
When O.K., open VAT klep between RBS-chamber and preparation room, about 10 turns until resistance is felt.
Take out the sample with the transfer rod. Careful! Change height and left-right position of the long magnetic manipulation rod, if necessary.
Turn sample holder to horizontal position (sample side up).
Pull sample back as far as possible.
Close the VAT klep.
Open KLEP1 to evacuate the top lock-in chamber. Wait till a pressure of about 5×10^-3 mbar is reached (TC1).
Close KLEP1.
Open the valve to the main lock-in chamber.
Take the sample over, onto the vertical transfer rod. Careful! Pull up the rod with the sample as high as possible.
Close the valve to the main lock-in chamber.
Let in air until atmospheric pressure. Watch TC1.
Lift off the top lid with the sample.
Turn off the light in the lock-in chamber.

Inserting a sample.

If necessary: type command 'insluis' (brings goniometer to the proper position).
Put flange with sample on top of the roughing lock-in chamber.
Close the air inlet valve.
Evacuate the top lock-in chamber by opening KLEP1, until TC1 is 5×10^-3 mbar.
Close KLEP1.
Open the valve to the main lock-in chamber.
Take the sample over onto the transfer rod and pull up the vertical transfer rod.
Close the valve between the roughing lock-in chamber and the high-vacuum lock-in chamber.
Wait until the vacuum in the high-vacuum lock-in chamber is less than 10^-6 mbar.
Open VAT klep, about 10 turns.
Install the sample. After uncoupling the transfer rod, give the sample a gentle push.
Withdraw transfer rod and close the VAT klep.
Turn off the light in the lock-in chamber. Turn off lights inside the RBS chamber, and close all windows.
In program 'control', type command 'mtr check'.
Put goniometer in desired position, e.g. 'MTR go 10 5'.
Check XY position, command 'XY ;'.
Check the vacuum, and, if O.K., open KLEP4.

Bringing the beam to the RBS setup
Starting up program control
Back to the INDEX

Performing a simple RBS measurement.

Open KLEP4.
Set beam computer control switch on COMP (below display COUNTRATE).
Preset time with:
```
    DMI
```
Start measurement:
```
    meet
```
Note: 'lgo' is not necessary, because the program 'meet' was already loaded by the command 'co'.
Inspect growing spectrum in the 'cocon' window:
```
    dis r1 0 500
```
Display spectrum in same frame without rubbing old spectrum:
```
    dix r2 0 500
```

Simulate 'life' display (refresh time 10 seconds):

    repeat; dis r1 300 600; sleep 10; endrep;

Stop 'life' display:
```
    exit
```
Write the total 8k spectrum sp on disk:
```
    wrt
```

Performing a channeling-RBS measurement.

Procedure TRIANGLE.

This procedure finds the precise motor coordinates of the string near to the normal crystal direction by performing a simple RBS measurement.

Find the boundaries of the windows in the spectra r1, r2, and r3:
```
    cur   (cursor)
```
Define the windows for the channeling experiment:
```
    lgo windows
```
(asks for boundaries of windows, saves in the files LL (lower levels) and UL (upper levels))

Start measurement:

    lgo triang

    phi      : -10  (start value of phi)
    Ntheta   : 100  (number of theta steps)
    Nphi     : 50  (number of phi steps)
    life time: 1000 (pulses per step)
    data-file:

The results are stored in:

    window 1 in spectrum scan[0],
    window 2 in spectrum scan[1], etc.

Sequence of theta,phi positions that are measured:

    0,-10;...(100)...+20,-10;...(50).....+20,+10;0,-10;....(100).....+20,+10

Display of 'life' results (in 'cocon'):
```
    repeat; dis scan[0]; dix scan[1]; dix scan[2]; sleep 10; endrep;
```
We see a number of planar channeling dips.
Analysis after the triangle measurement is finished (in 'co'):
```
    dips
```
Dip recognition criterion: 1.5 (if smaller, more dips will be found).

The triangle is displayed with positions of planar dips. Draw lines that connect dips: crystal planes. Choose the most probable theta,phi value of the intersection of the planes: the string. Use planes that intersect at correct crystallographic angles.

A scan through a string.

Give coordinates of the string:
```
    MTR GO (theta-value)  (phi-value)
```
For instance 'MTR GO 10.40 0.98'.