A ball-pen probe
is a modified
Langmuir probe
A Langmuir probe is a device used to determine the electron temperature, electron density, and electric potential of a plasma. It works by inserting one or more electrodes into a plasma, with a constant or time-varying electric potential between ...
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 The gyroradius (also known as radius of gyration, Larmor radius or cyclotron radius) is the radius of the circular motion of a charged particle in the presence of a uniform magnetic field. In SI units, the non-relativistic gyroradius is given by
:r_ ...
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
tokamak
A tokamak (; russian: токамáк; otk, 𐱃𐰸𐰢𐰴, Toḳamaḳ) is a device which uses a powerful magnetic field to confine plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being d ...
s such as CASTOR, (Czech Academy of Sciences Torus)
ASDEX Upgrade,
COMPASS
A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...
,
ISTTOK,
MAST,
TJ-K,
RFX,
H-1 Heliac,
IR-T1,
GOLEM
as well as low temperature devices as DC cylindrical magnetron in
Prague
Prague ( ; cs, Praha ; german: Prag, ; la, Praga) is the capital and largest city in the Czech Republic, and the historical capital of Bohemia. On the Vltava river, Prague is home to about 1.3 million people. The city has a tempera ...
and linear magnetized plasma devices in
Nancy and
Ljubljana
Ljubljana (also known by other Ljubljana#Name, historical names) is the Capital city, capital and largest city of Slovenia. It is the country's cultural, educational, economic, political and administrative center.
During antiquity, a Roman city ...
.
Principle
If a
Langmuir probe
A Langmuir probe is a device used to determine the electron temperature, electron density, and electric potential of a plasma. It works by inserting one or more electrodes into a plasma, with a constant or time-varying electric potential between ...
(electrode) is inserted into a
plasma, its potential is not equal to the plasma potential
because a
Debye sheath The Debye sheath (also electrostatic sheath) is a layer in a plasma which has a greater density of positive ions, and hence an overall excess positive charge, that balances an opposite negative charge on the surface of a material with which it is i ...
forms, but instead to a floating potential
. The difference with the plasma potential is given by the
electron temperature
Plasma parameters define various characteristics of a plasma, an electrically conductive collection of charged particles that responds ''collectively'' to electromagnetic forces. Plasma typically takes the form of neutral gas-like clouds or char ...
:
where the coefficient
is given by the ratio of the electron and ion saturation current density (
and
) and collecting areas for electrons and ions (
and
):
The ball-pen probe modifies the collecting areas for
electrons
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kno ...
and
ions
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
in such a way that the ratio
is equal to one. Consequently,
and the floating potential of the ball-pen probe becomes equal to the plasma potential regardless of the
electron temperature
Plasma parameters define various characteristics of a plasma, an electrically conductive collection of charged particles that responds ''collectively'' to electromagnetic forces. Plasma typically takes the form of neutral gas-like clouds or char ...
:
Design and calibration
A ball-pen probe consists of a conically shaped collector (non-magnetic
stainless steel,
tungsten
Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...
,
copper,
molybdenum
Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with l ...
), which is shielded by an insulating tube (
boron nitride
Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagon ...
,
Alumina). The collector is fully shielded and the whole probe head is placed perpendicular to
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
lines.
When the collector slides within the shield, the ratio
varies, and can be set to 1. The adequate retraction length strongly depends on the
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
's value. The collector retraction should be roughly below the ion's
Larmor radius. Calibrating the proper position of the collector can be done in two different ways:
#The ball-pen probe collector is
biased by a low-frequency voltage that provides the
I-V characteristics and obtain the
saturation current The saturation current (or scale current), more accurately the reverse saturation current, is the part of the reverse current in a semiconductor diode caused by diffusion of minority carriers from the neutral regions to the depletion region. This c ...
of electrons and ions. The collector is then retracted until the
I-V characteristics becomes symmetric. In this case, the ratio
is close to unity, though not exactly.
If the probe is retracted deeper, the I-V characteristics remain symmetric.
#The ball-pen probe collector potential is left floating, and the collector is retracted until its potential saturates. The resulting potential is above the Langmuir probe potential.
Electron temperature measurements
Using two measurements of the plasma potential with probes whose coefficient
differ, it is possible to retrieve the
electron temperature
Plasma parameters define various characteristics of a plasma, an electrically conductive collection of charged particles that responds ''collectively'' to electromagnetic forces. Plasma typically takes the form of neutral gas-like clouds or char ...
passively (without any input voltage or current). Using a Langmuir probe (with a non-negligible) and a ball-point probe (whose associated
is close to zero) the electron temperature is given by:
where
is measured by the ball-pen probe,
by the standard Langmuir probe, and
is given by the
Langmuir probe
A Langmuir probe is a device used to determine the electron temperature, electron density, and electric potential of a plasma. It works by inserting one or more electrodes into a plasma, with a constant or time-varying electric potential between ...
geometry, plasma gas composition, the
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
, and other minor factors (
secondary electron emission
In particle physics, secondary emission is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emi ...
, sheath expansion, etc.) It can be calculated theoretically, its value being about 3 for a non-magnetized hydrogen plasma.
In practice, the ratio
for the ball-pen probe is not exactly equal to one,
so that the coefficient
must be corrected by an empirical value for
:
where
References
External links
PhD thesis, Jiri Adamek(Czech and English language)
Overview: Ball-pen probe design, theory and first results at different fusion devices
Examination of plasma current spikes and general analysis of H-mode shots in the tokamak COMPASSElectrical Probe Measurements on the COMPASS TokamakProbe Measurements on the COMPASS TokamakScanning ion sensitive probe for plasma profile measurements in the boundary of the Alcator C-Mod tokamakDevelopment of Probes for Assessment of Ion Heat Transport and Sheath Heat Flux in the Boundary of the Alcator C-Mod Tokamak{Dead link, date=November 2018 , bot=InternetArchiveBot , fix-attempted=yes
Video: Temporal evolution of Type-I ELM in the divertor region on the COMPASS tokamak.
Plasma diagnostics