Abstract. Parameters of high-speed ablation of ultradeep channels by controlled pulse trains from a single-mode phase- conjugate dynamic cavity Nd : YAG laser emitting 20 ë 200- ns, 70 ë 250-mJ pulses at a pulse repetition rate in a train of 40 ë 250 kHz are studied. The optimal parameters of ablation are found, for which a long-lived region of a hot rareéed gas was maintained in the ultradeep channel, which suppressed the screening action of the surface plasma. The control of the lasing process during ablation optimises not only the heating and plasma formation, but also the removal of the processed material in the pause between laser pulses. Adaptive regulation of lasing parameters during ablation made it possible to obtain ultradeep channels of length 8 ë 27 mm and diameters 80 ë 300 lm of the input and output holes in metals (aluminium, steel and Inconel 718 nickel superalloy) and ultrahard ceramics (Al 2 O 3 , AlN, SiC). Keywords: ablation, ultradeep channel, surface plasma, multipulse irradiation, controllable Q-switching. 1. Introduction High-speed laser drilling of deep channels with a diameter of tens of micrometers is of considerable interest for using in various éelds of science and engineering [1, 2]. The geometry of such channels is usually characterised by the aspect ratio, i.e., the ratio of their depth to the diameter. The highest aspect ratios ( 300  600) are traditionally obtained during laser processing of polymer materials [2], having bulk absorption and a very low breakdown threshold that does not exceed the threshold of the surface-plasma production, which makes it possible to avoid plasma screening of laser radiation during drilling. However, the effect of plasma screening becomes quite perceptible during laser processing (deep drilling) of structural materials (metals, ceramics, etc.), and the losses and channel broadening caused by the screening due to lateral expansion of plasma reduce the aspect ratio of the obtained channels to 40 [3]. The eféciency of laser ablation and the drilling rate depend strongly on the parameters of the screening plasma and are reduced considerably, as a rule. For deeper channels, the density and screening action of the plasma are enhanced further due to restrictions on its expansion [4]. A decrease in plasma screening was earlier achieved by using femtosecond laser pulses [5] or a combination of pico- and nanosecond laser pulses with a high pulse repetition rate [3]. In the latter case, an increase in the drilling rate and a decrease in screening can be due to the formation of a long- lived rareéed gas region in the vicinity of the irradiation spot [6]. However, the formation of ultradeep channels with an aspect ratio exceeding 100 and a depth exceeding 2 mm even by using pulse trains requires a high laser pulse energy, which is usually achieved by generating giant nanosecond pulses. To optimise the process and to eliminate screening, it is necessary to control the operation of a high-power Q- switched laser. It was shown in [7] that a special passive LiF : F  2 Q switch with a variable initial transmission can be used to control continuously the pulse energy, duration and repe- tition rate of a high-power single-mode Nd : YAG laser with a dynamic loop phase conjugation (PC) resonator over a wide range. Due to self-compensation of intracavity dis- tortions during self-PC, the laser generates high-power pulse trains with a small divergence close to the diffraction limit. The control of lasing parameters by scanning a passive Q switch with a variable initial transmission during laser matching permits high-speed drilling of channels of depth more than 10 mm for a hole diameter of about 100 mm in various structural materials [8]. In this study, we analyse the parameters of laser ablation of ultradeep channels in metals, alloys and ceramics produced by controllable multipulse radiation from a self-PC Nd : YAG laser with a dynamic resonator. The analysis was aimed at developing and implementing a new and highly efécient technique of laser perforation of deep and ultradeep (with an aspect ratio exceeding 100) micrometer holes (of diameter 20 ë 300 mm) in structural materials, including hard and refractory materials. This laser technology is quite promising for improving the gas-dynamic properties of gas turbines and aircrafts, for production of fuel injectors, prospective ceramic engines, etc. T.T. Basiev, S.V. Garnov, S.M. Klimentov, P.A. Pivovarov A.M. Prokhorov General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 119991 Russia; e-mail: basiev@lst.gpi.ru; A.V. Gavrilov, S.N. Smetanin, S.A. Solokhin, A.V. Fedin V.A. Degtyarev State Technological Academy, ul. Mayakovskogo 19, Kovrov, Vladimir region, 601910 Russia; e-mail: ssmetanin@bk.ru Received 14 May 2007 Kvantovaya Elektronika 37 (9) 956 ë 960 (2007) Translated by Ram Wadhwa PACSnumbers:52.38.Mf;42.55.Rz;42.62.Cf ³ÓÑÚÐÑ ÒÓÑÚËÕÂÕß, ËÔÒÓÂÄËÕß ÑÛËÃÍË Ë ÒÑÊÄÑÐËÕß Ä ÓÇÆÂÍÙËá (499) 135 13 11 ËÎË (495) 132 66 66. DOI:10.1070/QE2007v037n10ABEH00 High-speed ablation of ultradeep channels by a phase-conjugate dynamically controlled passively Q-switched Nd : YAG laser T.T. Basiev, S.V. Garnov, S.M. Klimentov, P.A. Pivovarov, A.V. Gavrilov, S.N. Smetanin, S.A. Solokhin, A.V. Fedin 431/367ë MBë 30/xi-07 ë SVERKA ë 5 ÒÑÎÑÔ ÍÑÏÒ. å 1 Quantum Electronics 37 (10) 956 ë 960 (2007) ß2007 Kvantovaya Elektronika and Turpion Ltd