1. Pulsed Laser Deposition and Quantum Efficency of Mg films University of Lecce L. Cultrera

2. Outline PLD of Mg films improvements; –Vacuum system; –Target surface laser cleaning; –Deposition in inert gas; Quantum efficiency results; Necessary improvements for QE set-up; Future plans.

3. PLD tecnhique improvements Known problems related to Mg deposition Residual gas pressure should be decreased Improve the quality of residual gases Film thickness usually lower than 1  m Presence of debris on film surface Proposed solutions Ionic pump added to the deposition chamber Laser cleaning of target surface Deposition in low pressure inert gases Laser annealing of film surfaces in UHV

4. PLD Vacuum System In order to improve the vacuum level achievable by the actual pumping system (turbo pump, 250 l/s) of deposition chamber, a new ionic pump (40 l/s) has been installed during the first week of november. We expect a final vacuum less than 10 -6 Pa.

5. Target Laser Cleaning The laser cleaning of the target surface before the depostion process removes the most polluted and contaminated layers. Moreover the ablated pure Mg reacts with water, and oxygen giving a substantial improvement of the vacuum quality.

6. PLD in low pressure inert gas The initial dimensions of the laser plume are of the order of mm in the transverse directions, whereas in the perpendicular direction they are less than 1  m. As the velocities are controlled by the pressure gradients, the expansion is anisotropic in the direction perpendicular to the target surface. In presence of low pressure inert gases the plume expansion should be limited in transverse direction allowing more ablated material to reach the substrate surface. We make some deposition test in pure He atmosphere. UHV Low pressure inert gas

7. PLD in low pressure inert gas SampleTargetSubstrated T-S spot sizeHe P bg Laser pulsesLaser FluenceThickness Mg 6 MgSi (100)3.5 cm0.96 mm 2 5 Pa5000010 J/cm 2 2.5  m Mg 71 Pa5000010 J/cm 2 1.6  m The tests for the achievements of film thickness higher than 1  m with the plume confinement obtained in low pressure inert gas give good results. Mg thick films have been deposited. We expect that the use of inert backgruond gases should also improve the purity of the deposited material.

8. PLD deposited film for testing SampleTargetSubstrateT-S distanceSpot sizePressure (Pa)Laser pulsesLaser Fluence Mg001 Mg+C Cu 4.5 cm 1.1 mm 2 3.3x10 -6 30000+200010 J/cm 2 Mg002 30000+200010 J/cm 2 Mg003 30000+900010 J/cm 2 Mg004 5x10 -6 30000+900010 J/cm 2 Mg005 0.9 mm 2 30000+900014 J/cm 2 Mg006 30000+900014 J/cm 2 Mg007 Mg3.5 cm5 He50000 14 J/cm 2 Mg008 14 J/cm 2 Mg009 10 J/cm 2 Mg010 10 J/cm 2 In order to test the repeatability of the deposition a new series of twin samples were deposited. The first to complete the studies on the graphite protective coating, the second to start the studies on thick but unprotected films.

9. Quantum Efficiency Measurement Set-up Computer Camera Pump UHV Chamber Virtual Cathode Vacuum meter Photodiode Scope Ch.1 Scope Ch.2

10. Quantum Efficiency Measurement Results The best results obtained during QE measurements have been obtained for Mg samples covered with a graphite protective layer. 3X10 -4 E extr =1MV/m 4mm 2 SPOT SIZE QE>3X10 -4

11. Quantum Efficiency Measurement Results MIN. QE5.76 10 -5 MAX. QE8.02 10 -5 AVG. QE6.94 10 -5 STD. DEV.0.86 10 -5 The uniformity of the emission has been measured on non graphite covered thick sample. While the QE was lower than that measured For graphite covered samples, the uniformity is good enough. Cleaned area0.5 mm 2 Power density20 GW/cm 2 Energy density0.5 J/cm 2 Test areas0.04 mm 2

12. Morphological analysis on Mg samples ~2 GW/cm 2 ~20 GW/cm 2 Laser annealing induces crystallization Morphology depends on power density? No substantial surface modification

13. QE Measurement Set-up Improvements Known problems related to QE measurement set-up Normal incidence of the laser through the grid results in diffraction pattern. Gaussian spatial distribution of the laser beam is not good for uniform cathode activation or cleaning. High energy instability of the Q-switch mode locked Nd:Yag laser does not allow control of the irradiation parameters. Absence of any control in the laser cleaning depth before QE measurements. A new vacuum chamber has been built. Incidence of the laser beam will be 72°. Beam homogenizer is needed to obtain a flat-top spatial profile. Planning to modify the obsolete electronic circuits of the laser pulse picker with new ones. On line mass spectrometer will give the opportunity to monitor in “real time” the ablated chemical species.

14. Future Plans PLD of photocathode thin films: –To test other metallic cathodes (Y, Ba, Sm); –To test other protective coatings (Pd, SiO 2 ). QE measurements: –Improve the laser energy stability; –Improve the laser transfer line to cathodes; –Explore other ways for the cathode activation. Realize a S-band Mg cathode flange and test in rf-gun environment