CN102650663B - Method for acquiring voltage-current characteristic curve of plasma - Google Patents

Abstract

The invention relates to a method for acquiring a voltage-current characteristic curve of plasma. The method is characterized in that plasma parameters are probed by using a Langmuir probe under the condition of radio-frequency discharge mode; a point to be probed is probed by the Langmuir probe under the control of a scanning voltage to acquire sampling values of the parameters, thereby, the voltage-current characteristic curve of the plasma is acquired, wherein one-time probe working time comprises a plurality of discharge cycles; the stable scanning voltage is applied to the Langmuir probe during the same discharge cycle; and each discharge cycle corresponds to one different scanning voltage.

Description

A kind of acquisition voltage-current characteristic curve of plasma method

Technical field

The present invention relates to plasma field, particularly one obtains voltage-current characteristic curve of plasma method.

Background technology

As shown in Figure 1, Fig. 1 is traditional Langmuir probe operating diagram.In plasma, the evenly heat movement velocity of electronics is much larger than ion velocity, therefore, sprays electron number on detecting probe surface much larger than number of ions, when probe is in certain negative potential V when probe has just entered plasma in the unit interval _ftime can repel electronics attract ion, make probe current be zero, current potential V _fbe called floating potential.When reality is tested, probe and Plasma space potential V _sdifference V _pbe difficult to direct acquisition, the potential difference (PD) V between a reference electrode normally in measuring probe relative discharge system.If the potential difference (PD) of Plasma space potential and reference electrode is V _rif, reference electrode ground connection, V _rfor the space potential of plasma.Then have

V＝V _R+V _P(1)

As shown in Figure 2, Fig. 2 is the volt-ampere characteristic figure of plasma.Get V _pfor horizontal ordinate, probe current I is ordinate, can obtain the volt-ampere characteristic of probe.Suppose that the characteristic parameter of plasma is constant, the potential difference (PD) of Plasma space potential and reference electrode is V _rbe an invariant, the volt-ampere characteristic of so actual test gained is equivalent to the result after moving horizontally on the basis of Fig. 2.

Adopt simple condition can make the principle of work of electrostatic probe to explain clearly.Simple condition refer to ignore probe plasma interference, the secondary emission of detecting probe surface, plasma and probe material reaction and without magnetic fields etc.

The electric current I that electrostatic probe is collected is get to the electronic current I of detecting probe surface _ewith gas current I _isum, the direction that definition electric current flows to plasma from probe is just, then have:

I＝I _e-I _i(2)

If plasma is not by disturbance and current potential is zero, then probe current potential is V _p, make the electronics outside sheaths, ion concentration be respectively n _e, n _i, plasma electron can be obtained and ion velocity distributive law is according to Maxwell's distribution:

$f\left(V_e\right)=4\mathrm{\π}{\left(\frac{m_e}{2\mathrm{\πk}{T}_e}\right)}^{2/3}{V}_e^2\exp \left(\frac{{-m}_e{V}_e^2}{2{\mathrm{kT}}_e}\right)---\left(3\right)$

$f\left(V_i\right)=4\mathrm{\π}{\left(\frac{m_i}{2\mathrm{\πk}{T}_i}\right)}^{2/3}{V}_i^2\exp \left(\frac{{-m}_i{V}_i^2}{2{\mathrm{kT}}_i}\right)---\left(4\right)$

V _pduring > 0, probe surrounding space will form electronics sheaths, and the electronics entering detecting probe surface sheaths all arrives detecting probe surface under electric field action.Ion because speed is much smaller than electronics, and now also by the repulsion of electric field, thus can be out in the cold relative to gas current electronic current, so the electric current that probe is collected is:

$I=I_{e0}=\frac{1}{2}{n}_e\mathrm{eS}{\∫}_0^{\∞}{v}_{e}f\left({\mathrm{\υ}}_e\right)d{v}_e{\∫}_0^{\mathrm{\π}/2}\cos \mathrm{\θ}\sin \mathrm{\θd\θ}---\left(5\right)$

I _e0be called electronics saturated flow, θ is the direction of motion of electronics and the angle of detecting probe surface normal direction, and S is the surface area of probe, can obtain after formula being substituted into

$I_{e0}=\frac{1}{4}{n}_e\mathrm{eS}\sqrt{\frac{8k{T}_e}{{\mathrm{\πm}}_e}}\≈2.5\×{10}^{-14}{n}_{e}S\sqrt{k{T}_e}---\left(6\right)$

Wherein, kT _eunit be taken as electron volts.

V _p< V _ftime, electronic current constantly reduces, and probe current will be tending towards ion saturation current.Ion saturation current can not draw as the derivation of electronics saturation current, this is because probe current potential can form positive ion sheaths lower than positive ion after floating potential around probe, the formation of positive ion sheaths needs ion to enter the incidence rate u of sheaths _smeet Bohm criterion, that is:

$u_s\&GreaterEqual;\sqrt{{\mathrm{kT}}_e/m_i}---\left(7\right)$

And be called glass nurse speed.Slightly little at the ion density ratio plasma slab of positive ion sheaths boundary, general satisfaction:

n _is＝0.605n ₀(8)

So ion saturated flow can be obtained:

$I_{i0}=0.605{\mathrm{en}}_{0}S\sqrt{{\mathrm{kT}}_e/m_i}---\left(9\right)$

V _f< V _pthe scope of < 0 is often called zone of transition, is the region that in probe volt-ampere characteristic, curent change is the fastest.Electronics will reach detecting probe surface must overcome the resistance that electric field brings, and now the speed of electronics must meet the electronic current that now probe is collected is:

$I_e=\frac{1}{2}{n}_e\mathrm{eS}{\&Integral;}_0^{\mathrm{\&pi;}/2}\mathrm{d\&theta;}{\&Integral;}_{\sqrt{\frac{-2e{V}_p}{m_e}}\frac{1}{\cos \mathrm{\&theta;}}}^{\&infin;}{v}_{e}f\left(v_e\right)\cos \mathrm{\&theta;}\sin \mathrm{\&theta;d}{v}_e=I_{e0}\exp \left(\frac{{\mathrm{eV}}_P}{{\mathrm{kT}}_e}\right)---\left(10\right)$

So, the total current of probe collection can be obtained according to formula (11):

${I=I}_{e0}\exp \left(\frac{{\mathrm{eV}}_P}{{\mathrm{kT}}_e}\right)-I_{i0}---\left(11\right)$

V is worked as by formula (1) is known _pwhen=0, probe current potential equals Plasma space potential (reference electrode ground connection), and now I-V slope of a curve reaches maximal value, and this horizontal ordinate corresponding to point is space potential V _s.Usually V is determined from the extension cord intersection point of surveyed volt-ampere characteristic zone of transition straight line portion and electronics saturated flow when process data _pthe point of=0.

Confirm Plasma space potential just to revise the current potential of probe relative to plasma, can be tried to achieve by formula (11)

$\frac{{\mathrm{eV}}_P}{{\mathrm{kT}}_e}=\ln \left(\frac{{I+I}_{i0}}{I_{e0}}\right)---\left(12\right)$

Namely

${\mathrm{kT}}_e=\left|\frac{d\left({\mathrm{eV}}_P\right)}{d\ln (I+I_{i0})}\right|---\left(13\right)$

Therefore, the voltage of being tried to achieve by experiment and the semilog family curve of electric current just can try to achieve the electron temperature of plasma at the slope of zone of transition.Because probe current potential V is equivalent to V in practical operation _padd a definite value, the V therefore in formula (13) _pchange probe potential value V into and can not affect result.

From electronics saturated flow formula or ion saturated flow formula: after obtaining the electron temperature of plasma, by effective collection area S and the measured electronics saturated flow I of probe _e0or ion saturated flow I _i0the density of plasma can be tried to achieve.

In addition, V is worked as _p=V _ftime, probe current I=0, namely

$I_{e0}\exp \left(\frac{{\mathrm{eV}}_F}{{\mathrm{kT}}_e}\right)=I_{i0}---\left(14\right)$

Therefore, as long as floating potential V has been measured in experiment _f, electronics saturated flow I _e0and ion saturated flow I _i0also the electron temperature of plasma can be obtained.

What the linear hollow cathode discharge system of current magnetic confinement adopted is direct current high voltage pulses mode, discharge period is hundred microsecond ranks, because in the recurrence interval, plasma parameter can change, need the abundant scan period accurately could embody its real processes, therefore, if want that the mode by rapid scanning obtains the volt-ampere characteristic of probe, need sweep frequency up to about 100kHz, corresponding sweep frequency is 10 μ s.Actual technical difficulty has been suffered from when taking this scheme.First, under so high-frequency condition, ion saturated flow end is difficult to meet test request due to the differential mode ratio of the too little difference channel of electric current; Secondly, the undesired signal that high pressure pulse discharge process is brought is easy to test signal to flooding.Again, when scanning voltage frequency is too high, power amplifier carries out easily producing signal distortion in amplification process to the voltage signal that signal generator produces, and causes scanning voltage distorted signals.

Summary of the invention

The object of the invention is to, for solving the problem, proposing a kind of acquisition voltage-current characteristic curve of plasma method.

For achieving the above object, a kind of acquisition voltage-current characteristic curve of plasma method, the method detects by Langmuir probe plasma parameter under radio frequency discharge mode condition, described Langmuir probe carries out detection to the sampled value that gets parms to a sensing point under the control of scanning voltage, thus obtains the volt-ampere characteristic of plasma; Wherein, a probe face time comprises multiple discharge cycle, applies a stable scanning voltage in same discharge cycle to described Langmuir probe, the corresponding different scanning voltage of each discharge cycle.

The method employing of the described sampled value that gets parms gathers the parameter sampling value in the probe face time using radio frequency discharge voltage as trigger pip trigger oscillographic device.

Described parameter sampling value comprises: sparking voltage, discharge current, probe voltage and probe current.

Described plasma parameter comprises: plasma temperature, plasma density and floating potential.

The invention has the advantages that, under radio frequency discharge condition, when discharge frequency is too high cause to complete the scanning process of scanning voltage in a discharge cycle time, impulse discharge pulse is adopted to provide stable scanning voltage, multiple discharge completes the scanning process of scanning voltage, thus the complete volt-ampere characteristic obtained required for plasma parameter detection, obtain the detail parameters such as plasma density, temperature, floating potential." fast frame " oscillographic use provides technical support for impulse discharge measurement, thus can obtain in impulse discharge process, the plasma parameter of each time point, and then can obtain providing data foundation for analyzing large area plasma discharge process.The discharge frequency of radio frequency electric discharge is not particularly limited condition, namely under radio frequency discharge condition, the method plasma parameter all can be adopted to detect.The point getting some in data handling procedure in a stable region does on average, then can the effectively interference that brings of abating noises signal.

Accompanying drawing explanation

Fig. 1 is traditional Langmuir probe operating diagram;

Fig. 2 is the volt-ampere characteristic figure of plasma;

Fig. 3 is the operating diagram of acquisition voltage-current characteristic curve of plasma method of the present invention.

Embodiment

Below in conjunction with the drawings and specific embodiments, the present invention will be described in detail.

In the discharge process of short time, the plasma parameter of each discharge cycle and change procedure thereof can be thought identical.Can the voltage that in same discharge cycle, applying one is stable be given, the voltage that each cycle correspondence one is different, the electric current corresponding to each cycle be recorded simultaneously.Like this, the volt-ampere characteristic of plasma can be obtained equally.The specific implementation of this process mainly relies on " fast frame " (Fastframe) function that digital oscilloscope has, it often can come one and trigger just record one frame within a period of time, and every frame can comprise the data of all passages in this scan cycle.

As shown in Figure 3, Fig. 3 is the operating diagram of acquisition voltage-current characteristic curve of plasma method of the present invention.When practical operation, with the sparking voltage after decaying for trigger pip, oscillographic record frame number is set to 400 frames, and whole record period is then 400 × T _p, T _pit is discharge cycle.Is set to the time being slightly less than record period the scan period of scanning power supply, voltage is raised to+70V from-70V, the current value that oscillograph is corresponding under intactly recording different voltage.The oscillograph that experiment uses is TDS7104 model, and its each frame under 400 frames " fast frame " pattern can respectively record 500 sampled points to four sampling channels.These 500 sampled points can embody the situation of change of the plasma parameter in a pulse width time.And the point getting some in a stable region does on average, then can the effectively interference that brings of abating noises signal.

In a probe face process, oscillograph can collect the parameters such as sparking voltage, discharge current, probe voltage and probe current simultaneously.After a probe detection terminates, by outside switch data gathering line, another Langmuir probe can be converted to, the plasma parameter test making it carry out current location point.After probe data collection completes, regulate translation stage position by automatically controlled cabinet, the data acquisition of the second place can be carried out.Circulate this operation, namely can complete the two-dimensional parameter collection of whole plasma sheet.Wherein, Langmuir probe number is that 7, seven probe groups are in a row.Data analysing method adopts factorization voluntarily, the data of every frame corresponding point is taken out, obtains the discharge current under corresponding discharge voltage condition, and the corresponding data group of 400 frame data forms the volt-ampere characteristic of calculating plasma parameter.The parameter such as plasma density, temperature, floating potential of this sensing point can be obtained by family curve.The data all gathered are carried out such operation, can obtain by 7*N data point form whole two-dimentional plasma parameter, and then can by it carry out scientific analysis research.Wherein, N is the number of times of translation stage movement, i.e. longitudinal data density.

It should be noted last that, above embodiment is only in order to illustrate technical scheme of the present invention and unrestricted.Although with reference to embodiment to invention has been detailed description, those of ordinary skill in the art is to be understood that, modify to technical scheme of the present invention or equivalent replacement, do not depart from the spirit and scope of technical solution of the present invention, it all should be encompassed in the middle of right of the present invention.

Claims (4)

1. one kind obtains voltage-current characteristic curve of plasma method, the method detects by Langmuir probe plasma parameter under radio frequency discharge mode condition, described Langmuir probe carries out detection to the sampled value that gets parms to a sensing point under the control of scanning voltage, thus obtains the volt-ampere characteristic of plasma; Wherein, a probe face time comprises multiple discharge cycle, applies a stable scanning voltage in same discharge cycle to described Langmuir probe, the corresponding different scanning voltage of each discharge cycle;

Wherein, after a probe detection terminates, by outside switch data gathering line, another Langmuir probe can be converted to, the plasma parameter test making it carry out current location point; After probe data collection completes, regulate translation stage position by automatically controlled cabinet, the data acquisition of the second place can be carried out; Circulate this operation, namely can complete the two-dimensional parameter collection of whole plasma sheet; Offset signal is sawtooth wave, and in a probe face process, oscillograph can collect sparking voltage, discharge current, probe voltage and probe current simultaneously.

2. acquisition voltage-current characteristic curve of plasma method according to claim 1, it is characterized in that, the method employing of the described sampled value that gets parms gathers the parameter sampling value in the probe face time using radio frequency discharge voltage as trigger pip trigger oscillographic device.

3. acquisition voltage-current characteristic curve of plasma method according to claim 1, is characterized in that, described parameter sampling value comprises: sparking voltage, discharge current, probe voltage and probe current.

4. acquisition voltage-current characteristic curve of plasma method according to claim 1, it is characterized in that, described plasma parameter comprises: plasma temperature, plasma density and floating potential.