Changes

==Laser Beamline==

==Laser Beamline==

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process.

Figure 1 illustrates the arrangement of the ablation set up. A series of quartz plates are positioned immediately in front of the laser aperture so that a small sample of the beam (<5%) is reflected onto two separate energy meters labeled energy meter 1 and energy meter 2. Energy meter 1 is part of the laser’s on board energy feedback system which is used to control the output energy and stabilize the pulse-to-pulse variation to within 5%. Energy meter 2 measures each laser pulse incident on the diamond target during the ablation process. Figure 5a shows two columns of broad asymmetric patterns in a diamond sample cut using only a single lens for a varying number of laser pulses. If the focal spot that created these patterns was rastered over an entire diamond it would result in a radiator with large surface variations rendering it unusable for GlueX. The focus of the laser defines the cutting tool with which the diamond is shaped. An ill-defined focused will ablate non-uniformly as the diamond is rastered across it making it extremely difficult to cut uniformly to 20 µm thickness without cracking the thin diamond membrane. The geometry of the focus also determines the fluence (laser energy per cm^2 ) incident on the diamond surface. A tightly focused beam spot increases the available fluence, increasing the rate of ablation. It is therefore very important to measure the waist of the beam after L3 in the three lens system.  

[[Image:ablation_full.png|left|thumb|500px|Rendering of ablation beamline]]

[[Image:ablation_full.png|left|thumb|500px|Rendering of ablation beamline]]

The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.

The laser beam then passes through a series of lenses as shown in Figure 4. Lenses L1 and L2 are positioned with overlapping focal lengths so that the output of L2 is a highly parallel, expanded beam. This was to remove large spherical aberrations due to imperfections in the quartz lenses.

[[Image:spherabs.png|right|thumb|300px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|300px|Zygo image of pulses made on diamond after passing through the three lens system.]]

[[Image:spherabs.png|right|thumb|300px|Zygo image of pulses made on diamond after passing through a single focusing lens.]] [[Image:goodfocus.png|right|thumb|300px|Zygo image of pulses made on diamond after passing through the three lens system.]]

 

===Focal Spot Characterization===

===Focal Spot Characterization===

The focal spot created after the laser pulses pass through the optical setup illustrated in Figure 1 was measured using a harp scan in both the horizontal and vertical planes. Two mounts were machined with horizontal 8 and vertical v-grooves where 50 µm gold-tungsten wire was stretched and glued. The mount was anodized to insulate it from the gold-tungsten wire as well as the ablation chamber itself. As the laser irradiates the gold-tungsten wire electrons are freed via the photoelectric effect and a positive current flows through the wire which can be measured. The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2 mm and the z coordinate range is 12 mm. Each pass has a single value of z, and sweeps over the full 2 mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place.

The focal spot created after the laser pulses pass through the optical setup illustrated in Figure 1 was measured using a harp scan in both the horizontal and vertical planes. Two mounts were machined with horizontal 8 and vertical v-grooves where 50 µm gold-tungsten wire was stretched and glued. The mount was anodized to insulate it from the gold-tungsten wire as well as the ablation chamber itself. As the laser irradiates the gold-tungsten wire electrons are freed via the photoelectric effect and a positive current flows through the wire which can be measured. The total current produced is dependent on the flux of UV light incident to the wire and therefore proportional to the local intensity of the beam. Passing the wire through the beam waist creates a series of pulses from the gold-tungsten wire that rise and fall in amplitude, the peak defining the coordinate of the laser pulse maxima for that particular position of L3 (which is mounted on a translation stage moving in the direction of the beam path called z). An integrating circuit was designed and constructed to measure the sum of current off the gold-tungsten wire. The scans are done in pairs of 2d projections: xz (called x-scans) and yz (called y-scans). Between the scans the wire frame is swapped out because there are separate frames for the vertical (x-scan) and horizontal (y-scan) wires. The 2d scans consist of an inner loop over the transverse coordinate, and an outer loop over z. The transverse coordinate range is 2 mm and the z coordinate range is 12 mm. Each pass has a single value of z, and sweeps over the full 2 mm range in x or y. The output of the integrating circuit is connected to an ADC which is sampling continuously over that the whole time period the scan is taking place.