3. Diagnostics
3.1.4 Emissive Probe
An emissive probe (EP) is primarily used to establish the plasma potential, and can also be used to determine the amplitude of any rf oscillations in this potential. The tip of an EP consists of a small filament, which when heated, thermionically emits electrons into the plasma. The probe filament is typically heated using an external current supply, and after a certain heating current, will begin to emit. EPs are increasingly being used in a range of fields, including fusion experiments, where they offer a number of advantages over cold probes such as LPs [121, 122]. Essentially two different measurement techniques exist for use with an EP, known as the inflection point and floating potential methods respectively.
Inflection Point Method
The first method, known as the inflection point method, is regarded as the most reliable, and is perhaps the only known method to measure the plasma potential in a vacuum (that
is, with no plasma present) [112, 123]. If the filament is biased, then for biases above the plasma potential, all emitted electrons will escape the probe (we ignore space charge effects here). However, if the probe bias is less than the plasma potential, then these emitted electrons will be reflected and recollected by the probe [112]. This provides a means for establishing the plasma potential, since theIV characteristic of such a probe will show a sharp change near the plasma potential. The probe will also collect plasma electrons, thus changing theIV characteristic, but since the emitted electrons usually have a much lower temperature than the plasma electrons, a change in the characteristic will still be present [123]. The plasma potential is then found as the maximum of the derivative of the IV
curve (which corresponds to the inflection point of the original IV curve) [123]. Electron emission however can introduce space charge effects, which alter the IV curves, and thus the true plasma potential. Therefore a number of IV curves are taken at successively lower electron emission currents, and the plasma potential is found by extrapolation to zero emission [123]. In the presence of rf oscillations of the potential, this method can track both the lower and upper limits of the potential as a result of this oscillation [124]. Unfortunately, this method is slow and time consuming, and requires careful analysis of the resulting curves to distinguish the true inflection points from any noise in the signal, but it does preserve the life of the probe filament, since minimal heating current is needed (thus preventing overheating, and hence melting of the filament).
Floating Potential Method
The second measurement method, which is significantly simpler and easier to implement, is the floating potential method [125]. This method works in a strong electron emission mode, and uses the fact that emission can neutralize the sheath in front of the probe tip. When complete neutralization occurs, the probe filament floats to the plasma potential. By connecting the probe to an oscilloscope or multimeter, the plasma potential can be read directly with almost no analysis required. Additionally, the probe tip can be made much smaller than that of an RFEA for example, thus allowing measurement in small or difficult to reach locations within the plasma. This method has recently been used to map the potential of a DC Hall thruster, where its simplicity of operation allows rapid spatial measurements to be made [126]. The floating potential method however suffers from the fact that it requires strong emission to operate, which can cause probe damage and shorten the life of the filament. Additionally, because of the strong emission, space charge effects can occur which cause the probe to float to a potential different to that of the true plasma potential [121, 127], or the strong emission can perturb the plasma itself [128]. In order to function correctly, the probe needs to be able to emit sufficient electron current, thus if the filament is too small or there is insufficient heating current, this cannot occur, so that the sheath will not be completely neutralized [129].
§3.1 Plasma Diagnostics 73
EP Design
The probe used consists of a small 0.125 mm diameter tungsten filament, inserted into two holes of a 4-bore ceramic tube. These holes also house two copper wires that provide the heating current to the probe tip, and allow transmission of the measured signal current. Normal lead solder cannot be used to connect the filament to the copper wires, since it would melt at the temperatures experienced. Thus a mechanical connection is used instead, where a number of other smaller pieces of tungsten are packed within the holes around the filament and copper wire. This provides both the electrical connection, and also a heat transfer passage to cool the filament somewhat. Although this is a standard EP design [130], it is important that this mechanical connection be made very tight, since due to thermal expansion the electrical connection can become unreliable along the filament length. When this happens, emission can occur intermittently, or within the ceramic tube as opposed to the outside of the filament located in the plasma [130]. This can then severely distort the measured IV characteristics, and additionally can cause charging of the inner wall of the ceramic tube, causing further distortion. Finally, it is important that the filament used be of an even cross-section throughout, since if it is damaged or distorted, the local cross-section will be smaller than elsewhere, so that non-uniform emission can occur. An example of the probe tip used is shown in Fig. 3.11 (a). To create the rounded filament shape shown in this figure, the filament was bent around a smooth cylinder (such as the end of a drill bit) of the desired diameter.
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V 1 kΩ 120 Ω 120 Ω DAQ A B C D Probe (b) (a) Filament Ceramic tube Copper wire Tungsten insertFig. 3.11: (a) Sectioned schematic of the EP. The filament is placed within two holes in the ceramic tube, and heating current is supplied by the copper wires. The tungsten inserts are tightly packed to provide a strong electrical connection. (b) Electrical circuit schematic of the EP when used in the inflection point mode. The probe bias voltage is set from a sweeping power supply (A), and the emitted/collected current is measured across a 1 kΩ sense resistor using an isolation amplifier (B). The filament is heated from a power supply (C) that is isolated from ground using an isolation transformer (D).
−800 −40 0 40 80 4 8 12 Bias Voltage [V] dI/dV [x10 −3 mAV −1 ] −40 −20 0 20 40 60 80 −0.2 0 0.2 0.4 Bias Voltage [V] Current [mA] 36 38 40 42 0 1 2 V inf [V] I e0 /I c0 36 38 40 −0.02 0 0.02 0 0.2 0.4 0.6 0.8 1.0 1.2 0 20 40 60
Heating Current [A]
Floating Potential [V] (a) (b) (c) V p V p I e0→ 0 Heating current: 2.14−2.19 A
Fig. 3.12: (a) EPIV characteristics for a number of filament heating currents. The inset figure shows a magnified view of the curves near the zero axis. (b) Smoothed derivative of the charac- teristics in (a). The voltage at the peak of each of the curves is tracked, and the plasma potential is given by extrapolating this voltage in the limit of zero emitted electron current (illustrated in the inset figure). (c) Floating potential of the EP as a function of heating current. The plasma potential is defined as the knee of the second portion of the curve, as indicated.
The ceramic tube in Fig. 3.11 (a) is then attached to a metal probe shape, such as the one described in the beginning of Section 3.1, and is held together with a retaining ring and grub screw. To prevent plasma from entering the ceramic tube, Autocrete paste is used to cover the holes of the tube. Thus the only exposed part of the probe is the rounded filament tip. As mentioned in Section 3.1.2, magnetic field effects can be important in probe design, and to minimise these effects, the probe filament should be smaller than the electron gyroradius (B <<4.8√Te/d, whereB is the magnetic field, and dis the filament
§3.1 Plasma Diagnostics 75
diameter) [129]. This is because the emitted electrons tend to follow the local field lines, instead of being emitted isotropically, which can lead to increased space charge problems associated with the electron emission process itself. The maximum magnetic field used in this thesis is about 15 mT, which for a representative electron temperature of 5 eV, gives a minimum filament diameter of 0.72 mm. The probe filament diameter is chosen to be smaller than this (0.125 mm).
EP Electronics and Data Analysis
Figure 3.11 (b) shows the measurement and heating circuit used for the inflection point method. The heating current is supplied from a Good Will GPS-3010H regulated DC power generator, which is connected to a Ferguson TS 240/500 isolation transformer. This isolation is vital, since the heating supply cannot be referenced to ground, otherwise the heating current will form part of the measurement current, causing incorrect results at best, or damage to the DAQ system as worst. The outputs of the probe are then connected to two equal 120 Ω resistors, a sense resistor of 1 kΩ, and the HP 6827A power supply (which supplies the filament bias). The HP power supply is fed from LabView software and a DAQ system, and the probe is swept from −80 V to 80 V. The collected current is then measured across the sense resistor by using the Otter isolation amplifier described in Section 3.1.3, before entering the DAQ system for storage. As with the RFEA, 400 averages are taken at each data point with a sampling rate of 20 kHz. Note that because the heating current is continuously on, a voltage drop occurs across the filament. This voltage drop has a value of about 2−2.5 V, and thus the plasma potential is uncertain by about 1−1.25 V. Figure 3.12 (a) shows a number ofIV characteristics taken with the EP operating in the inflection point mode, while Fig. 3.12 (b) shows the derivative of these characteristics, together with the extrapolation of the inflection potentials to the plasma potential.
In the floating potential method, the DAQ, sense resistor, isolation amplifier, and HP power supply are all removed, and the probe is left to float, with this potential relative to ground measured with a high impedance Digitech QM-1320 multimeter. Initially for no or low heating currents, the probe is similar to a normal LP, and sits at the floating potential. As the current increases, the potential begins to increase gradually, before rapidly rising. As the current is further increased, the potential begins to saturate to a roughly constant value. Since the floating potential cannot rise to larger potentials (unless space charge effects are present) this provides a simple means to find the plasma potential. An example of this is shown in Fig. 3.12 (c). Here the plasma potential is defined as the knee of the second portion of the curve, as shown in the figure. This results in an additional uncertainty in the measured plasma potential of about ±1 V, for a total uncertainty (including the voltage drop due to the heating current supply) of about 2−2.25 V.