Quantifying Cryocooler Contamination

Contamination of the cryocooler working gas (usually helium) with other species results in performance degradation if those species freeze at or below the cooler’s intended process temperapassages in the cold zones1,2. Fortunately, avoiding contamination is fairly straightforward: use clean parts and low outgassing materials, and vacuum-bake (or hot dwell-and-purge1) the assemblies to remove the remaining volatiles. Verifying cleanliness, however, may require many months of runtime to establish temperature stability. For low-cost mass production and rapid R&D cycles, it is very desirable to have a method for actually quantifying the level of contamination in a cryocooler model of outgassing that leads to a quantitative estimate of available contaminants in milligrams, based on the accumulated vapor-pressure and the RGA measurements on evacuated units. This hence yielding a pass-fail test for cleanliness. INTRODUCTION The issue of contamination of working gas in cryocoolers is widely recognized1,2, as are many studied, as well1. The measurement of contamination level in cryocoolers is not as well understood, however. In the cited studies, the contamination is deliberately introduced, so its level is at least approximately known before the start of the test. On the other hand, contamination coming from within the cooler itself is challenging to measure, because it is stored not in the working gas but on surfaces, in non-metallic or porous solids, and in any trapped volumes. Analyzing a sample of the helium from a cryocooler does not tell us the actual contaminant level of interest, because: 1. the cryocooler acts as its own cryopump, so as long as the cooler is running, it continually freezes out any contaminants in the working gas. 2. when the cryocooler is not running, the partial vapor pressure of any contaminant is limited by the balance of release vs. reabsorption. What we really want to know is the quantity of contamination that could potentially outgas over a long period of time, e.g., how much would outgas if we could continually keep the vapor pressure low. THEORY To begin, we start by making some simplifying assumptions. By far the most common and stubborn contaminant is water; occasionally there are other volatiles such as methanol from cur631 Cryocoolers 19, edited by S.D. Miller and R.G. Ross, Jr. ©¶International Cryocooler Conference, Inc., Boulder, CO, 2016 C19_056 2 ing epoxies, but we can take the analysis pretty far by assuming water as the only contaminant of interest (for now). There may be several simultaneous outgassing processes, each with its own characteristics; nonetheless, it is generally true that the release of contaminants slows down as the amount available is reduced. If we make the very simplifying assumption that the overall outgas rate is proportional to the amount of (unfrozen) contaminant in a cooler, we can write: where m'(t) = the mass of contaminant frozen in the cold zones at time t; mo= the total mass of contaminant in the cryocooler available for outgassing at time t = 0; and = a time constant, characteristic of a given size and type of cryocooler. In other words, the outgassing process is assumed to obey a relaxing exponential, with an asymptote at m0, and where m'(t) reaches 63% of m0 at time t = . The buildup of pressure in an initially evacuated vessel due to outgassing is fundamentally similar, except that the pressure saturates not because the available contaminant has been exhausted, but because the release and reabsorption of contaminant are in equilibrium. Nonetheless, there is an important connection between them: if (vapor) pressure is uniform in a cryocooler during an outgas test (a big if, we should note) then the initial rate of pressure buildup roughly corresponds to the rate of frozen mass accumulation that the cryocooler would experience if running. In other words, if the cryocooler is evacuated, then isolated from vacuum, the pressure will rise according to (2) where the relationship between m0 from Eq. (1) and p(t) is Here V is the cryocooler internal gas volume, R is the universal gas constant, TH is the temperature of the outgas test (which is chosen to equal the expected, or, to be conservative, the hottest expected operating temperature of the warm parts of the cryocooler), and M is the molecular weight of the dominant vapor (presumed to be water in most of our analysis). Finding m0 and Without knowing , we can’t get m0 directly from Eq. (3). However, can be deduced from a sequence of two outgas tests, where the crycooler is vacuum baked at the same temperature TH for a time t in-between the two tests. In this case, the two outgas tests are only run long enough to establish an initial slope, for a much shorter time than t. Using the familiar dot notation for timederivative, and the subscript i for “initial”, we can say that In principle, the initial rate of increase of vapor pressure and the ultimate saturation pressure est. This redundancy permits independent calculations for the same value, which can be compared for consistency. Disagreement between the calculations can lead to insights and ways to improve the method beyond the very simple version presented here (for instance, how to handle multiple outgassing processes with different time constants). For example, if there exists a bakeout temperature TB > TH that can accelerate the bakeout process (in other words, if the cryocooler can be baked out at a higher temperature than its expected operating temperature) then m0 can be deduced from a sequence of two outgas tests at TH bracketing a dwell (where the vapor pressure is allowed to build) at TB. Knowing the volume, the temperature, (1)