emi and emc at extended facilities: examples from the

H.Kapitza (DESY-FLA)SwissFEL Meeting

PSI Villigen, November 4, 2010

EMI and EMC at Extended Facilities: Examples from the European XFEL


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SwissFEL Meeting, PSI Villigen, November 4, 2010H. Kapitza (DESY-FLA)

Outline

1. Electromagnetic field impedance2. Where do the currents flow?3. Grounding in extended facilities4. EMC examples from the European XFEL5. Conclusions

• ElectroMagnetic Compatibility is not the absence of ElectroMagnetic Interference, but the state of having it under control.

• An extended facility is large compared to the shortest wavelength of interest, typically > λ

/ 10.

Title Definition:


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1. Electromagnetic Field Impedance

EMI is mediated by electromagnetic fields. Fighting it is based on classical electrodynamics, not on magic.

Electromagnetic fields may be characterized by their impedance Z = E/H. [Hint: V/m / A/m = V/A = Ω]

The impedance depends on the source type (electric vs. magnetic dipole) and on the distance from the source (near vs. far field).

As an electromagnetic wave propagates through a material, its impedance approaches the intrinsic impedance of the material. In vacuum Z = 377 Ω.


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EM field impedance vs. distance from source

field impedance determined by source characteristics

field impedance determined by characteristics of the medium

High impedance source (electric dipole)

Low impedance source (magnetic dipole)

stored energy radiated energy

[= m @ 50 MHz]


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EMI severity vs. distance and vs. source type

EMI from far fields and electric near fields is much easier to handle than magnetic near field EMI.


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Shielding as an impedance matching problem

A low Z shield reflects most of a high Z field.

Radiated fields are easily shielded, c.f. WLAN in buildings.

Even more so are reactive electric near fields.

If these are your only problems, put shields around your stuff, ground at one point and you’re set.

A low Z shield lets most of a low Z field pass.

These are the real troublemakers in extended facilities.

Controlling reactive magnetic near fields means controlling their sources, the currents.

Control of the currents is a key issue in EMC.


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2. Where Do The Currents Flow?

Two very basic laws help finding this out:1. Currents always flow in closed circuits.

Current return paths to the source need most attention.

In practice it may be difficult to find all circuit segments.

Current measurements are an extremely helpful tool, much more than measuring voltages.

FLASH Gun Area Rogowski Coil


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Where do the currents flow?

2. Currents follow the path of least total impedance.

Impedance means resistance AND inductance. Hence the path may depend on frequency. Think high frequency!

Cable cutoff frequency ω

= R / L (= 2 kHz for RG58C/U):c s s

For ω

> 5ω

more than 98% of the current return through the shield, not through ground. Because this is the lower inductance path (loop area).

With rising frequency current flows in the circuit of least area.


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What is a grounding system?

Apply the two rules to design current return paths which shunt the noise away from the signal carrying conductors.

You best stay in control if you build low Z highways, not high Z roadblocks for the currents.

A designed low Z current return to the source: This is what we usually call a “grounding system”.


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3. Grounding in Extended Facilities

A grounding system may be viewed from different angles:

In the voltage view it

provides a voltage reference for signals,

but is usually not an equipotential plane.

As a safety ground it provides potential equalisation.

In the current view it

provides a designed low-impedance path for current return to the source.

As a safety ground it carries fuse blowing currents.


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Ground system topologies

• galvanic common impedance coupling: worst system w.r.t. EMI

• low cable cost: perfectly ok as safety ground system

• best signal ground system at low frequencies (at high cable cost)

• wire inductances and stray capacitances destroy star topology at high frequencies

• adequate grounding scheme at high frequencies using low Z ground straps to low Z ground plane

• ground loops and common impedance coupling at low frequencies

All of these are usually present in real facilities.


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Ground system size rules

In large facilities some wavelength-dependent design rules apply:

The size limit for a single point ground system (star topology) is about λ

/ 10 (= 30 m @ 1 MHz). This is

caused by wire inductances and stray capacitances.

The ground plane in a multi point ground system can be approximated by a grid with mesh size < λ

/ 50 (= 6 m

@ 1 MHz).

Following these rules large facilities should have a ...


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Distributed hybrid ground system

Local (< λ/10) single point ground areas are connected to a multi point facility ground.

Grounded cable connections are forbidden between local systems.

Facility Ground


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4. EMC Examples from the European XFEL


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The European XFEL

HallShaft

Tunnel

XFEL building types: halls, shafts, tunnels

Soil coverage 10-30 m

Quite high ground water level

Civil engineering in the Hamburg area has its challenges …


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XFEL shafts: Slurry walls

A modern technique for constructing building pits in wet areas


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XFEL shafts: Wall anchors

The pit must not collapse from earth and water pressure. So anchor it.


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XFEL shafts: Underwater concrete plates

Experimental hall XHEXP1 before and after pumping

Close the pit at the bottom and pump it out.


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4.1 XFEL Grounding Concept

Grounding was already considered during the XFEL design: Most favorable condition to properly implement EMC.

Building construction techniques impose boundary conditions on grounding and lightning protection (e.g. water- tight concrete). But they also offer opportunities.

Dimensions are mostly defined by norms, but also by the expected EMI.

It pays off to consult EMC experts early.

The building companies of course get nice drawings


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XFEL facility ground

In order to push its impedance as low as possible, the common facility ground (plane) should include as much through-going metallic infrastructure as possible (core iron, pipes, cable trays, ...)

Since these components will not really form a plane, aim at a mesh size of about λ(2 MHz)/50 = 3 m.

EMI signals at FLASH have frequencies < 2 MHz.

Due to its length >> λ(2 MHz)/10 = 15 m the XFEL facility needs a hybrid ground system.


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4.2 XFEL Tunnel Grounding

In inhabited areas tunnel boring is the only viable technique.

The tübbing segmentation of the tunnel wall naturally meets the mesh size of the EMC grid.

Tunnel XTD1 with 6+1 tübbings/ring


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Tunnel grounding: Tübbing connection

The tübbing reinforcement steel represents a sizable fraction of the metallic infrastructure used as XFEL facility ground.

Each tübbing has three outside connections to its metal cage.

Tübbings are inter-connected such that current can flow in the direction of the tunnel axis.

The internal cage structure is quite complicated, with many irregularities and varying quality welding points.


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Including the tübbing iron in the facility ground

Tübbing reinforcement cages Grounding connector Tübbing cage in mould

Produced tübbings Ground points of inserted tübbings Connected tübbings


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4.3 XFEL Pulse Cable Tests in FLASH

At XFEL the klystrons and pulse transformers will be located in the linac tunnel, close to the accelerator modules, while the modulators are installed in a hall above ground.

This scheme allows for modulator maintenance during XFEL operation without an expensive service tunnel.

The pulse transformers are connected to the modulators via pulse cables which are up to 1.5 km long. Max. pulse data: 11 kV, 1.8 kA, 1.7 ms.

EMI from these cables has been thoroughly tested since 2004. For the final test cables were routed through FLASH and then operated.


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Pulse cable routing for the tunnel test


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Different pulse cable supports in the FLASH tunnel

Cables entering tunnel at gun area

Cables behind ACC moduleWe’ll see in a minute what difference this makes...

standard cable tray

none

metal walkway


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Current measurements at FLASH

• Motivated by FLASH instabilities.• Klystron switched to load resistor →

work in tunnel allowed.

Beam tube section before BC2 (z=17 m)

Rogowski coils in actiontrigger: 100 mV/Ameasure: 50 mV/A

trigger

measure


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Current measurements at FLASH

z=17 m: no cable support0.5 A in beam tube

2.6 A not in PC bundle

z= 17 m

0.2 A in water pipe

z=29 m: cable tray

40 mA in beam tube

z=BC3: metal walkway

no PC related noisein beam tube


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Improving the stray current return path

Provide a better path for stray current return:Wrap cables with 0.1 mm Cu foil and connect through.


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Current measurements after ground improvement

2.5 A in unshielded PC bundle

0.2 A in shielded PC bundle

0.1 A in beam tube at z=17 m

• The cable shielding (= ground plane continuation) absorbs > 90 % of the stray current from the environment.

• At FLASH the beam pipe is a very attractive current route.

• Reason: The FLASH tunnel is composed of isolated concrete blocks and floor slabs. This makes a very poor facility ground.

• This is an example for fixing an EMI problem in a long grown non- optimal environment.


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5. Conclusions

EMC is based on electrodynamics, not on magic.

Low frequency magnetic fields cause most difficult EMI.

Controlling the currents is a key issue. But don’t build roadblocks – build highways!

An extended facility needs a hierarchical hybrid ground system, e.g. locally grounded stations connected to a facility ground mesh.

Civil engineering may impose constraints on the design of grounding and lightning protection.

Cable trays are electric conductors, not just mechanical supports.

EMC must be planned – the sooner, the better.

emi and emc at extended facilities: examples from the

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