A ball-pen probe is a modified Langmuir probe used to measure the plasma potential 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) ASDEX Upgrade, COMPASS, ISTTOK, MAST, TJ-K, RFX, H-1 Heliac, IR-T1, GOLEM as well as low temperature devices as DC cylindrical magnetron in Prague and linear magnetized plasma devices in Nancy and Ljubljana.^{[excessive citations]}

If a Langmuir probe (electrode) is inserted into a plasma, its potential is not equal to the plasma potential ${\displaystyle \Phi }$ because a Debye sheath forms, but instead to a floating potential ${\displaystyle V_{fl}}$. The difference with the plasma potential is given by the electron temperature ${\displaystyle T_{e}}$:

${\displaystyle \Phi -V_{fl}=\alpha T_{e}}$

where the coefficient ${\displaystyle \alpha }$ is given by the ratio of the electron and ion saturation current density (${\displaystyle j_{e}^{sat}}$ and ${\displaystyle j_{i}^{sat}}$) and collecting areas for electrons and ions (${\displaystyle A_{e}}$ and ${\displaystyle A_{i}}$):

${\displaystyle \alpha =\ln \left({\frac {A_{e}j_{e}^{sat}}{A_{i}j_{i}^{sat}}}\right)=\ln(R)}$

The ball-pen probe modifies the collecting areas for electrons and ions in such a way that the ratio ${\displaystyle R}$ is equal to one. Consequently, ${\displaystyle \alpha =0}$ and the floating potential of the ball-pen probe becomes equal to the plasma potential regardless of the electron temperature:

${\displaystyle V_{fl}=\Phi }$