* ar : - : I OFI ORNL P 153 . : . i i La ý · 45 . : 40 11:25 || 1.4 16 MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU OF STANDARDS -1963 This paper was submitted for publication in the open literature at least 6 months prior to the issuance date of this Micro- card. Since the U.S.A.E.C. has no evi- dence that it has been published, the pa- per is being distributed in Microcard form as a preprint. LEGAL NOTICE This report was prepared as an account of Government sponsored work. Neither the United States, nor the Commission, nor any person acting on behalf of the Commission: A. Makes any warranty or representa- tion, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, appa- ratus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this report. As used in the above, "person acting on behalf of the Commission”includes any em- | ployee or contractor of the Commission, or employee of such contractor, to the extent that such employee or contractor of the Commission, or employee of such contractor prepares, disseminates, or provides access to, any information pursuant to his employ- ment or contract with the Commission, or his employment with such contractor. - - - - - - - ع D7 دکر - ما Listi ir .. @ 1 1 109 Simple Manentim Probe for Plasma Studies* I. Alexeft and W. D. Jones This paper was submitted for publication in the open literature at least onths prior “o the issuance date of the Micro- card. Since the V.8.A.E.C. has no evi. dence that it has beer. publishod, the pa- per is being distributed in Microcard form as a preprint. We have been investigating a very simple magnetic probe for studying the properties f a plasma. The probs is merely a bare wire immersed in the plasua. When an. electrio current of high amperage flows through the wire, the wire is surrounded by a magnetic field which repels charged particles, By measuring the alectric current caused by the charged particles which are able to reach the wire, as a function of the magnetic Meld, we obtain pos information as to the momentum distribution of the charged particles in the plasma. In this respect, this probe differs from the Langmuir probe', which measures the energy distribution of the charged particles. Although other workers have used magnetic fields to analyze the momentum of particles emitted from & plasma?, apparently the use of the probe wire itself as the magnet is novel. The experimental apparatus is shown in Fig. 1. The probe was made of # 10 copper wire (0.259 cm diam) bent into a loop about 1 om in diameter, For a magnet supply, we used an ordinary soldering guns. This simple probe carried a peak magnet current of 425 A for one sec without severe heating. The peak potential drop in the wire exposed to the plasma was less than 0.1 v. The peak magnetic field at the surface of the wire was about 50 G. The experimental results obtained with this probe are shown in Fig.2. In Fig. 2, top, the upper ao trace represents the magnetizing current and therefore the magnetic field. The lower ac trace représents the electron current to the probe as a function of the magnetic Meld. The probe is biased approximately to plasma potential. The upper and lower horizontal traces *Rodsearch sponsored by the U, S. Atonic Energy Camission under contract ith the Union Carbide Corporation, . - - - - . - - represent zero probe electron current !circuit opon) and steady - state electron current (no magnet current), respectively. As the as magnetic field begins to rise, the proba electron current drops from approximately the steady - state value to zero. As the magnetic Meld dropæ , back toward zero, the probe electron current again rises. Note that tiie electron current varies at twice the frequency of the magnet supply voltage, a fact which may make this probe rather insensitive to power - lile pickup. The charaoteristic curve of the probe is shown in F4.g.2, botton. Here, the magnet supply voltage is used for the sweep of the oscilloscope, and the probe electron current, t'or the vertical deflection. Thus the curve is a plot of probe current vs magnetic field. The slight asymmetry betwzen the two halves of the ac cycle in this figure, and the slight displacement of the ac electron current trace from the two dc current traces in Fig. 2, top, aus thought to arise in the differential amplifiers. A simple, order - of - magnitude calculation shows that the probe is acting as it should. If an electron of velocity v (on secoT) is directed perpendicularly toward the wire, then a magnetic field 1 (Q) is needed to " prevent the electron from just reaching the wire, where H is given by H = mcy er in ' . : irisons pod istim In the above expression, m is the mass of the electron (em, c is the speed of light (a sec-1), e is the electronic charge (esu), ş is the radius of the - (oma wire (cm), and R is a cut - off distance for the magnetic Meld. In our experimental case, R is the distance at which the pot dependence of the magnetic field with radius changes to the r-> dependence of a lipole. . This change occurs at approximately two loop radii from the center of the loop of wire which forms the probe. : i in molimo b rest with Applying the above equation to our experimental conditions, we find that the plasma electron current to the probe should be strongly attenuated for a magnet current of 160 A. This theoretical prediction corresponds to an electron temperatura of 2.8 eV, a value previously obtained with this discharge tube under similar operating conditions. The above theoretical prediction agrees well with the experimental results, and corresponds to the electron current cut - off in the experimental curre of Fig. 2, bottom. Since this momentin probe appears to work, questions concerning its applications arise. We have thought of the following applications: 1. For studying the momentum distribution of the electrons in a plasma; 2. For studying the ions in a plasma by keeping the electrons from reaching the probe; 3. For suppressing the secondary electron emission from the probe; 4. For making a mass analysis of the various ionic species in a plasma. Finally, one can also vary the electrostatic potential on the magnetic probe in a manner analogous to a Langmuir probe. Thus, one can perhaps combine both the magnetic probe and the Langmuir probe into one, simple instrument. : . . · Acknowledgement The authors wish to express their appreciation to John G. Harris, who constructed most of the apparatus used in these studies.. . References 1. M. Mott - saith and Irving Langmuir, Phys. Rev. 28, 727 (1926). 2. G. Francis, Handbuch der Physik XXII, 61, (1956). K. G. Emeleus, J. Nuclear Energy, part C, 2, 69 (1961). 3. Weller Soldering Gun D - 440, 145 - 210 W, Weller Electric Corp., Easton, Pa. . . . ..: : . ... . .. .".... . i 4. . . Mortmore.. • rivier ook de en Figures Fig 1. Schematic of Apparatus The discharge tube is a sphere 25 om in diameter. The gas used was argon at a pressure of 0.3 Torr. The discharge voltage and current were about 50 V and 200 ma, respec'cively. The plane of the loop of the magnetic probe was perper. licidar to the anode - cathode axis. Fig 2, Oscillograms Showing Experimental Results Upper picture: The maximum magnet ourront show on the upper ac trace is 425 A. The maximus electron current shown on the lower ac trace is 2 ma, corresponding to the maximum downward deflection. The sweep is.2 millisec per large division. Lower picture; The Characteristic Curve of Thə Magnetic Probe. The horizontal axis corresponds to magnet current and represents 85 A per large division. The' vertical axis corresponds to plasma electron current, and represents 0.42 mA per large division. .:... r arion I Sie w dobro HOT. CATHODE CANODE :. . -50 V ..rs PROBE- TO TOP TRACE ON OSCILLOSCOPE VIA DIFFERENTIAL AMPLIFIER > 10.12 :: HIGH CURRENT Lee TRANSFORMER (SOLDERING GUNPOO TO BOTTOM TRACE ON OSCILLOSCOPE VIA DIFFERENTIAL AMPLIFIER ** 110 V AC. . - DISCHARGE TUBE WITH MOMENTUM PROBE Medicina Synonymer og .. .. .. .. . . . . . . 1 - ا :: .:.:.:.:.. . . . .. . . . . : البترین مہنہ تشن ہ مس.. ت مخ-- . . . . . . . .. . راسها • .:.. • • اندی به ":": ": | :: : |:. م . : . งานนน . .. . :.:. ۱ - :.:.:.::.:.:. . . . :: : :: : :: :: :: 11: مانسنه دلمه من : . . (ت . . . :.:... ... فنانة : ...بعد : : .. . " .. . : م نمنم - . | . . . | . * * * :: :: . . . . ! .. ... .. . . . 1 . - نت من تن منممنمه ستممممممممممممممه ،عصمه د ا ر END DATE FILMED 18/ 25 /65 ' .