BREAKDOWN IN RF CAVITIES

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BREAKDOWN IN RF CAVITIES

• Sergey Korenev

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Content

• 1. Introduction to problem.
• 2. High pressurized cavity with RF power.
• 3. Beam in high pressure gas matter.
• 4. High pressurized cavity with RF and beam.
• 5. Future works.

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Introduction to problem

• The study of ionization cooling led to idea of
  high pressurized cavity (Rolland Johnson).
• Experiments at MUONS Inc. have been shown very
  optimistic threshold electrical field ( E50
  MV/m 500 kV/cm) for Hydrogen pressurized cavity
  with f805 MHz.

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Discharge in pressurized RF cavity without beam

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Production of primary electrons

• Field emission.
• Forming of ecton ( plasma torch) with liquid
  phase.
• Thermoemission from liquid micro-torch.
• Generation of nanoparticles from electrodes
  material.
• Photo and collisions ionization.
• Dissociation ionization of hydrogen
• Kinetic energy of primary electrons is 1.5-2.5
  MeV for gradient of electrical field 500 kV/cm.
• Limitation of current by space charge ( Law
  Child-Langmuir).

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Ionization of gases molecules

• The places with generation of electron beams are
  not regularly on the surface of RF cavity with
  max gradient of electrical field.
• The size of these places is small on the level of
  few-ten microns.
• It leads to forming of plasma torches in the
  different places.
• The extraction of electrons from plasma
  torch leads to forming electron beam, which
  ionize gas at during propagation to opposite
  electrode. The forming of ionized heads from both
  sides and moving to electrodes lead to
  electrically connected electrodes in cavity. The
  velocity of propagations of front ionized wave
  (ionized gas) was measured in many papers and is
  about V 104-105cm/sec. The time of commutations
  t is tL/V. For MUONS Inc. experiments it is
  about 20 microseconds and has good experimental
  correlation.

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Ionization of gases molecules

• This effect we can for our goals.
• 1. We can make injection and accelerating of
  muons before of breakdown of cavity.
• 2. To use RF generator with pulse duration
  10-20 microseconds.
• The voltage can be increased to 1,5-2-3 times (
  Over-voltage effect) on dependence on parameters
  of RF generator.

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Example of discharge in high pressure gas

• Pictures of forming pulsed discharge for CO2
  with 5 nsec resolution.

10 micron
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Example of discharge in high pressure gas

• Pictures of forming pulsed discharge for CO2
  with 5 nsec resolution.

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Low temperature effect

• First time this effect was observed for Cu
  electrodes

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Breakdown for low temperature

• Our experiments with generation of cathode and
  anode plasma shown same effects.
• Series 2 for gap 15 mm.
• Series 3 for gap 10 mm.
• Positive polarity voltage has no dependence on
  temperature

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Breakdown for low temperature

• This effect was observed for vacuum case, but I
  hope this effect will be work for high
  pressurized cavity.
• This effect has good correlation with study of
  creation of cavity with High Temperature
  Superconductor coating (YBCO or other).
• The our presentation with R. Johnson on the
  AppAcc2007 about electron accelerator with broad
  beam based of toroidal cavities with HTS film had
  interest from industrial companies manufacturing
  electron accelerators and they have interest to
  creation of joint product.

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Conclusions

• The structure of high pressurized discharge has
  non-regular structure and presents streamer
  spark character with zones which have different
  parameters (concentration, temperature,
  irradiation and etc.)
• The few ways we can consider for increasing of
  threshold electrical gradient for breakdown in RF
  cavity
• The low temperature for electrodes (cavity),
  effect is about 1.3-2 times for nitrogen
  temperature.
• Decreasing of rise time and pulse duration
  of RF.
• Surface modification of electrodes.

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Beam in high pressure gas

• We will have ionization of hydrogen molecules by
  muons beam.
• The processes of ionization and recombination for
  pulsed injected beam to gas described by
  equation
• where j is current density, No is
  concentration of molecules, si is cross section
  ionization, nep is concentration of plasma
  electrons, ni is concentration of ions, Vd.ep is
  drift velocity for plasma electrons, nep, nip is
  concentration of electrons and ions in plasma, ar
  is factor of volume neutralization

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Ionization effects of gas by muons

• The concentration of electron components can be
  from 109 to 1013 cm-3 on dependence on number of
  muons.
• The compensation of space charge will be have
  place.
• The study can be start from nitrogen from safety
  simple conditions.
• The first experiments can be conduct for
  measurements of conductivity of ionized nitrogen
  using simple tested sensor.
• The sensor can be realized for measurements of
  plasma conductivity in cross section using
  section sensors.

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Measurements of plasma conductivity

• General principle

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Sensor of plasma conductivity

• Tests and sensors on Rhodotron

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Remark Conductivity of ionized gas

• Fusion research with Hydrogen plasma

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Conclusion

• Experiments with measurements of conductivity of
  ionized gas with high concentration ( pressure
  100-200Ata) allow to understand the real
  conditions for RF cavity.

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Cavity with beam

• Cavity with beam allows to consider situation of
  plasma in electrical field and magnetic field.
• It allows to consider as examples experiments
  with electron beam for study of high pressurized
  Spark Gaps.

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Ionization effects of gas by e-beam

Nitrogen, 50 Ata, e-beam 5 nsec, 150 KeV, 200 A)
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Ionization effects of gas by e-beam

• The study of e-beam ionization of effects in high
  pressurized Spark Gap have been conducted for low
  induction discharge system for production of
  sub-nanosecond rise time.
• The work with eximer lasers has same problems and
  solutions.
• Electron beams had current about 50-1000A, pulse
  duration 5-300 nsec, kinetic energy 50-200 keV.
• The diffusion processes are small in high
  pressurized gas and it allows to save dimensions
  of plasma channel.

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Conclusion

• The using of magnetic field and injection of
  small diameter of muons beam will allow to make
  insulated from electrodes of cavity ionized
  channel and absent of electrical breakdown of RF
  cavity.

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Future work

• The computer modeling of surface processes for
  electrodes in pillbox and spherical toroidal
  cavity with RF.
• The experimental study of breakdown phase in
  high pressurized discharge for RF cavities.
• The computer modeling of processes for plasma
  processes, including non-stability of plasma.

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Summary

• The potential increasing of gradient of
  electrical field for RF cavity can be realized by
  3 ways
• 1. Low temperature electrodes effect for
  breakdown.
• 2. The injection and accelerating of muons at
  during of time delay of breakdown or decreasing
  RF pulse duration.
• 3. Surface modification of electrodes materials.
• The accelerating of muons without breakdown of
  cavity can be realized by
• The electrical insulation of plasma channel by
  smaller muon beam diameter and using external
  magnetic field.