Transcript Pauli_3_High_voltage

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
F
mv2

 qE
mv2 2 Ek 2qVacc
E 


 2Vacc
q
q
q
2Vacc
2Vacc
Vacc
E 

 2d
V
E
Vbend
bend
d
p
B 
q
Bending voltage
d
Vbend  2 Vacc

E.g.
Vacc=130 MV
d = 5 mm
=1m
Vbend = 1.3 MV !!!!!!!!!!!
•Bending very difficult!!!
•Very high voltages
High voltage devices
Einzel
lens
Electrostatic
deflector
Power supplies
•P (+)
•N (-)
•R (+ or -)
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…
Sparking
•Sparking starts from the negative electrode (=electrons)
•Pay special attention on negative electrode surface
•Polish (mirror surface)
Connectors/cables
• High-voltage connectors
– SHV
• High-voltage cables
– Check the specifications before use
Sparking
• High pressure
–
–
–
–
Free electrons in residual gas
Atmosphere, dry air: 106 V/m
Vacuum: 107 V/m
Depends on geometry (slightly)
Sparking in the presence of B
• Magnetic field guides electrons
– B along E: no help
– B perpendicular to E: helps
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).
Multipactoring threshold
Vo = (2pd/l) 2 (mec2)/(pe)
where:
Vo is the voltage between the sides of the cavity
me= mass of electron
l = wavelength
d = spacing between walls
c = speed of light (3x108 m/s)
e = charge on an electron
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