Design and performance of a double-solenoid magnetic bottle photoelectron spectrometer for attosecond metrology

The design and performance of an in-house developed double-solenoid magnetic bottle (MB) time-of-flight photoelectron spectrograph are presented. A combination of a strong permanent magnet (Sm₂Co₁₇) with a soft iron cone and a double-solenoid geometry is used to generate MB configuration. The first solenoid (length ∼150 mm) is placed inside the vacuum, and the second solenoid (length ∼1 m) is placed outside the vacuum. The double-solenoid geometry improves the effective conductance and reduces overall material outgassing. Due to this, an ultra-high vacuum (∼5 × 10^-8 mbar) desirable for the working of the spectrograph was achieved using a small capacity (300 lps) turbo-molecular pump. An optimization of solenoid current generates a smooth magnetic field variation in MB, which keeps the adiabaticity parameter ∼0.6 at ∼25 eV photoelectron energy. The double-solenoid geometry also provides high collection efficiency as well as high energy resolution of the spectrograph. The experimentally measured energy resolution (ΔE) of the spectrograph is better than ∼60 meV at ∼15 eV photoelectron energy. The collection efficiency is estimated to be ∼25% under optimum conditions as compared with ∼10^-4 in field-free configuration. The calibrated MB spectrograph is used for the characterization of the attosecond pulse train using a cross-correlation "RABBITT" technique. The attosecond pulse train is generated from 15th to 25th odd high-harmonic orders, in argon filled cell. Attosecond pulses of average duration ∼260 as (FWHM) have been measured. The proposed MB electron spectrograph design provides a compact experimental setup for attosecond metrology and pump-probe studies with a relaxed requirement on vacuum pump capacity.