Abstract

Novel accelerator concepts such as the switched power linac call for devices which can deliver current densities exceeding 105 A/cm2 from an area of ~1 cm2 for a few picoseconds. Metal photocathodes driven by short intense laser pulses are ideal for these applications due to their inherently short response time and large electron density. Here we present the experimental results obtained with the metal photocathodes, gold, yttrium. samarium, tantalum, and magnesium. illuminated by 4.66-eV photons of 10-ps pulse duration and 10-Hz repetition rate. The schematic of the experimental arrangement is shown in Fig. 1. The electron yields for various photocathode materials were studied for various emitting areas from 0.05 to 6 mm2. For a given energy density and fields large enough to overcome the space charge, the yield increased linearly with the illuminated area. The yield also scaled linearly with the energy for a given illuminated area. The measured emission limited quantum efficiencies, defined as electrons per incident photon, for various cathode materials are listed in Table I. along with their published work function. The quantum efficiency appears to scale quadratically with the excess energy of the photon above the work function. The maximum measured charge, limited by the collection technique. was 330 pC with 3.1 μJ on Mg. The corresponding current density would be 66 kA/cm2. However, the applied electric field was not large enough to overcome the space charge. Based on the measured quantum efficiency, the emission limited current density for 3.1 μJ would be 84 kA/ cm2. Similar current densities can be obtained with samarium and yttrium photocathodes. Extension to much higher current densities would require higher electric fields and considerations of surface damage thresholdsNovel accelerator concepts such as the switched power linac call for devices which can deliver current densities exceeding 105 A/cm2 from an area of ~1 cm2 for a few picoseconds. Metal photocathodes driven by short intense laser pulses are ideal for these applications due to their inherently short response time and large electron density. Here we present the experimental results obtained with the metal photocathodes, gold, yttrium. samarium, tantalum, and magnesium. illuminated by 4.66-eV photons of 10-ps pulse duration and 10-Hz repetition rate. The schematic of the experimental arrangement is shown in Fig. 1. The electron yields for various photocathode materials were studied for various emitting areas from 0.05 to 6 mm2. For a given energy density and fields large enough to overcome the space charge, the yield increased linearly with the illuminated area. The yield also scaled linearly with the energy for a given illuminated area. The measured emission limited quantum efficiencies, defined as electrons per incident photon, for various cathode materials are listed in Table I. along with their published work function. The quantum efficiency appears to scale quadratically with the excess energy of the photon above the work function. The maximum measured charge, limited by the collection technique. was 330 pC with 3.1 μJ on Mg. The corresponding current density would be 66 kA/cm2. However, the applied electric field was not large enough to overcome the space charge. Based on the measured quantum efficiency, the emission limited current density for 3.1 μJ would be 84 kA/ cm2. Similar current densities can be obtained with samarium and yttrium photocathodes. Extension to much higher current densities would require higher electric fields and considerations of surface damage thresholdsNovel accelerator concepts such as the switched power linac call for devices which can deliver current densities exceeding 105 A/cm2 from an area of ~1 cm2 for a few picoseconds. Metal photocathodes driven by short intense laser pulses are ideal for these applications due to their inherently short response time and large electron density. Here we present the experimental results obtained with the metal photocathodes, gold, yttrium. samarium, tantalum, and magnesium. illuminated by 4.66-eV photons of 10-ps pulse duration and 10-Hz repetition rate. The schematic of the experimental arrangement is shown in Fig. 1. The electron yields for various photocathode materials were studied for various emitting areas from 0.05 to 6 mm2. For a given energy density and fields large enough to overcome the space charge, the yield increased linearly with the illuminated area. The yield also scaled linearly with the energy for a given illuminated area. The measured emission limited quantum efficiencies, defined as electrons per incident photon, for various cathode materials are listed in Table I. along with their published work function. The quantum efficiency appears to scale quadratically with the excess energy of the photon above the work function. The maximum measured charge, limited by the collection technique. was 330 pC with 3.1 μJ on Mg. The corresponding current density would be 66 kA/cm2. However, the applied electric field was not large enough to overcome the space charge. Based on the measured quantum efficiency, the emission limited current density for 3.1 μJ would be 84 kA/ cm2. Similar current densities can be obtained with samarium and yttrium photocathodes. Extension to much higher current densities would require higher electric fields and considerations of surface damage thresholdsNovel accelerator concepts such as the switched power linac call for devices which can deliver current densities exceeding 105 A/cm2 from an area of ~1 cm2 for a few picoseconds. Metal photocathodes driven by short intense laser pulses are ideal for these applications due to their inherently short response time and large electron density. Here we present the experimental results obtained with the metal photocathodes, gold, yttrium. samarium, tantalum, and magnesium. illuminated by 4.66-eV photons of 10-ps pulse duration and 10-Hz repetition rate. The schematic of the experimental arrangement is shown in Fig. 1. The electron yields for various photocathode materials were studied for various emitting areas from 0.05 to 6 mm2. For a given energy density and fields large enough to overcome the space charge, the yield increased linearly with the illuminated area. The yield also scaled linearly with the energy for a given illuminated area. The measured emission limited quantum efficiencies, defined as electrons per incident photon, for various cathode materials are listed in Table I. along with their published work function. The quantum efficiency appears to scale quadratically with the excess energy of the photon above the work function. The maximum measured charge, limited by the collection technique. was 330 pC with 3.1 μJ on Mg. The corresponding current density would be 66 kA/cm2. However, the applied electric field was not large enough to overcome the space charge. Based on the measured quantum efficiency, the emission limited current density for 3.1 μJ would be 84 kA/ cm2. Similar current densities can be obtained with samarium and yttrium photocathodes. Extension to much higher current densities would require higher electric fields and considerations of surface damage thresholds.

© 1989 Optical Society of America

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