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Techniques of Vacuum and Basics of High Voltage (3/3)

Techniques of Vacuum and Basics of High Voltage (3/3). Pauli Heikkinen Jyväskylä University. High voltage. DC Beam steering Beam focusing Reaction product guiding (e - ) High voltage barriers AC Acceleration Bunching. Electric rigidity. Bend the particle beam with an electric field.

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Techniques of Vacuum and Basics of High Voltage (3/3)

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  1. Techniques of Vacuum and Basics of High Voltage (3/3) Pauli Heikkinen Jyväskylä University

  2. High voltage • DC • Beam steering • Beam focusing • Reaction product guiding (e-) • High voltage barriers • AC • Acceleration • Bunching

  3. Electric rigidity Bend the particle beam with an electric field

  4. Bending voltage • E.g. • Vacc=130 MV • d = 5 mm • r = 1 m • Vbend = 1.3 MV !!!!!!!!!!! • Bending very difficult!!! • Very high voltages

  5. High voltage devices Einzel lens Electrostatic deflector

  6. Power supplies • P (+) • N (-) • R (+ or -)

  7. High voltage conditioning • At first the electrodes don’t keep the desired high voltage • Not necessarily a problem • “Teach” the electrodes = conditioning • Increase the voltage until the current jumps • Decrease the voltage so that the current (arc) drops • Increase… • Decrease…

  8. Sparking • Sparking starts from the negative electrode (=electrons) • Pay special attention on negative electrode surface • Polish (mirror surface)

  9. Connectors/cables • High-voltage connectors • SHV • High-voltage cables • Check the specifications before use

  10. Sparking • High pressure • Free electrons in residual gas • Atmosphere, dry air: 106 V/m • Vacuum: 107 V/m • Depends on geometry (slightly)

  11. Sparking in the presence of B • Magnetic field guides electrons • B along E: no help • B perpendicular to E: helps

  12. Multipaction • Multipaction is an effect that occurs with RF fields, usually in a vacuum or low pressure condition. • Results from an ion moving back and forth (in response to an RF field) and knocking other electrons off the sides when it hits. • If the transit time of the electron is nicely synchronized with the RF field, then just as it hits, the field is right to pull the new electrons towards the other side, and a cascading avalanche can result (if the electron emission coefficient (d) is >1).

  13. Multipactoring threshold Vo = (2pd/l) 2 (mec2)/(pe) where:Vo is the voltage between the sides of the cavityme= mass of electronl = wavelengthd = spacing between wallsc = speed of light (3x108 m/s)e = charge on an electron

  14. Remember also: • Induced high voltages at inductances (coils) • Abrupt current shut-off in a coil • High voltage capacitors in electronics • Can have HV even after power shut down

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