The Development of a Superconducting RF Electron Microscope

Fig. 2: 2-mode cavity We are developing a new type of electron microscope (EM), which adopts RF acceleration in order to exceed the energy limit of DC acceleration used in conventional EMs. It enables us to make a high voltage EM more compact and to examine thicker specimens, and possibly to get better spatial resolution. It also provides an ability to observe transient processes by employing a state of the art laser photocathode technology as electron source. Low energy dispersion ∆E/E, e.g. 1.0×10−6 or better, is required for good spatial resolution in EMs, while it is usually between 1.0×10−3 to 1.0×10−4 in accelerators. We have thus designed a special type of cavity that can be excited with the fundamental and second harmonic frequencies simultaneously; TM010 and TM020. With the 2-mode cavity, the energy dispersion of the order of 1.0×10−5 would be obtained by modifying the peak of accelerating field to be flattened. We also adopt a superconducting cavity in order to operate the EM in CW mode, so that we can obtain a beam flux comparable to conventional EMs. In addition, we can make the operation more stable. As the proof-of-principle of our concept, we are developing a prototype using a 300 keV transmission electron microscope (TEM), to which a new photocathode gun and the 2-mode cavity are attached(Fig.1). Now we have already manufactured the cavity(Fig.2). We have done the beam dynamics simulation using General Particle Tracer(GPT), using the realistic electromagnetic field of the gun and the cavity. As a result, it has been found that we can reduce the energy dispersion ∆E/E from 2.04×10−4 to 3.68× 10−5 with 2-mode cavity. Then we can get the spatial resolution of 376pm, which is only about 50% deterioration of the existing 300keV TEM, which is much smaller than 818pm, the expected value of 1-mode cavity. The required electric fields are 8.21MV/m for TM010, 9.25MV/m for TM020. The designed Q-values are 1.86 × 10 for TM010(1.3002GHz), 1.00 × 10 for TM020(2.5999GHz). Performance evaluation tests has shown that the maximum electric fields are (8.75±0.73)MV/m for TM010, (7.80±0.33)MV/m for TM020. And the measured Q-values are (1.41± 0.45)× 10 for TM010, (0.78± 0.12)× 10 for TM020. It proves that manufactured cavity’s Q-values are close to designed ones, therefore we can say that the manufacturing process of our specially-shaped cavity has been confirmed. The measured resonant frequencies are 1.2963GHz for TM010, 2.5851GHz for TM020, so that the frequency ratio is 1.994. Then next task is to modify the cavity in order to tune the frequencies. And gun is now under construction. The goal of this year is to acquire the electron beam from the cathode. APPC12 The 12th Asia Pacific Physics Conference