1

1) For existing systems, where the ferrite yoke needs to be shielded from the circulating beam, it is not generally possible to include ceramic tubes to support conductors (space limitations in the aperture).

Shielding of Ferrite Yoke from Beam

Interdigitated comb structure 20 mm spacing

2) For new systems it is not necessarily desirable to include ceramic tubes to support conductors, e.g. increased aperture size increased electrical stress on the pulse generator.

R&D is required to identify and develop effective beam shielding methods.

MKE magnets used an interdigitated comb structure – but this cannot be used on transmission line kicker magnets with “short” cells. In addition it is not as effective as, for example, the MKI beam screen.

Heat induced by the beam

Ferrite yoke

Beam

• The beam coupling impedance is reduced by providing a path for the beam image current, which screens the ferrite yoke from wakefields;

• Such a screen is installed between the beam and the ferrite yoke.

2

The eight LHC Injection (MKIs) magnets upgraded during LS1 are NOT expected to suffer from problems related to beam induced power deposition, of the ferrite yoke, during run 2 of the LHC. However, based on current HL-LHC beam parameters, additional upgrades are required, for implementation during LS2, to avoid problems of the ferrite yoke temperature exceeding the Curie Temperature: Study further means of further reducing beam induced power deposition:

3mm gap between ceramic tube and conducting cylinder (HV busbar side)

Metallized ceramic

1mm gap between ceramic tube and conducting cylinder (return [ground] busbar side).

End of metallization

SS cylinderCeramic

Screen conductors (graded lengths)Loverlap

1res

overlap

fL

MKI Shielding of Ferrite Yoke

3

Study further means of heat removal from the ferrite yoke, e.g. liquid cooling of HV busbar and/or higher emissivity of vacuum tank (see slide by Wim):

MKI vacuum tanks were electopolished (for septa)

emissivity of ~0.15. Note: visible light 400nm to

700nm wavelength.Plot courtesy of T. Bardo & S. Calatroni (TE-VSC)

TE-VSC have carbon coated SS316-LN and, with a ~600 nm thick coating, achieved emissivities of >0.8 (600nm thick C). A method of applying this to a “large” thank would need to be developed (e.g. by TE-VSC).

Cooling of Ferrite Yoke

4

Surface Flashover of Ceramic

It is thought that “high” pressure caused an electrical breakdown of an MKI in the LHC. Hence surface flashover of the ceramic tube, in a test (“Simi”) tank, is being/will be studied as a function of:1) Pressure of injected gas;2) Ionization of gas;3) Gas species (e.g. H2, H2O, CO, ….)Very important… e.g. could result in a “common mode” failure in all 4 MKIs, at an injection point!

5

1.4

aC

Cr2O3

Surface Flashover of Insulators

Uncoated ceramic has a maximum Secondary Electron Yield (SEY) of ~10. Coating the ceramic with Cr2O3 or Amorphous Carbon (aC) can reduce the SEY to less

than 1.4 (a magic number for the LHC, with 25ns beam). In addition Cr2O3 has been shown, by other researchers, to increase the voltage at

which surface flashover occurs. An order has been placed in industry to develop a technique to apply Cr2O3 to the

inside of a 3m long (MKI) ceramic tube. A shorter (50cm) long coated tube will be HV tested in the “Simi” tank.

Be careful of other potential problems, e.g. dust (UFOs)

6

FerriteEven with improved cooling of the yoke of a kicker magnet, there are advantages (and disadvantages, such as outgassing – see slide by Wim) of using a ferrite with a higher Curie Temperature (e.g. 180˚C, compared to ~120˚C for ferrite presently used). In addition future fast kickers (e.g. for the FCC) may need to provide a stronger field, and hence ferrites with a higher saturation flux-density will be required.Vacuum compatibility of suitable ferrites needs to be checked (by TE-VSC) and a prototype kicker magnet built and tested.

Very High Stability Pulse Power ModulatorsPrototype

inductive adder

Trigger inputs

1st protype of (low beam impedance, extremely high field uniformity [±0.01%]) striplines

A goal, for CLIC (DR), is to develop a kicker system with ±12.5kV (±250A), 160ns duration flattop, pulses with a stability of ±0.02%!!

An inductive adder, carefully designed, constructed and using sophisticated modulation techniques, gives promising results:

8

High Burst-rate Power ModulatorInitial “low” voltage prototype built to demonstrate basic concept:• A variant of the typical induction modulator topology (magnetic material) and full bridge inverter;• Cells produce alternating positive and negative polarity pulses, but load output is single polarity;• Flux alternates direction in the core, enabling pulse train output;DC in

DC in

To load

Q1Q2

Q3

Q4Cell 1

Cell 2

a) Required for CLIC CR;b) ~700 kHz burst rate (~100 pulses);c) 10 kV, 200 A;d) “Collaboration” started with SLAC

(Mark Kemp).

9

High Repetition Rate Marx GeneratorCLIC requires a high repetition rate (1 kHz, 12.5 kV) pulse generator for RF breakdown studies. A system, based on a Behlke switch, delivered by FPS in 2011, generally works well and has provided very useful statistics for breakdown studies: but the Behlke switch sometimes “dies” (the reason is not fully understood).

A replacement system, based on Marx Generator technology, is under discussion with Luis Redondo (Lisbon Superior Engineering Institute, Portugal):

Vdc

D2

C1 VMarxC2

D1

M1 M3

M2 M4

D3

C3

M5

M6

D4

C4

M7

M8

D5

C5

M9

M10

Out+

gate1

gate2

+

-

The Marx Generator is potentially a very interesting technology for CERN – and can provide a modular, fault tolerant, design with the ability to “shape” pulses.