US2852721A - Glow discharge circuits - Google Patents

Description

P 16, 1958 F. HARDERS ET AL 2,852,721

GLOW DISCHARGE CIRCUITS Filed Jan. 15, 1957 5 Sheets-Sheet 1 F. HARDERS ET AL 2,852,721

Spt. 16, 1958 GLOW DISCHARGE CIRCUITS 5 Sheets-Sheet 2 Filed Jan. 15, 1957 United States Patent GLOW DISCHARGE CIRCUITS Fritz Harders, Post Ergste, uber Schwerte (Ruhr), Helmut Kniippel, Dortmund-Lottringhausen, and Karl Brotzmann, Dortmund, Germany, assignors to Dortmund- Harder Hiittenunion Aktieugesellschaft, Dortmund, Germany Application January 15, 1957, Serial No. 634,232

Claims priority, application Germany June 16, 1954 7 Claims. (Cl. 315-243) This invention relates to improvements in apparatus for subjecting objects to the effect of a glow discharge, and is a continuation-in-part of our co-pending application Serial No. 515,426.

The use of glow discharges in physical or chemical processes, for example for treating metal objects, meets with the difiiculties that frequently the glow discharge changes over into an arc discharge, especially when the discharge is of high current strength, and this has a troublesome effect on the objects to be charged. The possibility of stablising the glow discharge by the use of suitably high limiting resistances connected in series with the discharge vessel is not practical because the process would then become too uneconomical owing to the high consumption of energy. On the other hand, however, the use of a low ohmic limiting resistance leads, when the discharge changes over to an arc discharge, to a momentary very large current which not only endangers the electric installation but makes an undesired burn mark on the object to be treated.

By means of the invention it is possible to overcome the above-mentioned disadvantages and to maintain a suitable glow discharge of high current strength A in continuous operation with a relative low consumption of energy. This result is obtained in that, by means of suitable electric circuit elements, the negative current-voltage characteristic of the arc is used, when the glow discharge changes over, to extinguish the are immediately. As the circuit element there is used, in accordance with the invention, a condenser which is connected in parallel with the glow discharge path, is so selected that, when the glow discharge changes over into an arc discharge, the latter causes the arc discharge to be quickly extinguished by utilising the dropping characteristic of the arc discharge. There may also be provided for the gas discharge vessel an ignition device of a known kind which is so dimensioned and designed that the gas discharge is initiated approximately when the necessary glow discharge voltage is reached, and is therefore, automatically set in operation. The current strength and the gas pressure in the discharge vessel are so adjusted that the glow discharge is operated as far as possible in the range of the abnormal cathode drop. v

Further details of the invention are illustrated, by way of example, in the drawing, Fig. 6, which shows the electric circuit.

In the drawing the discharge vessel is designated by the reference G, the parallel condenser by the reference C, the series limiting resistance by the reference R and the source of current, which is preferably adjustable, by the reference S. The limiting resistance R is so dimensioned that normally a glow discharge is maintained in the discharge vessel G. If now the glow discharge changes over into an arc discharge, then the condenser C is discharged to the voltage drop at the discharge path. Consequently, there flows through the limiting resistance R for a very short time a considerably stronger current than can be delivered by the source of current S alone.

2 This causes a further heavy drop in the operating voltage. However, the current which is now supplied only through the limiting voltage R is no longer sufiicient for the low operating voltage, so that the discharge is extinguished. This effect is still further promoted by the condenser C which is only charged again when the voltage increases.

It has been found by experiment that the duration for which the arc burns is, in general, so short that no burn mark is produced on the cathode on the discharge vessel. Further the occurrence of a burn mark can be avoided with certainty by connecting, in the circuit which contains the gas discharge path and is in parallel with the condenser C, a resistance R which is considerably smaller than the limiting resistance R which resistance R receives the energy of the condenser corresponding to the high current strength.

In order to produce a stable glow discharge of 50 am- R =7 ohms, R =0.02 ohms, C=40 pf.

In order to maintain the loss of energy which occurs when operating glow discharge as small as possible, it is advantageous to keep the limiting resistance connected in series with the discharge path as small as possible. In the above-mentioned circuit, however, the size of the limiting resistance has a lower limit owing to its co-operation with the condenser.

According to further features of the invention the above-mentioned limitation is avoided in that there-is connected in series with the limiting resistance an additional choke L which, when the glow discharge changes to an arc discharge, limits the momentary current which flows to a considerably greater extent than an ohmic resistance. It is then advantageous to dimensionthe choke L, the limiting resistance R and the condenser C in such a way that a damped oscillating circuit is formed, the first voltage maximum of which is so high that the glow discharge is again initiated without external ignition.

The limitation of the current flowing when the glow discharge changes over to an arc discharge when the choke L is present is no'longer effected solely by the limiting resistance R but to a considerably stronger extent at the moment of change-over owing to the inductance L. The ignition device Z, for again initiating the glow discharge after being extinguished can be omitted if, as above-mentioned R, L and C are properly dimensioned. The resistance R illustrated in the drawing is considerably smaller than the resistance R and for this purpose the ohmic resistance of the leads is generally sufficient. The resistance R serves the purpose of taking considerable part of the energy of the condenser C when the glow discharge changes over to an arc discharge.

For the continuous operation of a glow discharge of 200 amperes the circuit elements may, for example, be

dimensioned as follows:

It is well known that certain processes, such as nitrogenization or carbonizationof steel, may be carried out with considerable advantage in the presence of a glow discharge. In utilizing a glow discharge for purposes such as just indicated, the object to be treated is placed into a vacuum chamber, hereafter referred to as the vessel. The vessel includes suitable means for supporting the object, and it is equipped with at least two electrodes to which a voltage may be applied through an external electric circuit. Sometimes, the vessel itself may form one of the electrodes. After the object has so that thereupon it may be evacuated until the internal pressure drops to a value at which a glow discharge may bev maintained between the electrodes.

For most purposes, it is necessary to treat the object in an atmosphere of a gas different from air. For example, nitrogenization may be carried out in an atmosphere of ammonia, while a gas suitable for carbonization is methane. In such cases, the vessel is connected by one pipe to a vacuum pump and by another pipe to a gas supply, the latter pipe comprising a throttle. While the vessel is being evacuated, some gas from the supply is allowed to enter the vessel across the throttle at low pressure. Continuing this process for a suflicient length of time results in the vessel finally being filled with the gas, with only a negligible percentage of air remaining.

After the required gas pressure has been established inside the vessel, a voltage of suitable magnitude is applied to the electrodes and a glow discharge thereby ignited between them. Ignition of the glow discharge may be obtained just by raising the voltage sufficiently above the value which is required for subsequently maintaining the discharge. Sometimes, however, the difference between the voltage value required for ignition and the lower voltage value suflicient for maintaining the discharge is undesirably large. In such cases, an additional electrode may be provided, often referred to as ignition electrode.

For operating the glow discharge, either D.-C. or

A.-C. may be used. It was found, however, that D.-C. is preferable to A.-C. in many respects, the reason being that the most effective region within a glow discharge is that which is known to be the cathode drop, sometimes also referred to as cathode fall, which, in the case of D.-C. will at all times lie directly adjacent to the object provided the latter is connected to operate as cathode. The major elements of the external circuit usually comprise a source of electric energy and an impedance, both connected in series with each other and with the vessel. If D.-C. is used, the object is often connected directly or indirectly to the negative terminal of the source, thus becoming the cathode, which is done for the reasons explained above.

Glow discharges have the well-known tendency to break down to an arc discharge. Such a breakdown is particularly undesirable in the use of glow discharges for the purposes under discussion, because an arc discharge usually has a damaging effect upon the object under treatment if such a discharge is sustained for any appreciable time. To operate a glow discharge does not present any major diificulties if the discharge current is comparatively low, a simple resistor connected in series with the vessel then being a sufficient means to eliminate any danger of such breakdowns occurring. This is because in the lower current range a breakdown of the glow discharge to an arc discharge is impossible if the current is prevented from increasing as far as is required for igniting an arc, the arc discharge current being greater than the glow discharge current in the said range. However, if a glow discharge is operated at higher current the simple means of the resistor fails, because in the higher current range an arc discharge may exist at a current equal to or even less than the current required to sustain the glow discharge.

The border value separating what has been referred to here as the lower and higher current ranges depends to a certain degree upon circumstantial conditions, such as the nature of the gas, the gas pressure, the voltage, the configuration of the electrodes, etc. However, it is safev to say that a simple series resistor will in all probability fail to operate as a breakdown preventing means if the current exceeds the order of magnitude of 1 ampere.

It is one object of the invention to overcome this difiiculty and to allow the operation of a glow discharge without any substantial damage being caused to the ob- 4 ject under treatment by a possible are discharge occurring in operation.

It is another object of the invention to achieve the aforesaid result without the use of any mechanical switching apparatus, or apparatus comprising parts moving while in operation.

It is another object of the invention to reestablish the glow discharge immediately after a breakdown to an arc discharge has happened.

Other objects of the invention will become apparent from the following description of one embodiment of the invention, illustrated in the drawings. In the drawlugs Fig. 1 is a cross-section of a discharge vessel suitable for surface-treating metal objects.

Fig. 2 is a diagram illustrating the pipe connections and electrical connections made to the vessel shown in Fig. 1.

Fig. 3 is a circuit diagram.

Fig. 4 is a plot showing the voltage across a discharge vessel vs. the discharge current and is to illustrate certain basic differences between a glow discharge and an arc discharge.

Fig. 5. is a plot showing the voltage across the discharge vessel vs. the time during a' breakdown under conditions existing in the system according to the invention.

Referring to Fig. 1, a vessel suitable for surface-treating a metal object in the presence of a glow discharge consists of a cylindrical container having a lid 11 which may be fastened to a flange 12 of the container 10 by bolts 13, a gasket 14 serving to seal the interior of the container. The object shown is a hollow cylinder 15 of steel, and it is assumed that the inner surface 16 of this object is to be carbonized or nitrogenized. The cylinder 15 is supported by means of a bracket comprising a flange 17, a conical portion 18 and a rod 19 to which the cylinder 15 is fixed at 20. The conical portion 18 extends through an opening 21 of conical shape in such a way that there is a uorrow gap 22 formed between the two conical surfaces. The flange 17 is held in place by several clamps 23 with bolts 24, only one being shown, with a pair of gaskets 25 and 26 of insulation material serving as sealing means.

A metal rod 27 is arranged coaxially to and inside the object 15, and is supported by a bracket which extends through the sidewall of the vessel and consists of a metal rod 28 covered by a layer 29 of insulation material. Another bracket of similar configuration and thus comprising a rod 30 and an insulation covering 31 carries a metal pin 32 fixed to its inner end. Sleeves 33 and 34 are provided for sealing.

Two pipes, designated 35 and 36, connected to openings 37 and 38 in the side Wall are provided for evacuating the vessel and filling it with gas. The exhaust pipe 35 connects the vessel with a vacuum pump 39 driven by a motor 40. The supply pipe 36 is connected to a gas bottle 41 filled with whatever gas is to be used in the process. Each pipe is equipped with one valve, 42 and 43, valve 43 also serving as an adjustable throttle to control the pressure at which the gas enters the vessel. The pressure inside the vessel may be read from a pressure gauge 44.

Operation of the system will now be described first with no regard to the possibility of the glow discharge breaking down to an arc discharge.

After the lid 11 with the object 15 fixed thereto has been fastened to the cylinder 10, pump 39 is set in operation with valve 42 being open and valve 43 being adjusted to exert a certain amount of throttle action. This will cause the pressure inside the vessel to drop below atmospheric pressure, the pressure value finally reached depending upon the adjustment of throttle 43. Also, while the vessel is being evacuated the amount of air contained in it will decrease gradually whereas the percentage of gas will increase. If the pump is held in operation for a suificient length of time, the percentage of air inside the, vessel will have become negligible.

In the phase of operation succeeding this evacuation thepressure inside the vessel must be held within certain limits. In some applications, the margin set by these limits is relatively wide. In such cases, operation of the pump may be discontinued and the valve 42 closed. If the pressure then approaches the upper limit of the permissible pressure range before the process is completed the pump may again be operated with the valve 42 open, tolower the pressure sufficiently.

In cases where the aforesaid margin is relatively narrow the pump may be held in operation until the process to be described now is completed.

Leta D.-C. voltage be connected to suitable terminals 45' and 46 on flange 17 and rod 28 resp. in such a way that the object 15, being conductively connected to flange 17 assumes a negative potential relative to rod 27, the latter being conductively connected to rod 28. If the voltage magnitude is suitably selected, a glow discharge will be ignited between rod 27 and the inner surface 16 of object 15, the latter becoming the cathode. While this glow discharge is maintained, surface 16 and a layer just beneath it undergo certain changes in their molecular structure, and particles of the surrounding atmosphere, or components of such particles, are incorporated into the object within the said region. In this way, carbonization, nitrogenization or other treatment may be accomplished. Two examples will now be given, with numerical values that were found suitable:

. Example I For carbonizing the surface of a body of steel, the

vessel is filled with methane at a pressure of about 10' For nitrogenizing the surface of a body of steel, the vessel is filled with ammonia at a pressure of about 6 millimeters of mercury. The discharge current is then so adjusted as to give a current density of .002 ampere per square centimeter referred to the cathode surface which, according to experience, corresponds to a voltage of about 600 volts between cathode and anode. An average treatment yielding a satisfactory product requires about hours.

'A major difliculty in carrying out such and other treatments of the kind under discussion arises from the fact that the glow discharge shows a strong tendency to break down to an arc discharge. It is the purpose of the circuit now to be described to eliminate the damaging effect such breakdowns may have.

Referring to Figs. 2 and 3, a suitable circuit comprises a 3-phase transformer 47 whose primary 48 is connected to 3-phase power lines 49 over a suitable breaker 50. Three two-way rectifiers 51 are connected to the secondary 52 of transformer 47 with their D.-C. terminals being connected in parallel, the system as described so far representing a source of electrical D.-C. energy with output terminals 52 and 53, generally designated by the reference number 54. V

The negative terminal is directly connected to the cathode, or object 15 inside the vessel 10, by means of a lead comprising two sections 55 and 56 with a node 57'between them. A further lead 58 connects the positive terminal 53 with a node 59 across an inductance 60 and a'resistor 61. A capacitor 62 is connected across nodes 57 and 59. A lead 63 connects node 59 with the 65. The metal pin'32 is connected to the anode 27 in a node 66, the connecting lead 67 comprises a resistor 68. The vessel 10 is shown grounded.

In Fig. 4, two typical curves are shown, indicating the characteristic lines of a glow discharge (curve I) and an arc discharge (curve II) under comparable conditions, giving the relationship between the discharge current and discharge voltage, or voltage across the vessel. These curves intersect in P at a current i Assume that a glow discharge is operated at a current i less than i Let now the glow discharge tend to break down to an arc discharge. Such a breakdown is equivalent to a sudden decrease in the apparent resistance of the vessel, causing an increase in current and a decrease in voltage. Now, an arc discharge is possible only at values of the voltage and current lying on curve II. Hence, if a resistor would be connected in series with the vessel, of a resistance suflicient to limit the current so that it cannot reach curve II, the breakdown would be prevented from happening.

Let now a glow discharge be operated at a current i exceeding i Obviously, a resistor connected in series with the vessel could then not have the eflect just described. The resistor would again limit the increase in current occurring when the glow discharge tends to break down; nothing is gained thereby, however, because to the left of P an arc discharge requires a current equal to or even less than the current required by the glow discharge. It is thus seen that the conventional means to insure stable operation of a glow discharge and consisting of a series resistor connected to the vessel becomes ineffective if the glow discharge is utilized for surface treatment, because such treatment, in order to be efficient, is never carried out at currents lying to the left of P, the currents actually applied often being considerably larger than the current value corresponding to P.

The current i corresponding to P is not a constant. However, it was found that this current is always in the order of magnitude of l ampere.

It will now be described how the circuit shown in Fig. 3 operates in the case of a breakdown. Of the circuit elements shown, only source 54, resistor 61, capacitor 60, inductance 65 and the element represented by the vessel 10 need be considered. The operation will be explained with reference to Fig. 5, in which the voltage V across capacitor 62 as well as the voltage V between the anode and the cathode are plotted vs. the time. It will be noted that the time scale selected for the time interval from t to t is 5 times the scale selected for the time succeeding that interval, with a new counting beginning at t This was done in order to be able to show the complete process in one continuous plot.

The curves as plotted result from oscillographic measurements made in a setup in which the glow discharge was operated at V =500 volts and the discharge current amounted to approximately 50 amperes. Circiut data were as follows:

Inductance 60 0 Resistance 61 ohm-s 7 Capacitance 62 microfarads 40 Inductance 64 microhenrys 10 Resistance 65 0hms .02

The values given for inductance 64 and resistance 65 are approximate.

At t the glow discharge breaks down to an are discharge. The voltages V and V having been equal to each other other for some time preceding t now start changing as plotted. It is seen that V from its initial value of 500 volts suddenly drops to about volts, being forced down by the arc discharge. From t to t this voltage does not show any greater change, decreasing only slightly from about 75 to about 50 volts. However, during the same time interval, voltage V displays a change very similar to that of a damped oscillation.

anode, or rod 27, across an inductance 64 and a resistor 75 This was to be expected from the fact that the circuit mesh involved comprises capacitance, inductance and resistance, as represented by capacitor 62, inductance 64, resistor 65 and the vessel 10. A slight deviation from the text book curve of a damped oscillation results of course from the vessel resistance not being a constant.

Form t to 2 the slope of curve representing V is negative, and its lower peak within the first full period of the oscillation is reached at If voltage V would be capable now of continuing in the form of a damped oscillation, the slope of the curve, equalling the time derivative of V would change sign and the current, being displaced in phase to the voltage by about 90 degrees, as in any electrical oscillation, would be reversed. However, such a reversal of current direction cannot be survived by the arc discharge which, consequently, is extinguished at t;. Thereby, the current path through the circuit loop containing vessel 10 is broken inside the vessel, current now being able to flow only in the loop containing resistor 61 and capacitor 60. Voltage V;, once more being equal .to voltage V now starts increasing exponentially and comparatively slowly, as shown in the diagram. The glow discharge is then reignited as soon as voltage V V reaches a value suflicient for ignition, which in Fig. 5 occurs at a voltage value of about 540 volts at where the voltage drops to its initial value of 506 volts.

It might be mentioned here that the purpose of the auxiliary electrode represented by metal pin 32 is to reduce the difference between the voltage required for igniting the glow discharge and the voltage at which the glow discharge is operated subsequently. To make use of such means is of considerable advantage here because thereby the time elapsing from to t is greatly reduced. It is seen from Fig. 5 that the interval during which the arc is allowed to burn in this particular case is approximately 6x10 seconds. This is by far sufficiently short to prevent the are from doing any harm to the object under treatment or to other parts of the vessel, as it was found, that an are burning for even as long as 10* seconds would be tolerable in most cases. In fact, an arc burning for a relatively short time is often found to have useful effects. For example, a breakdown to an arc discharge is frequently caused by some impurity of solid nature on the surface of the object under treatment, and the arc then tends to remove such impurity by melting and burning it.

Measurements conducted in connection with circuits as shown in Fig. 3 but having numerical data different from those given above all have resulted in curves of the kind shown in Fig. 5.

It was found unnecessary in all physical setups operated so far to provide a coil in order to obtain sufficient inductance within the circuit loop containing vessel 10. Suitable values of that inductance lie between 1 and 100 microhenrys and care should be taken to avoid values considerably exceeding 100 microhenrys. Now, in a typical physical setup, the inductance inherent in the loop as such is mostly found suflicient for the purpose, being Within the numerical limits just indicated. This is due to the fact that the dimensions of a typical vessel are 4 to 10 ft. in height and 2 to 4 ft. in diameter, some even being larger, and that leads 56 and 63 for practical reasons are always arranged in some distance from each other. It is for this reason that the true nature of the process of extinction and rc-ignition as set forth above was not fully understood until exact measurements were made, since the circuits operated so far did not have any particular coils connected in series with the vessel.

It remains to explain the purpose of inductance 60. This inductance is not a vital element in the circuit, inasmuch as the system may be operated without any particular inductance being provided in the loop containing resistor 61. Inductance 60, which is provided in the form of a coil having an iron core, represents impedance in series with the resistance of resistor 61 and thus allows to use a resistor of magnitude less than that required in the absence of such a coil. In operation, energy is constantly dissipated" by resistor 61 whereas no energy to speak of is dissipated in the coil, Hence, by providing a coil and reducing the magnitude of the resistor correspondingly the economic efliciency of the circuit is increased.

The following recommendations as to how the circuit parameters should be selected result from experience made in the operation of various setups, all built in accordance with the invention.

It will be found useful to select the resistance of resistor 61 and the capacitance of capacitor 62 such that the product of the two is in the order of magnitude of 10- seconds.

However, in following this rule, the said capacitance should not be made less than 20 microfarads. As to resister 63, sufficient resistance will mostly be found inherent in the leads. Usually, a resistance of .02 ohm is by far sufiicient for a smaller setup while values considerably less than that will give satisfactory results in larger setups. Experience shows that optimal results are obtained if the resistance of resistor 61 is at least 200 times that of resistor 63.

The above recommendations primarily apply if no inductance is connected in series with resistor 61. If a coil is provided at 60, the resistance of resistor 61 may be decreased as explained above. Suitable values of both resistor 61 and coil 60 should then be determined empirically.

For economical reasons, the tendency will always be to make the circuit elements as small as possible. Hence, it seems unnecessary here to give any upper limits for the elements involved.

Having thus fully described the invention, it should be understood that the embodiments illustrated in the accompanying drawings are given by way of exemplifi cation and not of limitation and that various changes in the construction, shape, and combination may be made therein without departing from the spirit of the invention.

What we claim is:

1. In an apparatus for surface-treating metal objects in a vacuum vessel in the presence of a continuously burning glow discharge, in combination: a glow discharge vessel adapted to operate at currents exceeding 1 amp.; a pair of electrodes spacedl-y arranged inside said vessel; .3. source of electric energy; a resistor; a capacitor; said source of electric energy, said resistor and said capacitor being connected in series; an inductance; said inductance being connected in series with said electrodes; said series connection of said inductance with said electrodes being connected in parallel with said capacitor; the resistance of said resistor being less than the resistance across said electrodes when a glow discharge is present in said vessel.

2. In an apparatus as claimed in claim 1, a choke connected in series with said resistor.

3. In an apparatus as claimed in claim 1, the lower limit of the product of the resistance of said resistor and the capacitance of said capacitor being of the order of 10- seconds.

4. In an apparatus as claimed in claim 3, said glow discharge vessel being adapted to operate at a current of the order of 50 amps., said resistor having a resistance of the order of 7 ohms, and said capacitor having a capacitance of the order of 40 microfarads.

5. In an apparatus as claimed in claim 3, said glow discharge vessel being adapted to operate at a current of the order of 200 amperes, said resistor having a resistance of the order of 1 ohm, and said capacitor having a capacitance of the order of microfarads.

6. In an apparatus for nitriding the surface of an object in the presence of a continuously burning glow discharge and in an atmosphere of a gas containing nitrogen, in combination: a glow discharge vessel filled with a gas containing nitrogen adapted to operate at currents 9 exceeding 1 amp.; a pair of electrodes one of which is a steel work piece spacedly arranged inside said vessel filled with nitrogen; a source of electric energy; a resistor; a capacitor; said source of electric energy, said resistor and said capacitor being connected in series; an inductance;

said inductance being connected in series with said electrodes; said series connection of said inductance with said electrodes being connected in parallel with said capacitor; the resistance of said resistor being less than the resistance across said electrodes when a glow discharge is present in said vessel.

7. In an apparatus for carbonizing the surface of an object in the presence of a continuously burning glow discharge and in an atmosphere of a gas containing carbon, in combination: a glow discharge vessel adapted to operate at currents exceeding 1 amp.; a pair of electrodes one of which is a steel work piece spacedly arranged inside said vessel which is filled with a gas containing carbon; :a source of electric energy; a resistor; a capacitor; said source of electric energy, said resistor and 10 said capacitor being connected in series; an inductancei said inductance being connected in series with said electrodes; said series connection of said inductance with said electrodes being connected in parallel with said capacitor; the resistance of said resistor being less than the resistance across said electrodes when a glow discharge is present in said vessel.

References Cited in the file of this patent UNITED STATES PATENTS 2,258,149 Schutze Oct. 7, 1941 2,280,322 Anderson June 2, 1942 2,391,611 Back Dec. 25, 1945 2,535,886 Baker et al. Dec. 26, 1950 2,550,089 Schlesrnan Apr. 24, 1951 2,553,944 Schlesman May 22, 1951 2,716,198 McCallum Aug. 23, 1955 2,728,877 Fischer Dec. 27, 1955 2,762,945 Berghaus et al. Sept. 11, 1956 2,787,730 Berghaus et al. Apr. 2, 1957

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