335 Length Modulated Cell  (LMC)  Performance  The LMC  devices all show similar current excursions to the choppers, with subsequent recoveries to  nominal levels, as shown in Figure 5 . The current generally dropped over a period of 1- 2 years before  rising sharply back to the original current levels  for a brief period. This trend repeat ed in a slightly different  manner each time without any overall rise in current, so it can be  considered to be stable overall .  Choppers 1, 2 and 4 show some similar features  (Figure 3) . With the commonality of the LMC and  chopper designs, the settling behavior  is likely resulting from a change in friction of the retainer coupling  to the balls and races  (the choppers and LMC have the same bearing size and retainer, manufactured by  Draper Laboratories) . As the retainer wears or accumulates lubricant film, thresholds are reached where  the active site shifts to another ball location and proceeds to set tle again .  

         Figure 5.  LMC  Brushless -DC Motor Drive Currents  for a) LMCs 1&2, b) LMCs 3&4      

Optical cleanliness was one of the biggest design drivers for the LMC devices . In-orbit results  demonstrate that contamination control  was successfully managed as the long term radiometric gain  measurements showed no significant build- up of contamination on optical surfaces over the mission, with  the overall shift in gain being less than 1% in 16 years .  

Each of the Length Modulated Cells were fitted with a pressure transducer, using  high stability silicon -onsapphire sensor s (Sensotron) . Pressure readings for these cells were expected to drop gradually due to  gas leaks in the capillary bonded IR window seals (described previously in [4]), while  the gold and indium  plated Inconel seals around the largest perimeter of the gas cells c ould als o contribute. The long- term  trend for the pressure of LMC 3 and 4 showing the  expected pressure drop is presented in Figure 6 .    LMC  3 uses a molecular si eve to control the pressure and is showing a rate of pressure drop of about   1 kPa per year over the past few years . LMC  4 is a fixed pressure unit  (no sieve) , show ing a slightly more  rapid pressure decline since  2010 ( ~2.5 kPa per year) , most likely due to leakage via the aging epoxy in  the capillary bonds . LMC 1 lost about 10% of its pressure in the fir st 5 years . The LMC 2 pressure was  stable at about 75 kPa for one year and then showed the most dramatic loss with pressure reduced to   5 kPa by 2005. The capillary bond for the optical windows was a newly developed process for this  application, for which He leak testing, thermal cycling and ultrasonic inspection were performed to validate the engineering model hardware. The in- orbit results for two LMCs confirm that the bonding  method is capable of lasting for a long- duration mission, but that screening methods and aging factors  should be further considered (i.e. , radiation testing and life- testing of the process is recommended prior to  reuse) . Alternatively, a diffusion bonding process could be evaluated as an option.  

An anomaly was observed with S ieve 3 on 03 Oct 2009 (unrelated to the cooler anomaly)  due to the  heater for Sieve 3 turning off unexpectedly or failing. The heater can be activated to stabilize the pressure 2000 2002 2004 2006 2008 2010 2012 2014 20160.040.050.060.070.080.090.10.110.120.130.14      Time     red   LMC1 blue  LMC2 2000 2002 2004 2006 2008 2010 20120.040.050.060.070.080.090.10.110.120.130.14        Time      red     LMC3 blue    LMC4a) b) LMC Motor Current [A]