Lecture 8 Normal conducting RF. Copper Cavities. In copper cavities the shunt impedance, R, should be maximised in order to achieve a high accelerating gradient. Normal conducting cavities require nosecones and a small beam pipe in order to increase the shunt impedance factor. Average Heating.
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In copper cavities the shunt impedance, R, should be maximised in order to achieve a high accelerating gradient.
Normal conducting cavities require nosecones and a small beam pipe in order to increase the shunt impedance factor
Pulsed RF however has problems due to heat diffusion effects.
Over short timescales (<10ms) the heat doesn’t diffuse far enough into the material to reach the water cooling.
This means that all the heat is deposited in a small volume with no cooling.
Cyclic heating can lead to surface damage.
Once emitted this field emitted current can interact with the cavity fields.
Although initially low energy, the electrons can potentially be accelerated to close to the speed of light with the main electron beam, if the fields are high enough.
This is known as dark current trapping.
If we decrease the accelerating gap, while keeping the same voltage between the gap, the effective accelerating voltage increases due to the transit time factor.
As the gap is smaller the field strengths increase, which is good where the beam is but bad elsewhere.
To avoid problems we only decrease the gap near the beam. The narrowed gap region is known as a nose-cone.
Radio frequency Quadropoles are electrostatic quad’s for focussing the beam.
If the electrodes are specially spaced they can also accelerate the beam.
This is especially useful for low energy beams where space charge forces are large.
The most common type of structure is the p mode standing wave structure.
This can be a single cell or multiple cells coupled together.
These structures cannot have too many cells per cavity and means that for high energy accelerators many couplers are required. The cost of this is why these structures are overlooked for linear colliders like CLIC and NLC.
By measuring how the field varies between a cell and its nearest two neighbours we can use Floquet theorem to calculate the phase and reflections
E = field
P = cell length
R = reflection coefficient
y = required phase advance