Beam breakup in one-turn and two-turn energy recovery linacs

The capital and operating costs of an energy recovery linac (ERL) may be reduced by multi-turn operation, which uses more than one pass of acceleration and deceleration. However, the beam current limit due to multipass beam breakup (BBU) may be decreased. As an example, we study a 1-GeV ERL with average current of 100 mA, which is a possible source of bright synchrotron radiation. A one-turn design with a single superconducting L-band linac is compared to a two-turn single-linac design. In simulations, the BBU threshold currents of the two-turn design are about one-half as large as the one-turn thresholds. If the linac's strongest higher-order modes are damped to obtain quality factors of a few thousand, 100-mA currents are possible in both designs. Thus, a two-turn ERL may be a feasible path to reduce the linac cost and machine size.