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

accacck Vq

qV

q

E

q

mvE

qEmv

F

2222

2

accacck Vq

qV

q

E

q

mvE

qEmv

F

2222

2

bend

acc

bend

accaccE V

Vd

dVV

E

V2

22

bend

acc

bend

accaccE V

Vd

dVV

E

V2

22

q

pB q

pB

accbend Vd

V

2 accbend Vd

V

2

Bending voltage

E.g.

Vacc=130 MV

d = 5 mm

= 1 m

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 () is >1).

Vo = (2**d/) 2 * (me*c2)/(*e)

where:Vo is the voltage between the sides of the cavityme= mass of electron = wavelengthd = spacing between wallsc = speed of light (3x108 m/s)e = charge on an electron

Multipactoring threshold

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