US-12621920-B2 - Plasma control for spark optical emission spectroscopy

Abstract

An apparatus for plasma control is disclosed. The apparatus comprises: a plasma generator comprising two electrodes, an anode and a cathode, configured to produce a plasma between the two electrodes; a solenoid coil disposed to surround the plasma and configured to produce a magnetic field parallel to a longitudinal axis between the two electrodes; and circuitry configured for allowing independent timing of the magnetic field with respect to the production of the plasma. A method for plasma control in a spectroscopy system and an optical emission spectrometer using said method are also disclosed.

Inventors

• Patrick Lancuba
• Sean Kellogg

Assignees

• THERMO FISHER SCIENTIFIC (ECUBLENS) SARL
• FEI COMPANY

Dates

Publication Date

20260505

Application Date

20220509

Claims (18)

1. 1 . A method for plasma control in a spectroscopy system, the method comprising: generating a plasma; confining the plasma around a longitudinal axis by applying a magnetic field parallel to said longitudinal axis, wherein timing of applying the magnetic field is controlled separately from timing of generating the plasma; and observing a spectrum produced by the plasma at a location near or at the longitudinal axis.
2. 2 . The method of claim 1 , wherein the magnetic field is modulated and produced by a solenoid coil, surrounding the plasma, in which a variable current is discharged.
3. 3 . The method of claim 2 , wherein the modulation spans one of a nanosecond, a microsecond, or a millisecond regime.
4. 4 . The method of claim 2 , wherein the solenoid coil is made of a hollow metal tube in which a temperature management fluid flows.
5. 5 . The method of claim 4 , wherein the temperature management fluid actively cools the solenoid coil.
6. 6 . The method of claim 2 , wherein the solenoid coil is one of: a single turn coil or a multi turn coil and/or a single-layer coil or a multi-layer coil.
7. 7 . The method of claim 2 , wherein the solenoid coil has a length that is greater than its turn diameter.
8. 8 . The method of claim 2 , wherein a current used for producing the plasma is controlled by a first electronic circuit.
9. 9 . The method of claim 8 , wherein the current used for producing the magnetic field is controlled by a second electronic circuit separate from the first electronic circuit.
10. 10 . The method of claim 9 , wherein the current used for producing the magnetic field is modulated by the second electronic circuit in a way that produces a modulated discharge comprising a combination of an exponential decay and a sinusoid.
11. 11 . The method of claim 10 , wherein the current used for producing the magnetic field is modulated by the second electronic circuit to time it with respect to the current used for producing the plasma in such a manner that it optimizes a production of ionic and atomic spectral lines and decreases line interferences.
12. 12 . The method of claim 1 , wherein generating the plasma and/or applying the magnetic field is performed repetitively at a frequency from 1 Hz to 1 MHz.
13. 13 . The method of claim 1 , wherein the spectroscopy system is one of: a Spark Optical Emission Spectrometer (OES); an Inductively Coupled Plasma (ICP) OES; a plasma Mass Spectrometer (MS); a Laser Induced Breakdown Spectrometer (LIBS); and a Glow Discharge Mass Spectrometer (GDMS).
14. 14 . An apparatus for plasma control comprising: a plasma generator comprising two electrodes, including an anode and a cathode, configured to produce a plasma between the two electrodes; a solenoid coil disposed to surround the plasma and configured to produce a magnetic field parallel to a longitudinal axis between the two electrodes; and circuitry configured for allowing independent timing of the magnetic field with respect to the production of the plasma.
15. 15 . The apparatus of claim 14 , wherein the magnetic field production is timed in such manner that it induces an optimal tangential Larmor current on an external surface of the plasma which, combined with the magnetic field, produces a radial and inward Lorentz force that results in an optimal plasma compression.
16. 16 . The apparatus of claim 14 , the circuitry further comprising: a first electronic circuit that modulates a current used for producing the plasma; a second electronic circuit that modulates another current used for producing the magnetic field; and a logical connection between the first electronic circuit and the second electronic circuit, configured for allowing independent timing of the magnetic field with respect to the production of the plasma.
17. 17 . A spectrometer comprising the apparatus of claim 14 .
18. 18 . An optical emission spectrometer comprising: a plasma generator comprising an electrode, configured to produce a plasma between the electrode and a sample; a solenoid coil disposed to surround the plasma and configured to produce a magnetic field parallel to a longitudinal axis between the electrode and the sample; circuitry configured for allowing independent timing of the magnetic field with respect to production of the plasma; an optical system for dispersing emitted light from the plasma into discrete wavelengths; and a detection system for detecting intensity of the dispersed emitted light.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This is the U.S. National Stage of International Application No. PCT/EP2022/062432 filed May 9, 2022, which was published in English under PCT Article 21(2), which in turn claims the benefit of U.S. Provisional Application No. 63/212,475, filed Jun. 18, 2021. The provisional application is incorporated herein in its entirety. FIELD The present disclosure relates generally to systems, devices, and methods for plasma control, and more specifically to systems, devices, and methods for plasma control for spark optical emission spectroscopy. BACKGROUND Optical emission spectroscopy (OES), is a technique for the elemental analysis of samples and is particularly useful, for example, in the analysis of solid, metallic samples. The present disclosure relates to OES wherein a spark (herein used to refer to any electrical spark, arc or discharge) is used to rapidly vaporize a sample and excite elements in the vaporized sample, i.e. so-called spark OES. Light is emitted by the excited elements of the sample as transitions occur from an excited state to a lower energy state. Each element emits light of discrete wavelengths characteristic of its electronic structure, which are also termed spectral lines. By detecting the spectral lines, OES can provide a qualitative and quantitative determination of the elemental composition of the sample. A conventional spark optical emission spectrometer typically includes a spark generator for exciting the elements in the sample to emit light, an optical system for dispersing the emitted light into discrete wavelengths, a detection system for detecting the light intensity of the dispersed light and a data storage and processing system for storing and processing signals from the detection system representing the light intensity. To build up sufficient data for determination of the composition, a succession of sparks is typically employed and the resulting data generated from the sparks is accumulated for processing. A single measurement typically comprises several tens of seconds of analysis at a spark frequency of a few hundred hertz. This is necessary for two key reasons. The first reason is that the spark attacks different positions on the sample's surface depending on the presence of e.g. inhomogeneities or inclusions. This results in the spark not always being centered between the sample and the electrode. The second reason is that the spark oscillates as a function of time around the central electrode. This can be caused by a combination of, among others, the supersonic expansion of the plasma, turbulences in the gas flow, physical differences in a sample's composition (e.g. different melting points), etc. . . . This is illustrated in FIG. 2, where the plasma form and position is shown. Each of the subfigures (201 to 206) corresponds to a snapshot taken at a different time. To achieve statistically consistent and stable results, each spectroscopic measurement needs to be composed of thousands of individual sparks to compensate for these fluctuations and ensure high precision and high reproducibility. However, such strategy comes at a cost of having longer analysis times and of limiting the ability to measure low concentrations, as these will fall within statistical noise produced by the non-stable nature of the plasma. In other words, this limits the lowest concentrations that can be measured with Spark-OES technology. There is therefore a need for controlling the form-factor and time evolution of the plasma in order to a) stabilize the plasma as a function of time; b) improve the signal-to-noise ratio and, therefore, improve the limits of detection; and c) reduce overall analysis time, which would result in operational cost savings. BRIEF DESCRIPTION OF THE DRAWINGS The present disclosure will now be described in more detail by way of examples and with reference to the accompanying drawings in which: FIG. 1A is a schematic view illustrating the typical structure of an optical emission spectrometer; FIG. 1B is a schematic view illustrating the typical structure of a plasma between an electrode and a sample; FIG. 2 shows a series of photographs of a plasma taken at different times; FIGS. 3a-3e illustrate the concept of “theta pinch” described in the present disclosure; FIG. 4 illustrates a plasma control apparatus according to an embodiment of the present disclosure; FIGS. 5A-5B illustrate examples of solenoid coils according to embodiments of the present disclosure; FIG. 6 is a graphical representation of several types of current discharge as a function of time; FIG. 7 illustrates a method for plasma control in a spectroscopy system according to an embodiment of the present disclosure; and FIG. 8 is a schematic view illustrating the structure of an optical emission spectrometer according to an embodiment of the present disclosure. SUMMARY According to the present disclosure, there is provided a method for plasma control in a spectroscopy sy