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The Penning source is a cold cathode ion source that uses crossed electric and magnetic fields to generate ions, primarily molecular ions, at voltages between 2 and 7 kilovolts. Ions are accelerated through a potential difference and can achieve voltages up to 200 kV, striking a target and producing secondary electrons. To prevent damage from these secondary electrons, a suppressor voltage is applied to the accelerator electrode, which must be maintained at a minimum level to ensure safe operation.

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6 views1 page

Code Cathloe

The Penning source is a cold cathode ion source that uses crossed electric and magnetic fields to generate ions, primarily molecular ions, at voltages between 2 and 7 kilovolts. Ions are accelerated through a potential difference and can achieve voltages up to 200 kV, striking a target and producing secondary electrons. To prevent damage from these secondary electrons, a suppressor voltage is applied to the accelerator electrode, which must be maintained at a minimum level to ensure safe operation.

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accnaydungveviecriengtu
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Cold cathode (Penning)

[edit]

The Penning source is a low gas pressure, cold cathode ion source which utilizes crossed electric and
magnetic fields. The ion source anode is at a positive potential, either dc or pulsed, with respect to the
source cathode. The ion source voltage is normally between 2 and 7 kilovolts. A magnetic field,
oriented parallel to the source axis, is produced by a permanent magnet. A plasma is formed along the
axis of the anode which traps electrons which, in turn, ionize gas in the source. The ions are extracted
through the exit cathode. Under normal operation, the ion species produced by the Penning source
are over 90% molecular ions. This disadvantage is however compensated for by the other advantages
of the system.

One of the cathodes is a cup made of soft iron, enclosing most of the discharge space. The bottom of
the cup has a hole through which most of the generated ions are ejected by the magnetic field into the
acceleration space. The soft iron shields the acceleration space from the magnetic field, to prevent a
breakdown.[2]

Ions emerging from the exit cathode are accelerated through the potential difference between the exit
cathode and the accelerator electrode. The schematic indicates that the exit cathode is at ground
potential and the target is at high (negative) potential. This is the case in many sealed tube neutron
generators. However, in cases when it is desired to deliver the maximum flux to a sample, it is
desirable to operate the neutron tube with the target grounded and the source floating at high
(positive) potential. The accelerator voltage is normally between 80 and 180 kilovolts.

The accelerating electrode has the shape of a long hollow cylinder. The ion beam has a slightly
diverging angle (about 0.1 radian). The electrode shape and distance from target can be chosen so the
entire target surface is bombarded with ions. Acceleration voltages of up to 200 kV are achievable.

The ions pass through the accelerating electrode and strike the target. When ions strike the target, 2–3
electrons per ion are produced by secondary emission. In order to prevent these secondary electrons
from being accelerated back into the ion source, the accelerator electrode is biased negative with
respect to the target. This voltage, called the suppressor voltage, must be at least 500 volts and may
be as high as a few kilovolts. Loss of suppressor voltage will result in damage, possibly catastrophic, to
the neutron tube.

Some neutron tubes incorporate an intermediate electrode, called the focus or extractor electrode, to
control the size of the beam spot on the target. The gas pressure in the source is regulated by heating
or cooling the gas reservoir element.

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