Laboratory and Numerical Modeling of Internal Charging of Spacecraft Dielectric Materials

LABORATORY AND NUMERICAL MODELING OF INTERNAL CHARGING OF SPACECRAFT DIELECTRIC MATERIALS

Akishin A.I., Mileev V.N., Novikov L.S.

Skobeltsyn Institute of Nuclear Physics Moscow State University, Moscow, 119992, Russia e-mail: [email protected]

The basic parameters describing conditions of the ABSTRACT sphere irradiation and process of the electrical Results of laboratory studies of electrical discharges in breakdown are presented in table 1. Taking into account samples of optical glasses and polymethylmetacrylate at the effective range of electrons with Е~22 MeV in their irradiation by electrons with energy ~1-25 MeV PMMA is about 12 cm and the sphere was rotated and protons with energy ~50-100 MeV are presented. during the irradiation, it is possible to suppose that the Numerical simulation of depth distribution of the internal electric charge was distributed rather uniformly internal electric charge at irradiation of glass samples by on all volume of the irradiated sphere. In the near electrons with energies ~1-10 MeV was done using the surface layer of the sphere (thickness is 3-5 mm), the GEANT program package based on the Monte-Carlo concentration of the internal electric charge is probably method. In terms of the simulation results, it is possible sharply reduced because of the edge effect. to explain lower threshold fluences of origin of electric discharges in space (1010- 1011 cm-2) in comparison with Table 1 the fluences in laboratory experiments (1013-1014 cm-2). Parameter Value Electrical strength 1.8 MV/cm 1. RESULTS OF LABORATORY STUDIES Electron energy 22 МэВ MeV Beam current density 30-60 nA/cm2 1.1. Electrodischarge processes at charging of Critical electrons fluence 1013-1014 el/ cm2 dielectrics by electrons Discharge dose of radiation 104-105 Gr 2 3 At irradiation of samples by monochromatic beams of Dose rate 10 -10 Gr/s electrons with energies 1-25 MeV, the origin of Discharge duration 1 mks electrical discharges with formation of an electrical tree Diameter of the discharge channel 50-100 mkm Discharge current in the main (Lichtenberg figures) was observed [1]. In fig. 1, the channel 100-200 A Lichtenberg figure formed in polymethylmetacrylate Current density in the channel 106 A/cm2 (PMMA) sphere of 10.5 cm diameter after electrical Discharge energy 10-20 J breakdown of the internal electric charge injected into Discharge power 107 W its volume by irradiation with the 22 MeV electron Discharge power density 1011-1012 W/cm3 beam at the fluence of 1013 cm-2 is shown.

The edge effect exhibited itself in lack of tracks of Lichtenberg figures in the region of about 0.5 cm from edges of the PMMA plate irradiated by electrons was observed, too. In fig. 2, the photo of PMMA sample (1) is presented, on frontal plane of which the cylindrical channels (2) depth 1 cm and diameter 0.5 cm were drilled at equal distance of 1.5 cm. The sample was irradiated normally to the plane of the photo with 7 MeV electrons (fluence ~1013 cm-2). It is well seen that the electrodischarge channels (3) are absent both near the edges of the PMMA plate, and in a ring band with width ~0.5 cm from edges of the channels. This effect, at relevant experimentally chosen diameter and distance of disposition of the channels, was used for making an insulator intended for operation in conditions of radiation influences and stable against destructive action of electrodischarge processes. Fig. 1. Lichtenberg figures in the PMMA sphere.

______Proc. of the 10th ISMSE & the 8th ICPMSE, Collioure, France, 19-23 June 2006 (SP-616, September 2006)

At an irradiation by the electron beam of dielectric 1.2. Initiation of an electrical breakdown by samples (PMMA, optical glass) with thickness over the impulse laser radiation electron range, the electric breakdown occurs, as a rule, In experiments on initiation of the electric breakdown in to the nearest surface to area of the maximum internal radiation-charged dielectrics by impulse laser radiation, charge density. glasses of various composition (B-Si, B-La and P), in

which the internal charge was collected and maintained well [2] were studied. The charging of the glass volume occurred during impact of monoenergetic beam of electrons with energy 1-25 MeV, at the particle fluence 1012-1014 cm-2. The fluence value was chosen approximately 2 times lower, than it is necessary for spontaneous discharge of the charged sample. For initiation of electrical discharge in the dielectrics irradiated with electrons, the laser on AlY garnet: Nd3+ crystal or Ne glass in the modulated Q-factor regime (t=20 ns, λ=1.06 mcm) was used. Example of the Lichtenberg figures obtained in these conditions is given in fig. 4.

Fig. 2. A fragment of PMMA plate with cylindrical channels after an irradiation by electrons.

In a series of experiments, filming of the irradiated sample during irradiation and discharge was done in polarized light. The irradiated sample of optical glass or PMMA was placed between two polarizers. The light source was disposed before the first polarizer, and camera – behind the second polarizer (analyzer). At the crossed polaroids in lack of electric field in the sample, light does not pass through the system of polaroids and Fig. 4. Electrical breakdown in glass at laser initiation: 1 the sample (blacked area of photo). At origination of - charged sample, 2 - band of light discharge, 3 – strong electric field in the sample as a result of discharge channels nearest to the surface, 4 – irradiated accumulation of the internal charge during irradiation, side of the sample. rotation of the polarization plane due to the Kerr effect occurs, and the light areas corresponding to distribution At surface initiation, the main channel of the of the electric field in the sample are observed on the Lichtenberg figure goes out in the initiation point, and, photo (fig. 3). at initiation in volume, it transits through the point to the nearest sample surface. In fig. 5, the result of registration of glow of optical and electrical discharges obtained with the help of high-speed shooting is presented. Delay of the electric breakdown relatively to the initiating light pulse was observed in all experiments. Minimal delay value (2⋅10-8 s) was observed in the case of initiation from surface and energy of electrons 2 MeV. Maximum delays up to 5⋅10-7 s were fixed at internal initiation and energy of electrons 8 MeV. At initiation from surface, the delay increases with increase a b of distance from the surface to the area of the charge Fig. 2. Photos in polarized light of PMMA sample localization; at internal initiation, it grows with the irradiated with electrons with an energy 2.1 MeV at distance from the surface to the point of initiation. various fluences: a – 1.3⋅1012 cm-2; b – 3.9⋅1012 cm-2. formed Lichtenberg figures and discharge channel in the glass volume and didn’t resulted to the sample crack; the third breakdown initiated by irradiation into the central part of the sample has induced the sample crack into two fragments. The fourth and fifth irradiations were done on one of the fragments, and the discharges occurred have induced the cracking of this glass fragment, too.

Fig 5. Glow of optical (1) and electrical (2) discharges in glass.

Fig. 6. Character of mechanical fractures of a glass at an 1.3. Electrodischarge effects at irradiation irradiation by protons of a high energy. of inorganic glasses with protons Electrodischarge effects occurring at irradiation of The electric breakdown of the radiation-charged solid inorganic glasses by protons with energy 100 MeV were dielectric irradiated with electrons or protons of high studied on proton injector LI-100 in IHEP (Protvino, energy is accompanied with ejection of plasmoid from Russia) [3]. The main parameters describing energy the discharge channel going out on the sample surface. cumulation in the electric discharge channel are In fig. 7, the photo of the glass sample in which the presented in table 2. electric breakdown occurred at irradiation with 100 Table 2 MeV protons is presented. In the figure, the discharge Parameter Value channel (1) going out on one of edges is well seen. Discharge energy ~1 J Discharge time 10-7-10-6 s Discharge power 106-107 W Discharge current 1-100 A Current density 107 A/cm2 Rate of the current increase 108 A/s Energy density 104 J/cm3 Volume of discharge channel 10-5 cm3 Power density 1011-1012 W/cm3 Electric field 1.5 MV/cm Potential in the volume ~5 MV Temperature in the channel (3-5)⋅103 K Pressure in the channel 109-1010 Pa Plasma density 1020 cm-3

In these conditions, destruction of the BK-108 optical glass sample (40x35x8 cm) after electric breakdown of the proton charge implanted on depth 3.5 cm was studied. During irradiation, diameter of the proton beam was ~3 cm, fluence 1013 cm-2. The glass sample was subjected to a five-multiple irradiation in various areas of the frontal surface. The strong destroying effect at discharge is determined by the high energy of protons Fig. 7. The block of an optical glass with tracks of (100 MeV) and considerable square of the irradiation electrodischarge fracture after a proton irradiation: (~10 cm2). 1 - exit on a surface of the discharge channel. In fig. 6, the photo of the destroyed sample is shown. Electric breakdowns of the implanted proton charge occurred during the irradiation: two discharges have 2. MATHEMATICAL MODELLING OF THE 2.2. Calculation results INTERNAL CHARGE FORMATION On the basis of the technique developed, the computer 2.1. Internal charging simulation technique in simulation of the internal charge accumulation in the terms of the Monte-Carlo method case of monochromatic beams of electrons and electrons with energy spectra typical for radiation belts of the Computer simulation of the internal charging process in Earth was done. Glass slab of width 0.5 cm was used as spacecraft dielectric materials under the impact of a sample. For modeling of laboratory conditions in electrons with energies 0.1-10 MeV was done in terms accelerators, the energy of the collimated electron beam of the Monte-Carlo method [4]. was 1.0-10.0 MeV. The energy spectrum of electrons of As the base of algorithms and programs package for radiation belts of the Earth incident on the sample simulation, the GEANT-3 software package developed isotropically, was described by exponent with mean initially for solution of fundamental problems in high- energy 0.5 MeV which is similar to models of energy physics was chosen [5]. This package does not conditions of “the worst case” for an internal charging contain the program module taking into account the developed in NASA [6] and DERA [7]. influence of the electrical field created by the The depth distribution of the internal charge is one of accumulated internal charge on motion of the charged the main characteristics determining the phenomenon of particles inside the dielectric. It is important to include dielectric material charging under impact of the high the influence of the electric field in simulation energy electrons and possible discharge processes in procedures because the field significantly determines dielectrics. In fig. 8 and fig. 9, results of calculation of the depth distribution of the injected charge in number of the stopped electrons versus depth of slab dielectric. So, the program module for simulation of under various environments are presented. Two influence of the internal electrical field on process of calculations are executed without taking into account charged particles passing in the substance was internal electric field. In fig. 8a, results of calculation developed. for the omnidirectional mono-energetic beam of For simulation of electron motion in the substance, the electrons with energy 2 MeV are shown, and fig. 8b approximation of continuous energy losses on ionization illustrates the similar effects for the isotropic flux of in multiply scatterings with small energy transmissions trapped electrons of the Earth’s radiation belts. in each collision and discrete processes with secondary Comparing fig. 8a and 8b it is seen, that distribution of electron and photon formation among which the the stopped electrons in these cases have different dominating role is played by knockout of δ-electrons character – for a mono-energetic beam the maximum of from energies above 10 keV was used. For computation number of the stopped electrons lays on depth of 0.12 of distribution of an accumulated charge in dielectric, it cm (i.e. on a path length of electrons under the influence is necessary to take into account formation of an excess of internal electric field), and for space environment this positive charge ("holes") after ionization of atoms also. maximum is displaced closer to the surface of the In algorithm of simulation of dynamic accumulation of sample on depth less than 0.1 cm. It is caused by two the internal charge in dielectric using the Monte-Carlo reasons: presence of low energy particles in the method, approximation of "large" particles was used. In spectrum and significant fraction of particles with small the method, a set of N particles with a charge eZN and a incident angle in the case of isotropic distribution. given energy and incident angle distribution function of In fig. 9a and fig. 9b, results of similar calculations particles falling on the target in time Δt corresponds to taking into account of the self-consistent electric field of each event. After modeling of one event and tracking of internal charge in dielectric are shown. For convenience primary and secondary particles in the target, the of data presentation, the scales of depth h in these increment of distribution function of the internal charge figures are distinct from the scales on fig. 8a and 8b. Δρ r)( in given points is evaluated. In terms of the From comparison of fig. 8a and 9a it is seen, that the calculated distribution of the internal charge ρ r)( , account of the electric field of the internal charge for the case a mono-energetic beam of electrons leads to electric field intensity E ( ) and potential U ( ) are r r formation of the additional internal charge at small calculated at the same point which will be used at depths ( <0.01 cm), caused by retarding of incident simulation of following event. Thus, the series h particles and transport of secondary electrons in a description of time development of the internal charging dielectric. Results of the similar calculation executed for process, and also self-consistent calculation of the space requirements (fig. 9b) show, that in this case internal electrical field and its influence on motion of almost all charge is concentrated in a thin slab <0.005 primary and secondary charged particles is carried out. h cm at the irradiated surface of the target. This difference is caused by distinction in energy and angular characteristics of incident particles and influence of the electric field on formation of the internal charge.

Number of particles Number of particles

Depth, cm Depth, cm a a Number of particles Number of particles

Depth, cm Depth, cm b b

Fig. 8. Number of stopped electron distribution versus Fig. 9. Number of stopped electron distribution versus depth of sample without account of internal electrical depth of sample with account of internal electrical field: field: a - for a mono-energetic beam of electrons with energy a - for a mono-energetic beam of electrons with energy 2.0 MeV at normal incident angle; 2.0 MeV at normal incident angle; b - for the Earth’s radiation belt electrons spectrum with b - for the Earth’s radiation belt electrons spectrum with isotropic angular distribution. isotropic angular distribution.

Computations of the electric field strength in laboratory 3. CONCLUSION (fig. 10a) and space (fig. 10b) conditions yield depth distribution close to each other as in curve shape, as in Thus, characteristics of the internal electric charge absolute value. Significant difference in the electric considerably differ in case of laboratory simulation field intensity magnitude arise at small depth (lower experiments in accelerators and in space conditions of 0.01 cm) where the magnitude value in space increases spacecraft charging in the Earth radiation belts. The the value in laboratory conditions by a factor of 2. indicated differences allow to explain the origin of internal electric discharges in dielectrics in space conditions at considerably lower values of electron x10-1 fluencies as contrasted to the laboratory experiment condition.

REFERENCES 1. Akishin A.I., Vitoshkin E.A., Tyutrin Yu.I. Discharge in Electron Irradiated Glasses. Radiat. Phys. Chem.,Vol. 3, No 3, 305-306, 1984. 2. Akishin A.I., Goncharov Yu.S., Novikov L.S., Tyutrin Yu.I., Tsepliaev L.I. Discharge Phenomena in Electron Irradiated Glasses. Radiat. Phys. Chem., Vol. 23, No 3, 319-324, 1984. 3. Akishin A.I., Tsepliaev L.I. Destruction and Discharge Phenomena in the Irradiated Glasses. J. Nucl. Mater. Vol. 233, 1318-1320, 1996. 4. Mileev V.N., Novikov L.S., Tasaikin V. Application of the GEANT Tools for Spacecraft Internal Charging Simulation. th Electric field strength (MV/cm) (MV/cm) strength Electric field Proceedings of 7 Depth (cm) Spacecraft Charging Technology Conference, a ESTEC, Noordwijk, The Netherlands. SP-476, 207- 210, 2001. -1 x10 5. Brun R., et al. GEANT User's Guide, 1993 6. Avoiding problems caused by spacecraft on-orbit internal charging effects, NASA Technical Handbook, NASA-HDBK-4002, 1999. 7. Wrenn G.L., Rodgers D.J. and Buehler P. Modeling the outer belt enhancements of MeV electrons, J. Spacecraft and Rockets, Vol. 37, No 3, 408- 415, 2000. 8. Weber K.H. Nucl. Inst. Meth., 25, 261, 1964 9. Trenkel C. Comparison of GEANT 3.15 and ITS 3.0 radiation transport codes, ESA working paper, EWP 1747, 1993.

Electric field strength (MV/cm) (MV/cm) strength Electric field Depth (cm)

b

Fig. 10. Electric field strength distribution on depth of the sample. a - for monochromatic beam of electrons with energy 2.0 MeV at normal incident angle; b - for Earth radiation belt electrons spectrum with isotropic angular distribution.