Siddharth Karkare | Cornell University (original) (raw)

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Papers by Siddharth Karkare

Abstract The surface roughness of GaAs photocathodes used in the injector prototype for the ERL a... more Abstract The surface roughness of GaAs photocathodes used in the injector prototype for the ERL at Cornell University was measured and compared to that of the atomically polished GaAs crystal surface using the atomic force microscopy (AFM) technique. The results show at least an order of magnitude rise in the GaAs surface roughness after subjecting it to heat cleaning, prior to activation. An analytical model for photoemission that takes into account the effect of surface roughness has been developed.

Abstract The Cornell ERL Photoinjector prototype has recently demonstrated successful operation a... more Abstract The Cornell ERL Photoinjector prototype has recently demonstrated successful operation at 20 mA for 8 hours using a bi-alkali photocathode grown on a Si substrate. The photocathode film was grown off center, and remained relatively undamaged; however, upon removal from the gun, the substrate at the gun electrostatic center displayed significant visible damage.

Abstract A broad-range R&D program is pursued at Cornell University aimed at preparation and char... more Abstract A broad-range R&D program is pursued at Cornell University aimed at preparation and characterization of high quantum efficiency photocathodes for the Energy Recovery Linac photoinjector. The currently investigated photoemitters include both positive and negative electron affinity materials such as bi-alkali antimonides and III-V semiconductors activated with Cs and either O or F.

Abstract The requirements for high quantum efficiency in the visible spectral range and an increa... more Abstract The requirements for high quantum efficiency in the visible spectral range and an increased lifetime as compared to cesiated GaAs can be met by multi-alkali photocathodes, either CsKSb or NaKSb. In this paper we detail the procedures that allow the growth of these thin films suitable for the ERL photoinjector operating at Cornell University. Quantum efficiency, spectral response, and surface characterization of deposited samples are presented. A load-locked multi-alkali cathode growth system is also described.

The Cornell University energy recovery linac (ERL) photoinjector has recently demonstrated operat... more The Cornell University energy recovery linac (ERL) photoinjector has recently demonstrated operation at 20 mA for approximately 8 hours, utilizing a multialkali photocathode deposited on a Si substrate. We describe the recipe for photocathode deposition, and will detail the parameters of the run. Post-run analysis of the photocathode indicates the presence of significant damage to the substrate, perhaps due to ion back-bombardment from the residual beam line gas. While the exact cause of the substrate damage remains unknown, we describe multiple surface characterization techniques (x-ray fluorescence spectroscopy, x-ray diffraction, atomic force, and scanning electron microscopy) used to study the interesting morphological and crystallographic features of the photocathode surface after its use for high current beam production. Finally, we present a simple model of crystal damage due to ion back-bombardment, which agrees qualitatively with the distribution of damage on the substrate surface.

Applied Physics …, Jan 1, 2011

Thermal emittance measurements of a CsK 2 Sb photocathode at several laser wavelengths are presen... more Thermal emittance measurements of a CsK 2 Sb photocathode at several laser wavelengths are presented. The emittance is obtained with a solenoid scan technique using a high voltage dc photoemission gun. The thermal emittance is 0.56 ± 0.03 mm mrad/mm(rms) at 532 nm wavelength. The results are compared with a simple photoemission model and found to be in a good agreement.

Chaos, Solitons & Fractals, Jan 1, 2009

Physical Review E, Jan 1, 2010

We report the generation of complex toroidal nanoscale vortices in nanochannel flows, through the... more We report the generation of complex toroidal nanoscale vortices in nanochannel flows, through the employment of patterned surface patches with contrasting wettability characteristics. Through extensive moleculardynamics simulations, we further delineate that complicated interactions between the wettability gradientdriven vortex structures and surface roughness-influenced flow features may give rise to interesting patterns in the flow field. We also establish the orientation specificities of these nanoscale vortices, and demonstrate that the surface roughness effects may potentially act to alter the flow rotationalities realized out of modulated surface patterning effects to a considerable extent, which may have interesting consequences toward designing nanoscale mixing elements.

Applied Physics …, Jan 1, 2011

Measurements of the intrinsic emittance and response time of a Cs 3 Sb photocathode are presented... more Measurements of the intrinsic emittance and response time of a Cs 3 Sb photocathode are presented. The emittance is obtained with a solenoid scan technique using a high voltage dc photoemission gun. Photoemission response time is evaluated using a RF deflecting cavity synchronized to a picosecond laser pulse train. We find that Cs 3 Sb has both small mean transverse energy, 160 6 10 meV at 532 nm laser wavelength, and a prompt response time (below the resolution of our measurement) making it a suitable material for high brightness electron photoinjectors.

Applied Physics Letters, Jan 1, 2011

High quantum yield, low transverse energy spread and prompt response time make GaAs activated to ... more High quantum yield, low transverse energy spread and prompt response time make GaAs activated to negative electron affinity (NEA), an ideal candidate for a photocathode in high brightness photoinjectors. Even after decades of investigation, the exact mechanism of electron emission from GaAs is not well understood. We show that a nanoscale surface roughness can affect the transverse electron spread from GaAs by nearly an order of magnitude and explain the seemingly controversial experimental results obtained so far. This model can also explain the measured dependence of transverse energy spread on the wavelength of incident light.

Abstract The surface roughness of GaAs photocathodes used in the injector prototype for the ERL a... more Abstract The surface roughness of GaAs photocathodes used in the injector prototype for the ERL at Cornell University was measured and compared to that of the atomically polished GaAs crystal surface using the atomic force microscopy (AFM) technique. The results show at least an order of magnitude rise in the GaAs surface roughness after subjecting it to heat cleaning, prior to activation. An analytical model for photoemission that takes into account the effect of surface roughness has been developed.

Abstract The Cornell ERL Photoinjector prototype has recently demonstrated successful operation a... more Abstract The Cornell ERL Photoinjector prototype has recently demonstrated successful operation at 20 mA for 8 hours using a bi-alkali photocathode grown on a Si substrate. The photocathode film was grown off center, and remained relatively undamaged; however, upon removal from the gun, the substrate at the gun electrostatic center displayed significant visible damage.

Abstract A broad-range R&D program is pursued at Cornell University aimed at preparation and char... more Abstract A broad-range R&D program is pursued at Cornell University aimed at preparation and characterization of high quantum efficiency photocathodes for the Energy Recovery Linac photoinjector. The currently investigated photoemitters include both positive and negative electron affinity materials such as bi-alkali antimonides and III-V semiconductors activated with Cs and either O or F.

Abstract The requirements for high quantum efficiency in the visible spectral range and an increa... more Abstract The requirements for high quantum efficiency in the visible spectral range and an increased lifetime as compared to cesiated GaAs can be met by multi-alkali photocathodes, either CsKSb or NaKSb. In this paper we detail the procedures that allow the growth of these thin films suitable for the ERL photoinjector operating at Cornell University. Quantum efficiency, spectral response, and surface characterization of deposited samples are presented. A load-locked multi-alkali cathode growth system is also described.

The Cornell University energy recovery linac (ERL) photoinjector has recently demonstrated operat... more The Cornell University energy recovery linac (ERL) photoinjector has recently demonstrated operation at 20 mA for approximately 8 hours, utilizing a multialkali photocathode deposited on a Si substrate. We describe the recipe for photocathode deposition, and will detail the parameters of the run. Post-run analysis of the photocathode indicates the presence of significant damage to the substrate, perhaps due to ion back-bombardment from the residual beam line gas. While the exact cause of the substrate damage remains unknown, we describe multiple surface characterization techniques (x-ray fluorescence spectroscopy, x-ray diffraction, atomic force, and scanning electron microscopy) used to study the interesting morphological and crystallographic features of the photocathode surface after its use for high current beam production. Finally, we present a simple model of crystal damage due to ion back-bombardment, which agrees qualitatively with the distribution of damage on the substrate surface.

Applied Physics …, Jan 1, 2011

Thermal emittance measurements of a CsK 2 Sb photocathode at several laser wavelengths are presen... more Thermal emittance measurements of a CsK 2 Sb photocathode at several laser wavelengths are presented. The emittance is obtained with a solenoid scan technique using a high voltage dc photoemission gun. The thermal emittance is 0.56 ± 0.03 mm mrad/mm(rms) at 532 nm wavelength. The results are compared with a simple photoemission model and found to be in a good agreement.

Chaos, Solitons & Fractals, Jan 1, 2009

Physical Review E, Jan 1, 2010

We report the generation of complex toroidal nanoscale vortices in nanochannel flows, through the... more We report the generation of complex toroidal nanoscale vortices in nanochannel flows, through the employment of patterned surface patches with contrasting wettability characteristics. Through extensive moleculardynamics simulations, we further delineate that complicated interactions between the wettability gradientdriven vortex structures and surface roughness-influenced flow features may give rise to interesting patterns in the flow field. We also establish the orientation specificities of these nanoscale vortices, and demonstrate that the surface roughness effects may potentially act to alter the flow rotationalities realized out of modulated surface patterning effects to a considerable extent, which may have interesting consequences toward designing nanoscale mixing elements.

Applied Physics …, Jan 1, 2011

Measurements of the intrinsic emittance and response time of a Cs 3 Sb photocathode are presented... more Measurements of the intrinsic emittance and response time of a Cs 3 Sb photocathode are presented. The emittance is obtained with a solenoid scan technique using a high voltage dc photoemission gun. Photoemission response time is evaluated using a RF deflecting cavity synchronized to a picosecond laser pulse train. We find that Cs 3 Sb has both small mean transverse energy, 160 6 10 meV at 532 nm laser wavelength, and a prompt response time (below the resolution of our measurement) making it a suitable material for high brightness electron photoinjectors.

Applied Physics Letters, Jan 1, 2011

High quantum yield, low transverse energy spread and prompt response time make GaAs activated to ... more High quantum yield, low transverse energy spread and prompt response time make GaAs activated to negative electron affinity (NEA), an ideal candidate for a photocathode in high brightness photoinjectors. Even after decades of investigation, the exact mechanism of electron emission from GaAs is not well understood. We show that a nanoscale surface roughness can affect the transverse electron spread from GaAs by nearly an order of magnitude and explain the seemingly controversial experimental results obtained so far. This model can also explain the measured dependence of transverse energy spread on the wavelength of incident light.