Landau cavities at MAX II
Contact person: Mattias Georgsson (e-mail: mattias.georgsson @maxlab.lu.se)
Passive Landau cavities have been built, installed and taken into regular operation in the MAX II ring. The purpose of the Landau damping system is twofold: to decrease the energy spread of the beam induced by coupled bunch oscillations and to increase the Touschek life-time. The cavities are of the pillbox type operated at 1500 MHz, the third harmonic of the main RF.
The MAX II ring has been operated for several years and all the target specifications have been met, except that for the energy spread of the electron beam. The increased energy spread is due to undamped coupled bunch oscillations and would deteriorate the performance of the undulators when used at higher harmonics. A feedback or a HOM damping system has been considered, but this should imply a short Touschek lifetime when the ring is operated at low coupling. In this situation a passive Landau cavity system was chosen to solve both problems, such a system should damp the coupled bunch instability and prolong the Touschek lifetime.
The Landau cavity system operated at an harmonic of the RF will provide strong nonlinearities and thus damp the longitudinal coherent oscillations and also increase the bunch length by reducing the voltage gradient at the bunch position. This increased bunch length gives a decreased electron density and thereby higher Touschek lifetime. The Landau cavities are passive elements and fed by the electron beam itself.
In operation
When the Landau cavities are tuned far from resonance, the beam is unaffected by the Landau cavities as seen in fig. 1a. Closer to resonance the beam size decreased as seen in fig. 1b. This picture, from the diagnostic beamline, is then used for beamsize measurements.
Figure 1a The beam in MAX II with the Landau cavities detuned. Horizontal polarisation.
Figure 1b The beam in MAX II with tuned Landau cavities. Horizontal polarisation.
From these beamsize measurements the beam energy spread is obtained. The natural beam energy spread is 0.7× 10-3. With full current and detuned cavities the energy spread is 4.5× 10-3 due to the coupled bunch oscillations, but with tuned Landau cavities the energy spread decreases down to 1.0-1.8× 10-3. Fig 2.
Figure 2 The beam energy spread versus beam current with and without Landau cavities.
Figure 3 Beam lifetime. When the Landau cavities are tuned in the current times lifetime increases.
Effect of the Landau cavities is seen on the beam lifetime. The lifetime times current at normal operation is about 3 Ah. The lifetime is Touschek limited. When the bunch lengthening effect occur, with the tuned cavities, the intrabeam scattering decreases and the lifetime increases to almost 6 Ah. Fig 3. This is during the whole fill from 250 mA to 100 mA. The landau damping is effective down to a ring current of 80 mA.
Results The design criteria are matched with good agreement to the specifications. The results can be summarised:
- The energy spread decreases from 5× 10 -3 to 1.0-1.8× 10 -3.
- The Touschek lifetime increases by a factor of three due to incoherent elongation of the bunches.
- Possibility to run undulators at higher harmonics.
The Landau damping system is now used in routine operation and another important effect is that only one fill a day is sufficient instead of the previous two.
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