Energy Saving Measures in the High-Power RF System of the European XFEL

The high-voltage modulators of the 26 klystrons in the XFEL represent the largest power consumer of the accelerator facility. The beam energy is usually 14 GeV, but a few weeks a year, lower and higher beam energies are also produced. For example, 11.5 and 16.3 GeV. Until mid-2022, the modulator voltages were set so that enough RF power for 16.3 GeV beam energy could be provided at all times. In order to save energy, the modulator voltages will now be lowered to a sufficient level for beam energies of ≤ 14 GeV. In this way, the previous power consumption of the modulators of 5 MW can be reduced up to 1 MW depending on the operation mode. Over the course of a year, this will save several GWh of energy. In the following, the relationship between the output voltage and the power consumption of the modulators is described. Afterwards, it is reported how the power consumption was reduced in various operating conditions in 2022.


Introduction
97 accelerator modules and one electron gun are supplied by 26 RF stations with 0.7 to 6.5 MW RF power.Figure 1 shows an RF station in the XFEL tunnel with exemplary, approximately correct and easily comprehensible parameters.The provided input signal with a frequency of 1.3 GHz is amplified to 100 W by a preamplifier and then fed to the klystron.The klystron then amplifies this signal to 5 MW which is delivered via 2 waveguide arms.The RF power is then fed via a waveguide distribution to the 32 cavities that accelerate the electron beam [1,2].To generate this RF power, the klystron needs a high operating voltage.In our example, 8 kV are generated by a so-called high-voltage modulator and transformed up to 96 kV with a pulse transformer.Thus, 10 MW of electrical power must be made available for the klystron operation (96 000 V × 104 A ≈ 10 MW).But this is only done 10 times per second for a period of 1.7 ms each.Therefore, the average power required is only 170 kW.In order to achieve the maximum possible beam energy, slightly more power is required on average and power losses of the overall system must be compensated.Therefore, the total power requirement for all 26 RF stations is 5 MW.This makes the RF system by far the largest energy consumer in the XFEL.

Power Consumption
In the following, we will show what the energy consumption primarily depends on.Figure 2 shows the voltage source, the modulator and the load, the klystron, in the model.The perveance of our klystron is given by The klystron current depends on this perveance and the klystron voltage The klystron can thus be seen as a voltage-dependent resistor, which, as will be shown later, is a decisive factor for the power consumption.Neglecting the duty cycle and the overall efficiency, the power on the flattop is and For the klystron voltage one can also write where ü is the transmission factor of the transformer.In equation ( 4) we insert equations (3), ( 5) and ( 1) Here we again insert equation ( 5) and we get an equation that depends only on the modulator voltage Now we take into account the duty cycle D and the overall efficiency  and get the power consumption of one modulator For the RF stations A1 to A25 we can assume the following parameters  = 10  • 1.7  = 0.017  = 0.935 The normal conducting gun has a smaller duty cycle.The analytical formula equation ( 9) is in good agreement with the voltages and power consumption measured in the operating modulators (figure 3).The modulator voltages shown are sufficient to achieve the maximum possible gradient in the XFEL of 17.5 GeV.
According to equation (9), an energy consumption of 251 kW results with a modulator voltage of 9.1 kV.
If the modulator voltage is lowered by 31 % to 6.3 kV, the energy consumption drops significantly more by 60 % to 100 kW.This is because the power consumption of the modulator is This makes it clear that the high modulator voltages should be lowered as much as possible in order to achieve a large savings effect.This is possible if the maximum gradient is not required.

Power Reduction
In mid-2022, the beam energy was lowered from 17.5 to 14 GeV as planned.The klystrons have to supply less RF power for this, and it was possible to lower the modulator and thus also the klystron voltages.The modulator voltages were reduced by 0.4 to 1.2 kV in 13 of 26 RF stations.This led to a reduction in power consumption of 24 to 65 kW per station.In total, about 0.5 MW was saved, which corresponds to 10 %.In August 2022, the beam energy was then lowered from 14 to 11.5 GeV.In order to keep the effort for setting new operating points of the RF stations small, the already reduced klystron voltages were kept and 3 RF stations were switched off completely.This led to an additional reduction in power consumption of 0.5 MW.

Energy Saving
For the next years, it is planned to operate the accelerator with a beam energy of 16.5 GeV for one third of the operating time (a gradient reserve of 1 GV is kept in reserve to compensate for the failure of one RF station).Another third with 14 GeV and the remaining operating time with 11.5 GeV.The measures presented will then lead to energy savings of about 3.6 GWh per year.

Summary
No investments had to be made for the implementation of these energy-saving measures, and they do not restrict the performance of the European XFEL.There are also two positive side effects for the klystrons when the voltage is reduced: firstly, they run more stable and secondly, their lifetime is extended.

Figure 1 .
Figure 1.Power supply of the klystron and generation of the RF power.

Figure 2 .
Figure 2. Model of modulator and klystron.By U and I are meant voltages and currents on the flattop of the high voltage pulse.

Figure 3 .
Figure 3. Measured and calculated -modulator voltages and power consumptions.