Experimental investigation of multibunch, multipass beam breakup in the Jefferson Laboratory Free Electron Laser Upgrade Driver

In recirculating accelerators, and, in particular, energy-recovery linacs, the maximum current can be limited by multipass, multibunch beam breakup (BBU), which occurs when the electron beam interacts with the higher-order modes (HOMs) of an accelerating cavity on the accelerating pass and again on the energy recovering pass. This effect is of particular concern in the design of modern high average current energy-recovery accelerators utilizing superconducting rf technology. Experimental characterization and observations of the instability at the Jefferson Laboratory 10 kW free electron laser (FEL) are presented. Measurements of the threshold current for the instability are made under a variety of beam conditions and compared to the predictions of several BBU simulation codes. This represents the first time in which the codes have been experimentally benchmarked. With BBU posing a threat to high current beam operation in the FEL driver, several suppression schemes were developed. These include direct damping of the dangerous HOM using cavity feedback and modifying the electron beam optics so as to reduce the coupling between the beam and mode. Both methods were shown to increase the threshold current for stability. Beam optical suppression techniques, in particular, have proved to be so effective that they are routinely used in the normal operations of the FEL Upgrade Driver.

Figure 1

Schematic of the experimental setup for simultaneously measuring the HOM power and voltage from a particular cavity.

Figure 2

A screen shot of an oscilloscope showing the HOM voltage (red) and power (blue) of the 2106.007 MHz HOM in cavity 7 of zone 3 during BBU.

Figure 3

FFT of a pure 2106.007 MHz signal (top) and FFT of the HOM voltage from cavity 7 during BBU (bottom).

Figure 4

The resonance curve for the 2106 MHz HOM as a function of average beam current with nominal, decoupled optics. Note that the effective $Q$ of the curve increases as the current increases. This indicates the system is unstable.

Figure 5

A plot of $1/Q_{\mathrm{eff}}$ versus average beam current from the data in Fig. 4. The intersection of the least squares fit (functional form given on the plot) with the horizontal axis determines the threshold current to be 2.4 mA.

Figure 6

The resonance curve for the 2114 MHz HOM as a function of average beam current with nominal, decoupled optics. Note that the effective $Q$ of the curve decreases as the current increases. This indicates the system is stable.

Figure 7

A plot of $1/Q_{\mathrm{eff}}$ versus average beam current from the data in Fig. 6. The least squares fit is used to determine that the mode is stable.

Figure 8

A plot of the HOM power of the 2106 MHz mode as a function of time for three different values of macropulse current (note the logarithmic scale of the vertical axis).

Figure 9

A plot of the three values of $1/{\tau }_{\mathrm{eff}}$ corresponding to each macropulse current from Fig. 8 versus the macropulse current. The threshold current is 2.2 mA and is extracted in the same manner as the BTF measurements.

Figure 10

Measured response of the 2106 MHz HOM due to vertical (red) and horizontal (blue) displacements through cavity 7.

Figure 11

A plot of the threshold current versus the change in quadrupole strength showing the effect of point-to-point focusing. At approximately 250 G the change in phase advance makes the $M_{34}$ element of the recirculation matrix from the cavity back to itself equal to zero before changing sign and hence the threshold current to be negative.

Figure 12

The effect on the $Q$ of the 2106 MHz mode with the cavity-based, narrowband feedback off (red curve represents $Q=6.2\times {10}^6$) and on (blue curve represents $Q=1.3\times {10}^6$).