Ball-pen probe

A ball-pen probe[1] is a modified Langmuir probe used to measure the plasma potential[2] in magnetized plasmas.

The ball-pen probe balances the electron and ion saturation currents, so that its floating potential is equal to the plasma potential.

Because electrons have a much smaller gyroradius than ions, a moving ceramic shield can be used to screen off an adjustable part of the electron current from the probe collector.

Ball-pen probes are used in plasma physics, notably in tokamaks such as CASTOR, (Czech Academy of Sciences Torus)[1][2][3] ASDEX Upgrade,[4][5][6][7][8][9][10] COMPASS,[6][7][11][12][13][14][10][15][16][17][1] ISTTOK,[10][18] MAST,[19][20] TJ-K,[21][22] RFX,[23] H-1 Heliac,[24][25] IR-T1,[26] GOLEM[27] as well as low temperature devices as DC cylindrical magnetron in Prague[21][28][29][30][31] and linear magnetized plasma devices in Nancy[32][33] and Ljubljana.

[21][28][34][excessive citations] If a Langmuir probe (electrode) is inserted into a plasma, its potential is not equal to the plasma potential

because a Debye sheath forms, but instead to a floating potential

The difference with the plasma potential is given by the electron temperature

is given by the ratio of the electron and ion saturation current density (

) and collecting areas for electrons and ions (

The ball-pen probe modifies the collecting areas for electrons and ions in such a way that the ratio

and the floating potential of the ball-pen probe becomes equal to the plasma potential regardless of the electron temperature:

A ball-pen probe consists of a conically shaped collector (non-magnetic stainless steel, tungsten, copper, molybdenum), which is shielded by an insulating tube (boron nitride, Alumina).

The collector is fully shielded and the whole probe head is placed perpendicular to magnetic field lines.

When the collector slides within the shield, the ratio

The adequate retraction length strongly depends on the magnetic field's value.

The collector retraction should be roughly below the ion's Larmor radius.

[citation needed] Calibrating the proper position of the collector can be done in two different ways: Using two measurements of the plasma potential with probes whose coefficient

differ, it is possible to retrieve the electron temperature passively (without any input voltage or current).

is measured by the ball-pen probe,

is given by the Langmuir probe geometry, plasma gas composition, the magnetic field, and other minor factors (secondary electron emission, sheath expansion, etc.).

It can be calculated theoretically, its value being about 3 for a non-magnetized hydrogen plasma.

for the ball-pen probe is not exactly equal to one,[5] so that the coefficient

Ball-pen probe used on tokamak CASTOR in 2004. A stainless steel collector moves inside a ceramic (boron nitride) shielding tube.
Schematic picture of a single ball-pen probe. Ions (in red) have a large gyromagnetic radius and can reach the collector more easily than electrons (in blue).
Potential and ln(R) of the ball-pen probe for different positions of the collector