Active EMI Compensation for Pulsed Kicker Systems in Particle Accelerators

Tobias Stadlbauer (SY-ABT), Fritz Caspers (EP-UGC)

Kicker systems in particle accelerators operate at high voltages, reaching up to 80 kV, and currents up to 30 kA, with risetimes as fast as 10 ns, producing pulses comparable to lightning strikes in terms of current amplitude, length, and spectrum. The kicker generator connects to the kicker magnet via coaxial transmission lines. Due to the system design, these transmission lines are grounded at both ends, the generator and the magnet site. This connection to the building ground at multiple points can lead to significant electromagnetic interference (EMI) by conducted emissions due to a not distinctly defined current return path. The same issue arises when kicker magnets are installed in vacuum tanks connected to the building ground, and when the outer conductor of the feedthrough is in electrical contact with the vacuum tank, which is generally the case.

The return current of the generator does not exclusively follow the desired return path through the outer conductor of the transmission line (Tx-Outer). Instead, it is partially diverted into the grounding network (R.GND) and into the return conductors of sensitive measurement cables (Tx-Measure-Outer).

This unwanted current flow causes a voltage drop in the measurement cables, which is superimposed onto the measurement signal, degrading the signal’s accuracy.

Conventional common-mode chokes (CMCs) can only be applied directly to the measurement cables and do not address the EMI at its source. The transmission lines of the generator are too large in diameter to be wound around a magnetic core to increase the common mode impedance. Moreover, it is impossible and inefficient to equip the many measurement cables with individual CMCs.

To overcome these challenges, a novel approach has been proposed that actively compensates for EMI by inducing a counter voltage into the transmission lines. This compensation voltage cancels out the differences caused by the finite conductivity of the return conductors, functioning similarly to both a passive common-mode choke and an active electrical transformer.

The coaxial transmission line is fed through the magnetic core with only one winding, with an additional compensation winding applied to the core. With this setup a counteracting voltage can be induced into the transmission lines, to significantly reduce EMI at the source.

Under the specific conditions of our systems, it was demonstrated that by choosing the induced voltage (V_ind.outer) to be equal to the voltage drop over the outer conductor of the transmission lines (V_Tx.outer) the voltage difference (V_GND) between the generator ground and the magnet ground can be eliminated and thus no kicker current is flowing in the grounding network anymore.

A high-voltage power supply charged a capacitor, which was then switched to the compensation coils via an IGBT. The IGBT was operated in linear mode, controlled by an operational amplifier and function generator, ramping up the magnetic flux density to induce a counteracting voltage during the generator pulse. The analysis demonstrated that the induced counter voltage effectively neutralized the voltage differences, leading to improved measurement accuracy. The active EMI compensation method addresses the root cause by substantially reducing EMI at its source.

In the test system, the common-mode current was reduced by over 96%, from 280 A to below 10 A.

Consequently, the need for EMI protective measures on auxiliary systems connected to the building ground at multiple points is eliminated, offering a practical and effective solution for mitigating EMI in kicker systems.

The active EMI compensation system can be retrofitted into current operational systems. C cores can be opened and fitted around existing cables without disconnecting them. The compensation can be switched on or off depending on operational needs, and in case of a fault, the kicker system remains unaffected, with EMI simply returning to uncompensated levels.

The method preserves the integrity of the kicker pulse and is compatible with various cable types and pulse lengths, demonstrating a versatile and robust solution to the longstanding EMI problem in kicker systems at CERN.

Future research will focus on optimizing the compensation method and exploring its application to other high-current systems, potentially extending its benefits beyond the current scope.

CDS Note, CERN-ACC-NOTE-2024-0016