Reducing the heat load on the LCLS 120 Hz RF gun with RF pulse shaping

The LCLS injector must operate at 120 Hz repetition frequency but to date the maximum operating frequency of an S-band rf gun has been 50 Hz. The high fields desired for the LCLS gun operation limit the repetition frequency due to thermal expansion causing rf detuning and field redistribution. One method of addressing the thermal loading problem is too reduce the power lost on the cavity walls by properly shaping the rf pulse incident on the gun. The idea is to reach the steady state field value in the gun faster than the time constant of the gun would allow when using a flat incident rf pulse. By increasing the incident power by about a factor of three and then decreasing the incident power when the field reaches the desired value in the gun, the field build up time can be decreased by more than a factor of three. Using this technique the heat load is also decreased by more than a factor of three. In addition the rf coupling coefficient can be increased from the typical critically coupled designs to an overcoupled design which also helps reduce the field build up time. Increasing the coupling coefficient from 1 to 2 reduces the heat load by another 25% and still limits the reflected power and coupling hole size to manageable levels. At this time the highest operational repetition rate for a 1.6 cell LCLS prototype RF gun has been 50 Hz [1] at 100 MV/m at the University of Tokyo. At higher repetition rates and fields, the gun field distribution may be significantly perturbed due to thermal expansion. The thermal expansion is not uniform in all dimensions due to the position of the cooling channels which leads to a small but different frequency shift in each cell of the gun. In addition the LCLS gun will utilize a load-lock system for installing new cathode plates and may limit the amount of cooling available at the cathode plate creating strong thermal gradients. The LCLS gun will exhibit a small field redistribution as the cell resonant frequencies change due to the thermal distortions. The most noticeable change in the fields is the ratio of the on axis fields in each cell. The field perturbations may lead to an electron beam exiting the gun with lower than optimal beam brightness since the frequency and field distribution tuning is necessarily done at low power with a network analyzer or similar measurement set up. The effect is especially noticeable in the beam longitudinal emittance where the correlated energy spread depends on the cell field ratio [2]. There are several known methods to solving or mitigating this problem. The ideal method is to first study the energy deposition in the rf gun and provide appropriate cooling at the necessary locations without compromising structural integrity to eliminate frequency shifts and the associated field redistributions. Preliminary analysis of this type has been conducted on the LCLS prototype at BNL [3] as well as SLAC [4] indicating there will be a significant heat load and temperature gradient leading to substantial frequency shifts and a modified cell field ratio. Optimizing the cooling channel location has not been completed but the preliminary analysis indicates it will be difficult to design the cooling channels to eliminate the frequency shift at the required input power level. A second method involves measuring the fields in each cell as a function of time and tuning the gun such that the desired field ratio is achieved at high power when the laser is fired instead of low power on the bench. Measurements can be performed utilizing capacitive probes located outside the coupling or laser ports on the full cell and half cell respectively. This method requires retuning the gun each time the field level, rf pulse shape, or repetition rate is changed. The author expects that the field measurements will be a good diagnostic to indicate the field ratio has been altered, but should not be relied on as the sole method of maintaining the cell field ratio. Instead the gun design should include a method to reduce the frequency shifts as much as possible to reduce the amount of high power tuning necessary. Thus the desired method to reduce the resonant frequency shifts of the gun cells is to reduce the heat dissipated in the gun. The dominant heating mechanism of the gun is resistive heating due to the finite conductivity of the cavity walls. Other energy deposition (such as the photocathode drive laser) and extraction (such as the electron beam) mechanisms are negligible compared to the rf power heating since they involve considerably less energy. The stored energy in the cavity fields is 6.7 J for the LCLS design of 120 MV/m and assuming a Q0 of 12000 and a critically coupled cavity as are typically obtained with the LCLS prototype RF photocathode gun. However a 6.2 MeV, 1 nC beam only extracts 5.7 mJ of energy and the drive laser supplies around 0.5 mJ of UV energy. Thus both of these effects have a negligible effect on the energy lost to the cavity walls and can be ignored in this calculation. In order to reduce the amount of energy lost on the cavity walls, one must either reduce the resistivity of the walls or reduce the amount of energy incident on the gun. Since the LCLS prototype gun is already constructed out of Cu specifically because of the high conductivity, the only additional benefit is to operate the gun at 30 C instead of the traditional 45 C for SLAC accelerators to reduce the resistivity by about 10% [3]. However, this only reduces the energy lost in the walls by 10% and the benefit is reduced if temperature tuning the gun frequency is required after a cathode replacement. Alternatively one can drive the gun to its operating voltage in a shorter period of time and thereby reducing the energy lost to the walls. Since the cavity is designed to operate as an efficient accelerator, the cavity losses are kept low to maximize the accelerating voltage in the gun for a given klystron power input. In other words the gun is designed to have high shunt impedance. However, because the gun cavity is operated as a standing wave structure, the filling time becomes