EMP Sielding and Damage Mitigation at ELI-NP

Transcription

1 Experimental Programme Workshop at ELI-NP EMP Sielding and Damage Mitigation at ELI-NP Marius GUGIU, IFIN-HH, October 3-5, 2012, Magurele - Romania 1

2 Electromagnetic pulse Generation mechanisms Laser - target interaction creates copious amount of relativistic electrons charged particles and hard X rays. EMP is generated by impulse of relativistic electrons which excite the target chamber. Chamber rings at its natural frequencies which can extend from a few MHz to many GHz. EMP is emitted from the chamber through ports and wiring into the surrounding room. The EMP is also emitted from the chamber through beam tubes to the laser component. System Generated EMP (SGEMP) can be produced outside the chamber by penetrating radiation, but is generally less significant than the EMP generated within target chamber. Damage mitigation Semiconductor devices in equipment are susceptible to damage by EMP. Electronic equipment must be protected by screened enclosures or removed from the room. Need to ensure that the level of EMP in occupied areas outside the target is safe for personnel (below the limits for human exposure).

3 EMP Measurement from Titan PW Class Laser at LLNL EMP was measured using moebius loop antennas. Signals was captured and then integrated on order to obtain the magnetic field [1]. Journal of Physics: Conference Series 112 (2008)

4 EMP Magnitude Data from Titan Laser at LLNL The peak strength of EMP as measured by a high frequency B-dot probe as function of energy. 4

5 EMP Magnitude Data from Titan Laser at LLNL The data measured at LLNN at 2 GHz show an increase in peak E field with target thickness [1]. The data measured at LLNN with a compact high energy spectrometer show an increase in number of electrons with target thickness and we also see that energy of electrons extends out of 25 MeV [1]. [1] LLNL-TR

6 EMP Magnitude Estimation from Vulcan and Astra Gemini Lasers at RAL Specular reflectivity into ω 0 and 2 ω 0 given by the relativistically oscillating mirror model and measured experimentally [1]. Up to W/cm 2 the electrons and ions are in equilibrium and not much EMP is generated. Above W/cm 2 hot electrons are ejected from the plasma, generating a significant EMP. Measurements show only small variations in laser energy absorbed from to W/cm 2. The fraction of energy absorbed is typically > 0.35 for solid targets but less for gas targets. Most of the absorbed energy is converted to hot electrons. As the pulse is shorted the electron energy increases but fewer electrons are generated. [1] New Journal of Physics 13, (2011). 6

7 Estimate of EMP for ELI-NP Need to known EMP magnitude & frequencies to design shielding and mitigation! Proprieties of EMP generated in laser facilities depends on the Energy (FY08 LDRD Final Report), pulse duration and intensity of laser. Titan with 12µm aluminium foil targets: Energy = 335J, Pulse Duration = 2ps, Beam size 20µm, EMP = 400kV/m, Power = 335/(2x10-12 ) = 0.17PW, Intensity 0.17x10 15 /(πx10-10 ) 5x10 19 W/cm 2 Titan data shows the EMP scales with energy and is proportional with target size (reduced electrostatic field associated with large plasma area allowing more electrons to escape). Based on these data, RAL (RAL-TR ) estimate the fraction of electrons in the fast spike as function of target size. They observe a steady increase of this fraction as the target size, correlated with the increase in the EMP observed. Some theories suggest EMP scales with Power or square route of Intensity. As we saw, the data analyzed form RAL shows an EMP scaling only with Intensity. Scale by: Energy = 600J Power = 20PW Square route of Intensity = W/cm 2 EMP (kv/m) The above is a simple approach ignoring target types. Table gives a low estimate and a high estimate. It is prudent to use: The optimistic case for safety critical parts and those which can t be added later. The pessimistic estimation for non safety parts and expensive parts which can be added later. The frequency range is estimated to be between 30 MHz and 20 GHz. EMPs produced at 1PW and 0.1 PW laser beams are expected to be lower by factors 20 7 and 200 respectively.

8 EMP Limits for Electronic Personnel and Equipment Personnel Limits given by the ICNIRP Guidelines which are expected to become EU law. Limits are clearly defined for CW fields but only suggested for short pulse fields. Should work to the suggested limits which means EMP field 900V/m [1]. Persons with medical implants should be excluded from the building when high power lasers are operating because the effect of the EMP on these can t be predicted. Equipment Susceptibility of electronic equipment must meet limits set by EMC directive. This does not consider short pulse fields. Based on limited testing the peak EMP field in areas where unshielded electronic equipment is used such as oscilloscopes and PCs is below 1 kv/m [2],[3]. Conclusion For areas occupied by personnel or unshielded electronic equipment: EMP 900V/m. [1] Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic and Electromagnetic Fields, International Commission on Non-Ionizing Radiation Protection (ICNIRP), Health Physics, April 1998, Volume 74, No 4. [2] EN , Generic Standards- Immunity Standard for Industrial Environments. [3] R. Hoad et al., HPEM and HEMP susceptibility assessments of computer equipment, Electromagnetic Compatibility, EMC-Zurich th International Zurich Symposium on, page

9 Shielding Strategy [4] Primary shielding should be provided by vacuum vessel. Secondary shielding should be provided by room. Additional shielding can be added later if it is needed. Outside Target Room <900V/m: Target Room 700V/m 50kV/m Outside Target Room <900V/m RAM Lining Beam Tube Target Diagnostic Target Chamber 700kV/m 50,000kV/m Target Positioner EMP Filters EMP Filters Electronic Equipment In Shielded 19 inch Rack Radiation Shielding Door EMP Door Metal Pipes vacuum, water & nitrogen EMP Filters Copper Cables 400V power & interlocks Basement <900V/m Insulating Cables fibre optics [4] Report no. 1999/ : EMP Shielding and Damage Mitigation, prepared by Dr. Michael John MEAD. 9

10 Shielding Strategy for Vacuum Vessel Target chamber could provide good electromagnetic shield if conducted and radiated emission through vessel penetrations are strictly controlled. This depends on configuration of target chambers which is determined by the users who often create significant EMP emission. Experiments which have optical windows for diagnostics allow EMP radiation. Windows are needed for alignment, but these could have shutters. Time available to set up experiments is limited, so users make quick temporary connections to diagnostics inside the chamber allowing conducted emission. Optical windows and poor target chamber wiring SE 20dB. No windows & EMP filters at vacuum feed-throughs SE 60dB. Radiated emissions through beam tubes to laser equipment is a problem. EMP Estimates Total SE Required Target Chamber SE Target Room SE Low 700kV/m 20log(700/0.9) = 58dB High 50,000kV/m 20log(50,000/0.9) = 95dB 20dB 60dB 20dB 60dB 38dB 0dB 75dB 35dB 10

11 Room Shielding by Using Thick Concrete Walls The screening effectiveness of 2m concrete is extrapolated as: Frequency 100MHz 500MHz 1GHz 2GHz 4GHz 8GHz 20GHz 203mm wall 10dB 22dB 28dB 31db 52dB 70dB >70dB 2m wall 100dB 220dB 280dB 310dB 520dB 700dB >700dB National Institute of Standards and Technology (NIST) Construction Automation Program, Report No. 3 Electronic Signal Attenuation in Construction Materials, NISTIR

12 Problem of Penetrations 12

13 Beam Penetration 4m Concrete Wall EMC Gasket 0.6m TBD (1m) Beam Tube 1.5m TBD (1.2m) Concrete blocks built round beam tube Central rebars welded to penetration frame Bolt holes in penetration frame to be drilled & tapped after installation of beam tube All bolts M6 at 50mm spacing 13

14 Vacuum Vessel Shielding Vacum joints KF flanges with O-ring rubber seals do not provide a good metal to metal seal (these should provide SE at least 30 db). SE can be increased by adding conductive tape or EMC gaskets. CF flanges have metal seals which should provide SE 60 db. Windows For alignment viewing window we sugest pneumatically operated interlocked shutters. For diagnostic windows (diagnostic in air) we propos shield round diagnostic attached to Vacuum Vessel. Electrical Wiring Must be used vacuum feed-throughs which take EMP shielded plugs. Low frequency control and instrumentation through filter connectors. High frequency instrumentation by coaxial shielded cables connected to shielded equipment. 14

15 Vacuum Vessel Shielding Use filters connectors & vacuum feed-through 15

16 Beam Tubes - Radiated Emission Reduced by Dispersion, Reflection & Energy Loss Dispersion Different frequency components of EMP excite different modes of propagation and travel at different velocities - EMP is dispersed and amplitude reduced. Discontinuities Departure from ideal circular waveguide cause reflections and coupling between modes. Energy Loss Proportional to 1 where σ is conductivity of the inner surface of waveguide. σ Attenuation is low for metals even for low conductivity metal like stainless steel. Reduction Measured for Vulcan Laser (RAL) A 30dB reduction in EMP measured between target chamber and compressor input. 16

17 Circular Waveguide Attenuation for 300mm carbon lined Attenuation db/m plotted versus frequency in GHz TM01 TM02 TM11 TE01 TE

18 Use of Graphite for Damping May Be a Good Idea Beam Tubes Use graphite lining to reduce emissions. Inside Target Chamber LLNL tried aluminium shielding round target. EMP reduced by factor of 2. They should have tried graphite. Vacuum Considerations Graphite tiles can be used in ultra high vacuum systems. Higher reduction may be obtained using RAM. 18

19 Other important issues of EMC Electromagnetic interference (EMI) is an other ever-present challenge at laser facilities. Shielding : The diagnostics themselves can be a strong source of EMI[1]. A adequate shielding design in order to reduce the coupling path between source and victim devices can lead to a strong reduction of EMI. The solution can be placed at either end of coupling path, or between, thus making the source less emitting and the victim less susceptible. Grounding: Improper grounding is a major contributor to intersystem EMI problems. There are two reasons for grounding devices, cables, equipment, racks and systems: One: - to prevent a shock hazard. - to protect against lighting damage. Two: - to reduce the EMI due to either electric field or magnetic flux coupling. - to provide a reference voltage for all operating voltage levels. - as well as to prevent circulating ground-current loops resulting in common-mode impedance coupling. [1] LLNL-CONF , April

20 CONCLUSIONS: Are required more precise estimates and measurements of strength, time duration and frequency dependence of EMP inside and outside of target chamber by studding the effect of varying laser energy and distribution of escaping electrons. To evaluate of SCEMP in surrounding target room and in coaxial cables. Should be found better solutions to reduce EMP emission down beam tubes. To design shielding to mitigate the EMP as much as possible closer to source, it is required the strict control of diagnostic, probe positioner equipment and experimental apparatus to be introduced into target chamber, including the supply and signal transmission of these devices. 20

21 Thank you for your attention! 21