Download Electron-beam tubes

Electron-beam tubes - an introduction
Electron-beam tubes are miniature particle accelerators. They are a type of Cathode Ray Tube (CRT)
commonly used in oscilloscopes and for display screens for radar, some computer monitors and older
televisions. The tubes designed for science investigations enable the properties of electrons to be demonstrated in fascinating ways. Unfortunately, the use of such equipment in schools has declined. Many
tubes are tucked away in their cardboard boxes but still available for use. This leaflet is designed to
provide easy, step-by-step, guidance to encourage science teachers and technicians to put an electronbeam tube back into operation.
Our experience is that, once they see a tube working, teachers are keen to use it in their lessons. In due
course, some may then wish to find out more about the variety of more-advanced work that can be
carried out with these devices.
This leaflet is most clearly viewed in colour, either on the CLEAPSS CD ROM (2008 or later), or printed on a
colour printer.
1.
Electron-beam tubes available
Manufacturers produce a variety of electron-beam tubes1. By far the most common type found in UK
schools is the ‘Teltron® Classic’ range2.
Electron-beam tubes used in school science can be divided into two groups.
•
Tubes, including fine-beam, diode and triode tubes, which require a High-Tension (HT) power
supply3, operating at a relatively high current (around 30 mA). This type of power supply presents a significant electrical hazard, similar to that associated with mains electricity. Therefore, good
user knowledge and proper control measures, including special HT leads and plugs, are essential.
This leaflet does not cover the use of these tubes.
•
Tubes, including Maltese-cross, Perrin and Electron-deflection tubes, which require an ExtraHigh-Tension (EHT) supply, but current-limited to 2 mA or less4 (and therefore much safer to use
than an HT supply). Each tube incorporates a fluorescent screen, which gives an impressive
indication of the presence of an electron beam. Instructions to set up these tubes are given below.
Figure 1: Teltron® electron-beam tubes, with fluorescent screens, which require an EHT supply.
Maltese-cross tube
Perrin tube
Electron-deflection tube
1
Technical information about different electron-beam tubes is provided in section 12.6 of the CLEAPSS Laboratory Handbook.
2
Teltron® tubes and accessories are now available from UK 3B Scientific. See section 7 of this leaflet.
3
Information on the safe use of HT supplies is in the CLEAPSS Laboratory Handbook, section 12.9.3.
4
EHT supplies for science teaching are normally current-limited to 2 mA, but older units, supplying up to 5 mA, may still be used.
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2.
Principle of operation
Electron-beam tubes use a ‘gun’ to accelerate electrons. The electrons are first ‘evaporated’ from a hot
cathode (negative electrode), which is similar to the tungsten filament in a low-voltage light bulb. This
process is known as thermionic emission. The resulting cloud of electrons is attracted towards a cylindrical,
high-voltage anode (positive electrode). A hole at the end of the anode allows a proportion of the rapidlymoving electrons to ‘shoot’ through, producing an electron beam. An illustration of an electron gun,
fitted inside the ‘neck’ of a typical Teltron® tube is shown in Figure 2.
Figure 2: Electron gun in a Teltron® tube
Once created by the gun, the electron beam proceeds through a vacuum inside the tube until it collides
with a fluorescent screen. Here, energy from the electron beam is released in the form of light, indicating
the presence of the beam.
Three simple demonstrations are easily achievable with most tubes containing a fluorescent screen.
•
The technique used for accelerating electrons (a particle accelerator).
•
The effect of changing the accelerating voltage.
•
The effect of introducing a magnetic field, using permanent magnets.
Other, more-advanced, investigations, will be dependent on the particular tube in use and more-detailed
information (not included in this leaflet) will be needed. See section 7 for the technical help available.
3.
EHT power supply
A combined Low-Tension (LT) and Extra-High-Tension (EHT) power supply is required to operate the
electron gun. For safety reasons, the power supply must be one designed for use in school science, eg, by
Griffin, Harris or Unilab. This will ensure that:
•
the maximum EHT voltage is no more than 5 kV (5 thousand volts)5,
•
the EHT output will be internally current limited to less than 2 mA (2 thousandths of an amp) and
•
there is a separate, fixed, low-voltage, ac output of about 6 V (typically 6.3 V ac).
As with all portable, mains-operated, electrical appliances, the EHT power supply must have passed a
standard electrical inspection and test6. If you are uncertain about the suitability of a particular power
supply, please contact the CLEAPSS Helpline.
There is a high risk of electric shock, or permanent damage to the tube, if an inappropriate
power supply is used.
5
EHT supplies for science teaching are normally voltage-limited to 5 kV, but older units supplying up to 6 kV may still be used.
6
Inspection and testing of portable electrical appliances is explained in section 6.4 of the CLEAPSS Laboratory Handbook.
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© CLEAPSS , The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk
Figure 3: Typical school EHT power supplies used to operate an electron-beam tube
4.
Connecting leads
When HT power supplies are used to operate some types of electron-beam tube (see section 1), leads with
an appropriate voltage rating and well-shrouded connectors are essential.
However, when standard, current-limited, school EHT supplies are used, as described in this leaflet,
well-maintained, general-purpose, low-voltage leads with standard 4mm plugs and sockets are sufficient.
x
The risk of harm by electric shock from a
current-limited EHT supply is minimal.
However, an electric shock from such a supply
is still unpleasant. The presence of bare pins, at
high voltages, should always be avoided.
If only standard leads with plugs at both ends
are available, exposed pins should be protected.
This is best done with firm-fitting insulated
sleeves (eg, plastic plug bodies) as shown.
This should only be a temporary arrangement,
to check that a tube operates properly.
Once a tube is in regular use, leads with sockets
should be used, where appropriate7.
7
Ready-made ‘extension-test’ leads, fitted with a 4 mm plug at one end and a 4 mm socket at the other, are available from various suppliers,
eg, Rapid Electronics, order codes 17-0374 (black) and 17-0376 (red); www.rapidonline.com.
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© CLEAPSS , The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk
5.
Circuit diagram
A simple circuit, which will operate a Teltron tube, requires only four or five leads, as shown below.
Figure 4. Basic circuit to operate a Teltron tube.
6.
Step-by-step set up procedure
The procedure described uses a ‘Teltron® Classic’ Maltese-cross, electron-beam tube, the type most
commonly found in schools. Connections to the electron gun will be identical for similar tubes with
fluorescent screens, which may be substituted for the Maltese-cross tube.
The step-by-step set-up procedure is also effective, when used by teachers, to explain the principle of the
electron-beam tube to students.
•
Electric shocks from school EHT power supplies are not fatal, but usually memorable.
•
All wiring should be connected with the mains switch on the power supply turned
off.
•
There can be a delay in EHT being removed from the output, once the supply is turned
off. Therefore, it is recommended to wait a few seconds before touching connections
(check the display of a meter, if fitted to the power supply).
•
Electron-beam tubes are made from glass and are evacuated. They must be handled with
extreme care. Even minor knocks or drops may cause a tube to shatter.
Implosion risks are similar to those for a large light bulb.
•
Before moving a tube, it should be allowed to cool, in order to prolong the life of the
heater.
In the sequence A to K overleaf, connections are made according to the circuit diagram, as shown in
Figure 4.
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Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk
A. Check the electron-beam tube for physical damage (internal and external).
A rattling tube does not necessarily mean that it will not work, but loose or missing parts may require repair,
or the tube may need to be returned to the manufacturer for refurbishment.
B. Secure the tube
carefully in the stand.
Avoid putting stress on the
tube mountings, as they
sometimes become loose.
Slide any fixing devices into
place.
C. Connect the heater.
Use two black leads
between the 6 V ac output
on the power supply
(sometimes labelled
‘heater’ or ‘filament’) and
the two heater supply
sockets on the back of the
electron-beam tube.
Voltages greater than about 6 V, applied to the heater, will destroy it
and render the tube useless.
D. Switch on the power
supply, to check the
heater.
A glow/light from the heater
is usually visible at the back
of the tube.
Reducing ambient light
gives a more impressive
display.
Turn off the power supply at the main switch after this step.
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© CLEAPSS , The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk
E. Connect the cathode to
the EHT(-).
At the power supply, use a
black lead to link one of the
black heater leads to the
EHT(-) socket.
F. Connect the anode to
the EHT(+).
Connect a red lead to the
anode of the tube. At the
power supply, plug the
other end of this lead into
the EHT(+) socket.
Make sure that no metal pins at EHT voltage protrude
(see section 4, Connecting leads)
G. For the Maltese-cross
tube, connect the cross
to the EHT(+).
Connect a red lead to the
tube.
At the power supply, plug
the other end of this lead
into another EHT(+) socket
(the second socket is often
connected to the first via an
internal resistor). If only one
EHT(+) socket is present,
use stackable plugs.
or
This connection stops
electrons charging the
cross, ensuring that a
sharp-edged shadow is
produced.
Make sure that no metal pins at EHT voltage protrude
(see section 4, Connecting leads)
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© CLEAPSS , The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk
H. Turn on the electron
beam.
Set the EHT voltage control
to zero. Turn on the power
supply at the main switch to
operate the heater. Gradually turn up the EHT
voltage.
At a few kV, the fluorescent
screen should begin to
glow as the electron-beam
strikes it.
The higher the EHT voltage, the brighter the fluorescence. The electrons
gain more kinetic energy,
when the accelerating
voltage is higher.
I. Hold a strong magnet
near the screen.
A ceramic or neodymium
magnet works well. The
electron beam will be
deflected by the magnetic
field.
Initially concealing the
magnet in the hand may
increase interest. Alter the
distance of the magnet from
the tube, its polarity and its
position along the tube. The
effect on the electron beam
should be discussed.
J. Point two strong magnets (check that they attract) toward one another, on either side of the neck of the
tube.
The electron beam should move up or down, perpendicular to the magnetic field. Reversing the polarity of the
magnets (by turning them both over) should make the electron beam deflect in the opposite direction. This
demonstrates the motor rule (Fleming’s left-hand rule).
In the specific case of the Maltese-cross tube, the light from the heater/filament should cast a light, faint shadow of
the cross on the screen. The shadow associated with the white light is unaffected by the magnets, but the shadow
associated with the electrons, which produce the green glow on the screen, moves.
K. Turn the EHT voltage down to zero before switching off the power supply.
Allow the tube to cool before handling.
7.
More-advanced work
New electron-beam tubes are usually supplied with detailed circuit diagrams and suggested investigations. Some suppliers offer technical help and direct support to schools. In particular, UK 3B Scientific
supplies Teltron® electron-beam tubes and includes resources on its web site at www.3bscientific.co.uk.
Enquiries can be emailed to [email protected].
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®
© CLEAPSS , The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk