Feasibility of Using a High Power C 02 Laser as an Alternative Source to Test High Heat Load X - ray Optics

Feasibility of using a high power C02 laser as an alternative source to test high heat load x-ray optics The proposed Undulator A at the Advanced Photon Source (APS) will deliver peak power densities of 150 watts/mm 2 , with total power up to 3800 watts, incident on the first crystal of a double crystal monochromator. The XFO-OP group has undertaken an extensive research program to find efficient methods to dissipate the resulting high heat loads while preserving the diffracting characteristics of the first crystal. Testing of the cooled optics is an integral part of the research effort. Present synchrotron radiation sources can provide neither sufficient power nor power density to perform tests under conditions similar to the expected output of Undulator A. For example, prototypes for the cooled first crystal have been tested at CHESS [1] with 379 watts of power and a peak power density of 48 watts/mm 2 , and at NSLS [2] with 38 watts total power and 118 watts/mm 2. No synchrotron radiation source currently in operation can achieve the combination of total power and power density expected at the APS undulator beamlines. We have studied the possibility of using a high power carbon dioxide laser as an alternative heat load source to test cooled x-ray optics. The CO 2 laser at the Laser Applications Laboratory (LAL) at ANL has an output power from 800 to 6700 watts. The focusing optic can be selected to achieve different laser beam sizes; one of the available integrating lenses produces a 4 mm by 4 mm beam spot, with power densities as high as 400 watts/mm 2 , This laser has been successfully used for testing APS beamline front end components that will also be subject to large heat loads. The main question to be answered is whether the power deposition profile for the laser in silicon can be adjusted to resemble that of x-ray absorption. At the C02 laser wavelength of 10.6 microns, the absorption coefficient in silicon (N-type, resistivity p = 1 Qcm) is 0.2 cm-1 [3] (see Figure 1). In contrast, the 1/e absorption length for the undulator x-ray beam incident on the first crystal of the monochromator is approximately 20 cm-1 ; at typical diffraction angles (8B= 10-20 degrees), practically all of the x-ray absorption occurs in the top 1-mm layer of the silicon crystal. A meaningful test of the cooled x-ray optic requires …