SOURCES, SURVEYS and SOLUTIONS
Curt R. Dunnam
Linear Research Associates, Trumansburg, NY
©Today Enterprises (Microscopy Today)
Part 1: Introduction
A decade of energetic debate regarding possible hazards of corporeal a.c. magnetic field exposure has at least alerted us to the fact that significant time-varying magnetic fields exist in most workplace environments. Although the personal risk from exposure to such alternating magnetic fields on the order of 1 to 20 milligauss is demonstrably small (if it exists at all), peak-to-peak field variations in this range can easily wreck havoc at electron microscopy sites, particularly when FEG's are involved. Complicating the a.c. magnetic field interference problem is the fact that environmental fields are seldom steady-state, instead changing frequently as electrical loads in a building are switched on and off. A major consequence is that the resulting EM interference symptoms are often intermittent and correspondingly difficult to diagnose and remedy. Even in the steady-state case, resolution loss caused by low-level a.c. magnetic fields is often mistakenly attributed to conducted vibration or some ill-defined instrument maladjustment. In other instances, subtle resolution loss can arise without the operator's cognizance when new equipment and/or power wiring is installed in the same building. Even nearby elevators can create significant magnetic interference by distorting the geomagnetic field during passage. Obviously, in any of these instances, correct initial diagnosis of time-varying magnetic fields as the interfering agent will save considerable time, effort and money.
In the next few issues of MT we will present a brief serialized primer in which common EM site time-varying fields and typical EM interference thresholds are described, and which will include practical tips on surveying and magnetic field attenuation. This series will be largely drawn from a half-day seminar which the author has presented before client organizations during the past year. Explanations of magnetic source and field phenomena will be on the level of "physics for non-majors", where physical causes and effects are presented without the benefit (or burden) of extensive analytical review. Nonetheless, this series should instill confidence in those readers who wish to conduct meaningful a.c. magnetic field surveys and correctly interpret the data collected. In the course of this series, we will illustrate various physical principles of magnetic sources and fields by examining several interesting, arguably bizarre, site interference examples drawn from actual survey and mitigation work.
Part 2: ACMF/EM Antipathy
A.C. magnetic field [ACMF] sources and their EM interaction mechanisms are discussed in this installment, which continues our July/August series on preventing and resolving site magnetic field problems. This article carries forward the theme of a primer on the basic physical causes and effects of a.c. magnetic fields, with special emphasis on the manner in which such fields may affect site EM instrumentation. Experience has demonstrated that a basic understanding of the various aspects of ACMF sources will often prove helpful in identifying the sources which actually pose a threat to EM site operations.
Like most space in commercial buildings, EM sites are subject to simultaneous ACMF's from many sources external and internal to the site building itself. External sources may include power transmission lines, substations and local stepdown transformers. The major internal sources are more numerous and varied, but typically include vaulted transformers, high-current primary busses, ballasted lighting, and motor-driven apparatus such as chillers, ventilation blower and air compressors. Common site-related ACMF sources are illustrated in Figure 1.
When estimating field influences at the EM location, it is useful to note that relatively high local fields from magnetic-core devices such as transformers and motors [BT] decrease most quickly with distance, fields from high-current distribution busses [BC] decrease less rapidly and fields due to so-called ground currents through conductive paths such as the building structure [BS] and metallic drain line(s) [BD] decrease the least rapidly of all (see Table 1).
Each alternating-current source produces a time-varying ACMF which can be characterized by peak strength, temporal waveform (which depends on harmonic content), orientation and magnitude-distance dependence. Typical site sources, ACMF values and dependence on radial distance "r" are summarized in Table 1. In perusing the Table, you will see that the worst actors for high-valued proximate fields are devices such as transformers and motors. Coincidentally, these sources exhibit the highest rate of field decrease with added distance. This turns out to be a very fortunate state of affairs for EM users!
Using Table 1, it is fairly easy to make a rough determination of which sources might or might not pose a likely threat to a particular EM site. Such work requires only a measurement of the average ACMF value near any given field generator at which point its contribution to the actual or proposed EM location ACMF may be estimated in a straightforward manner. This is accomplished by utilizing the distance dependence function for that type of source. For example, a transformer which produces a measured field of 100 milligauss [mG] at a distance of 1 m should produce only 0.5 mG near an EM located 6 m distant. Any field generator estimated to produce in excess 0.7 mGrms at the EM location should be considered a problem source. Ground current loops, which exist throughout most buildings and exhibit a 1/r dependence, commonly fall into this category.
|External Sources-Large||@ 15 m||@ 30 m|
|Internal Sources-Moderate||@ 5 m||@ 10 m|
|Internal Sources-Low||@ 1 m||@ 2 m|
At the EM, the resulting time-varying ACMF is determined by a vector summation over all individual source fields, although in practice only one or two source field will be dominant. It is this summation ACMF vector which directly influences the EM instrument's charged particle beam. To understand the effect of the time-varying magnetic field on the EM instrument, it is necessary to understand the nature of three significant ACMF-EM interaction mechanisms.
All ACMF interactions with an EM are described by the F=q[BXv] Lorentz force you heard a lot about in undergraduate physics. By far the most prevalent of the three mechanisms is EM transverse beam displacement. Beam motion of this type is imparted by the ACMF vector component which lies in a plane normal to the beam axis. For vertical-column EM's this imaginary surface is horizontal and is universally defined to be the xy plane. Most EM's (both transmission and scanning) arrive from the factory exhibiting threshold xy-plane ACMF values of around 1.0 mGrms. The actual threshold value of course depends on beam energy (EM susceptibility is inversely proportional to its square root , EM internal shielding, ACMF harmonic content and mean path length over which a normal ACMF component exerts an influence on the beam).
Less frequently seen in practice is the EM beam defocusing effect which may be caused by a strong ACMF z-axis component. This influence is usually ascribed to distortion of the EM objective lens fields. Because focusing sensitivity is approximately an order of magnitude lower than the previously described transverse effect, the transverse effect usually dominates. Z-axis ACMF defocusing thresholds range from 10 to 15 mGrms.
With the increasing use of field-emission (electron) guns [FEG's], a third ACMF interaction mechanism is more often observed which produces symptoms quite similar to the defocusing effect mentioned above. In such instances, FEG resolution loss may occur at isotropic ACMF threshold values in the vicinity of 1mGrms. The isotropic factor implies that both the emissive spot and subsequent electron trajectories are affected. ACMF components exceeding 0.5 mGrms in any axis should be avoided for optimum FEG employment.
In this article of the ACMF series the point has been made that measurable ACMF's exist throughout buildings with energized a.c. distribution systems. But the relevant question is, do these inevitable ACMF's exceed EM threshold limits at a given site? Armed with a basic knowledge of sources and EM interaction mechanisms, in the next article we will explore basic a.c. magnetic field survey data collection methods for meaningful site ACMF analysis.
Part 3: Survey Instrumentation and Methods
A.C. magnetic fields exist throughout every EM site. For a given site, then, how does one predict when EM interference problems are likely to occur? Obviously, an accurate, meaningful magnetic field survey is the first step toward evaluating site EM magnetic field compatibility. Consequently, this article in our series will describe measurement equipment and techniques for achieving accurate, unambiguous magnetic field characterization.
In the previous article [ Part II: "ACMF/EM Antipathy", MT, November '95], we examined several types of sources which produce alternating current magnetic fields [ACMF's]. In near proximity, or at sufficiently high power levels, any of these sources (e.g., transformers, power cabling, ground currents, and/or combinations thereof) can affect EM operation. In this article, we will review specific instrumentation and methods for accurate magnetic field survey data collection and interpretation, a necessary prior step for EM interference prediction and resolution.
Moving beyond conventional a.c. magnetic field survey equipment techniques, we will also briefly examine low-frequency [LF] survey methods. Modulation of the ambient geomagnetic field is often troublesome in urban or high-rise settings and, in many instances, it is the sole disruptive factor. LF magnetic field perturbations are due to the movement of trains, vehicular traffic and elevators or, less frequently, the proximity of high-power d.c. equipment. Because LF field variations caused by these sources do not register on standard a.c. magnetic survey apparatus, special equipment and measuring procedures are required.
Insofar as line-operated a.c. magnetic field sources are concerned, it should be obvious that an understanding of source ACMF characteristics can prove useful when planing a site. Buss trays and conduits, for example, can be routed away from a prospective EM location prior to actual construction. Similarly, power distribution transformers can be relocated, etc. (ref. previous article for applicable data). However, not every ACMF source can be anticipated. In practice, it is nearly impossible to identify significant ground-current-induced a.c. magnetic sources by inspection, even in the case of new construction. This difficulty stems mainly from the unpredictability of electrical and magnetic induction into, and resultant current paths through, building grounding systems and structural elements. Even in recently-constructed buildings where reasonable attention has been paid to routing of busses and location of distribution transformers, the unpredictability of ground currents always mandates one or more ACMF surveys prior to site certification.
Goals of an ACMF survey are twofold; first and most important is accurate determination of environmental field levels, including low-frequency geomagnetic variations if the site is situated in an urban or large-building setting. The second goal is laying a foundation for efficient resolution of any high-field problems which may be uncovered. Attaining the first goal is straightforward and may be accomplished, for line-frequency ACMF's, with sufficient accuracy by employing apparatus as simple as a hand-held gaussmeter. Achieving the second goal implies data-taking methods of adequate resolution to permit unambiguous identification of problem sources and, in some cases, identification of a usable alternate location within a site. Complete data records will also aid in determining the feasibility of active and/or passive shielding solutions.
Equipment and methodology are of course related. A tri-axial hand-held milligaussmeter [viz. Teslatronics Model 70] is the instrument of choice for a quick sweep of the proposed EM site (so-called "field coils" are a distinctly inferior alternative for this application due to their frequency sensitivity and directivity). Sweeping is an essential initial survey step in any formal site qualification procedure. A sweep implies smoothly working the hand-held instrument back and forth across the area to be surveyed, mentally noting high, low and average values for an imaginary plane at approximately 1 m (waist) height. Any anomalous local "hot spot" readings are noted for future reference and, using the same instrument, traced back to the source or sources. If a FEG installation is anticipated, the ceiling area at arm's reach should also be swept and the same recording and tracing procedure followed.
With the site ACMF's at least qualitatively understood and any problematic local sources identified, the survey engineer divides the room into four or six measurement quads using a small marker to identify the center of each quad and, next, prepares a survey form. Our standard survey form includes a sketch of the room with subdivision lines and a reference coordinate system, to which the survey engineer adds dimensions, important room details and notation describing known ACMF sources. Information indicating the type of measurement equipment used, range settings and signal weighting is also entered before continuing with the survey work.
For line-frequency [1.6 Hz to 1.6 KHz] ACMF data collection, we use either a single-axis (or selectable single-axis) hand-held gaussmeter or a tri-axial [Bartington MAG-03] probe-based fluxgate system such as illustrated in Figure 1. In either case, x-, y- and z-axis values are recorded for each quad and at the proposed EM column location for a 1.0 m height.
If the measurement equipment displays Bxy or Bxyz vector magnitudes these figures are also recorded; if not, they are calculated later. Generally, mid-morning or afternoon collection of ACMF values presents a reasonable assurance of capturing worst-case data. However, if it is suspected that ACMF values are changing appreciably over time, the more versatile fluxgate "a.c." system can be configured with an optional chart recorder. Time-log measurements should be taken either at the proposed EM column position or, alternatively, near the center of the room away from strong local sources.
A final series of low-frequency "d.c." measurements is required in urban or large-building environments. In these settings, 0.5 - 2.0 uT peak-to-peak (5 - 20 milligauss) magnetic variations covering a range of .001 to 1 Hz are inevitably present. Consequently, an a.c.-line fields survey is almost certain to yield an incomplete picture of possible magnetic field-EM interference. Fortunately, however, low-frequency measurements are straightforward and may be made with the same tri-axial fluxgate probe used in line-frequency ACMF surveys. Therefore, a large additional capital investment is not required. A high-accuracy, relatively inexpensive d.c. to 1.6 Hz fluxgate probe system based on this principle is illustrated in Figure 2. In this "d.c." survey configuration, lowpass filter and offset summer modules are used to define the frequency range
of interest and cancel out axial geomagnetic field components. With the fluxgate probe set in place at the room center and adjusted to 1.0 m height, system outputs representing two (observed) worst-case axes are routed to a dual-channel chart recorder. For a given site, a relatively high resolution (0.5 cm/min.) time-log is recorded for two hours during the business day followed by a 24 hour record at a somewhat lower (4 cm/hr.) rate. In subsequent analysis of the chart tracings, both short-term and diurnal information are relevant to EM site qualification.
In the next article we will discuss interpretation of a.c.-line and LF magnetic field survey data and the role of survey data analysis in resolution of excessive EM site ACMF's.
Part 4: Survey Data Analysis
Up to the present waypoint in this series on EM site magnetic fields, we have identified typical sources of time-varying magnetic field intensities, examined salient field characteristics and illustrated correct survey methods. Our goal this month is to analyze data collected at a proposed site and answer the key question of whether or not the candidate site is, as far as magnetic fields go, acceptable for EM use. In the process of analyzing the magnetic field survey data we will define some of the interpretive techniques involved and observe the distinction between localized (a.c. power) and non-localized (geomagnetic) time-varying fields. Finally, we will discuss the implications of EM susceptibility threshold vs. measured field ratios when considering remedial site shielding.
Unambiguous, accurate field survey data is required as a basis for EM site acceptability if analysis of that data is to be the final arbiter of whether a site can be "fixed" or must be abandoned. Interfering magnetic fields which fall below well-defined levels, for example, can often be adequately reduced by shielding the site. Since the alternative option of relocating a proposed (or operating) site may be costly in terms of physical, financial and political tradeoffs, it is obviously important to correctly analyze the magnetic field situation before a final decision is made.
Even at relatively low intensities, a.c. magnetic fields (and slower field variations, in the case of elevator or vehicular geomagnetic modulations) can interfere in subtle ways with EM operations, particularly at sites employing FEG, high resolution and/or low beam energy equipment. In the previous article of this series (Part III, "Survey Instrumentation and Methods", Microscopy Today, January/February '96) we reviewed survey data collecting instrumentation and methodology, and there made a careful distinction between magnetic fields related to a.c. power usage as being quite distinct from "quasi-d.c." fields attributable to vehicles moving through the earth's relatively static field. In the following example data set analyses we will accordingly analyze these two field categories separately.
Presuming an urban, large-building setting, we find ourselves armed with two sets of data magnetic field survey work performed in Part III of this series. Let us first consider the "a.c. magnetic field" [ACMF] survey data, Table 1, which is comprised of powerline-related magnetic field
values recorded at several points throughout the site. The room is rectangular with a bird's-eye aspect of approximately 3 to 2 and is divided into roughly square cells which are consecutively lettered A, B, C and D, E, F proceeding from left to right and top to bottom. As noted in the previous article, the tabulated values are recorded with the aid of a calibrated tri-axial fluxgate probe and have been verified by comparison with values observed on a hand-held teslameter.
ACMF's may be characterized in two ways. First, they possess interference components which range from 16 Hz (e.g., Scandinavian electrified trains) to 720 Hz (12 harmonic of 60 Hz). Also, significant third-harmonic magnetic field energy at 150/180 Hz is quite common. The second important characteristic of ACMF's is that they are frequently produced by local sources such as transformers, motors and video display monitors. Often, transformers and motors are hidden by walls and/or partitions and are only easily "seen" with the hand-held teslameter. Analysis of "hot spot" data noted during a sweep will indicate if a source is contributing in a significant way to magnetic fields at the proposed EM column location. If so,a marginal site may be salvaged by relocating the offending electrical apparatus further away from the EM site.
In the hypothetical room represented by the above table, peripheral equipment imposes strict limitations on instrument placement and we can locate the EM column only where the corners of cells A, B, C and D, or cells C, D, E and F touch. Let us assume initially that our EM is a standard non-FEG instrument, and exhibits a typical susceptibility threshold of 250 nTp-p [2.5 mGp-p] in the XY plane and 500+ nTp-p [5.0 mGp-p] in the Z (vertical) axis. Reviewing the above table figures, we observe that we are be on fairly safe ground as far as the environmental ACMF fields are concerned. If, on the other hand, we are expecting to operate a FEG-equipped or high-resolution or low eV instrument in that room, some type of shielding will be necessary. Under the BXYZ conditions listed in the final column of Table 1, and with a typical 150 nTp-p [1.5 mGp-p] isotropic interference threshold, such instruments would probably not deliver full resolution specs.
Next, we look at the "quasi-d.c." [QDC] data which we prudently collected (having previously noted that our building is full of exotic equipment and a subway line runs beneath it). As described in our last installment, the initial survey step for QDC fields is to fix a probe near the center of the EM room and manually note all axial field variations occurring in a frequency range of 0 to 1.6 Hz over a period of two minutes or more. That data is tabulated (Table 2) and immediately analyzed
|AXIS||LOW||HIGH||�, ABS||�, REL,||%|
to determine which two axes are to be monitored for a total period of at least 24 hours using a dual-channel chart recorder (ref. Part III). In this case, we have chosen axes Y and Z. With our completed chart (or charts) in hand, the statistics and magnitude of peak-to-peak low-frequency magnetic field variations occurring within any eight minute window (i.e., corresponding to -20dB with respect to the lowest frequency of interest) are carefully noted. Let us assume here that the relevant peak-to peak QDC variations discerned on the chart are 1.5 �Tp-p [15 mGp-p] in the Y axis and 2.5 �Tp-p [25 mGp-p] in the Z axis. It is readily apparent that these variations are over an order of magnitude greater than the ACMF EM threshold specs. Worse still, the EM conductive shrouds and UHV containment are relatively ineffective in blocking QDC field variations below 16 Hz, and the EM's exhibit up to 30% more sensitivity to magnetic field variations in this frequency range. All factors considered, our measurements indicate field modulations in the room are approximately 24 times greater than the interference threshold for a FEG instrument! From the standpoint of probable magnetic field interference, this site is clearly unacceptable in its present state for any of the previously mentioned instrument classes.
Nonetheless, since the observed magnetic field variation to EM susceptibility ratio is less than 25, the site may in fact be usable if magnetic shielding is employed. We will discuss that encouraging prospect in the next article of this series.
Part 5: Solutions!
This month's installment completes our primer series on magnetic fields by defining conditions where remedial EM site shielding is in fact technically feasible and by illustrating examples of both passive and active shielding methods for attaining lower site fields. For those readers who may have missed one or more previous articles, we have thus far presented some basic physics describing magnetic source fields (Microscopy Today, November, 1995), examined magnetic survey equipment and methods relevant to EM interference thresholds (MT, January, 1996), and, in Part IV ("Survey Data Analysis", MT, May, 1996), suggested techniques for interpretation of typical EM site survey data. Our continuing goal through this series is to educate the EM community about magnetic fields and how to measure, evaluate and cope with them. This final article completes the task with a look at technologies for reducing fields in situ.
When magnetic shielding is required to bring a site into specification, knowledge of ambient field sources, levels and frequency characteristics is essential in specifying cost-effective field reduction. This month, we will use ambient field information previously gathered at an example site as a basis for selecting and estimating costs of appropriate magnetic shielding for reduction of both localized and pervasive field sources.
First, it is useful to note that any source which generates a time-varying magnetic field within a frequency range of approximately 1 millihertz to 1 kilohertz at a magnitude exceeding 100 nanoteslas [1 milligauss] peak-to-peak may be considered a potential EM interference factor. Next, we need to segregate field sources into two types-those which are physically small to moderate-size and may be located in or very near the site, and those which are physically large and are situated relatively far from the site. Vaulted transformers, motors, video-display monitors and fluorescent lighting ballasts are examples of the former while large transmission lines, substations, pad-mounted transformers and major disturbers of the geomagnetic field (such as elevators and subway trains) are examples of the latter. For convenience, we will categorize the smaller, physically manageable sources as localized and the larger, incorrigible sources as pervasive (magnetic fields from the larger sources may be termed pervasive in the sense that their influence usually exceeds a building-sized volume).
Data available to us from our earlier Part III and IV survey activities will be used in preliminary source identification. Further, to better illustrate combined shielding techniques, let's add to our previous readings a large a.c. magnetic field [ACMF] contribution from a 20 KVA 480/208 stepdown transformer recently installed just down the hallway. As before, the EM room is divided into square cells A through F [corrected from Part IV to read sequentially A and B, C and D, E and F, from top to bottom, left to right; proposed EM column location at the junction of A, B, C and D]. Initially, we look at sweep data taken with a hand-held teslameter and observe that readings at the proposed EM column location are approximately 4500 nTp-p [45 mGp-p] and also that the readings drop off rapidly as one moves away from the far corner of cell B. This is clearly a high-gradient field and a sign of a nearby localized source, and it is in fact an easy task to "home in" on the offending transformer with the hand-held teslameter. Once the source has been located, we can readily determine remaining ACMF fields at the EM site by switching the transformer power off for a short interval (in the early morning hours, if necessary!) in order to complete our measurements.
Noting at this point that worst-case magnetic fields measured in the site are approximately 36 times greater than the susceptibility threshold for our proposed FEG instrument, one may well wonder if the site can be salvaged for EM work.
Fortunately, it can. To arrive at a good solution, however, we first need to consider the shielding options available and their relative costs. The two main categories of magnetic shielding-active and passive-may be used either separately or together as required. Passive shielding can be defined as surrounding a volume with sheet material of either high magnetic or electrical conductivity. At the relatively low frequencies which affect particle beam instruments such as EM's, however, it is most efficient to employ shielding material such as MuMetal® which exhibit high magnetic conductivity (i.e., permeability). Because passive shields may be constructed in a tiered fashion (Figure 1) using a combination of high saturation and high permeability mu-metal materials, they are particularly effective at shunting elevated field levels near
localized sources. Electronic, or active-feedback shielding, on the other hand, is most useful in reducing lower-level, low-gradient pervasive fields circulating from larger, distant sources. Its positive characteristics are relatively low cost per unit of shielded volume and physical unobtrusiveness, with tradeoffs of limited maximum field capability and attenuation coefficient. Active shielding is therefore a technique that works best for full-room shielding of affected instrumentation once strong local sources have been moved or passively shielded.
In our example EM site, we note from data analysis that the 4500 nTp-p ACMF reading is comprised of around 4300 nTp-p from the transformer source and 200 nTp-p of remaining low-gradient pervasive field. From our passive-shielding materials data sheet and close-in measurements of the transformer's leakage flux, along with supplemental engineering assistance from the shielding vendor, we find that a single-layer medium-permeability enclosure can be specified which provides a shielding factor of slightly greater than 120. This relatively high shielding coefficient reduces the transformer's contribution of time-varying magnetic field at the EM column to a negligible level. With this part of the solution in place, the remaining site magnetic fluctuations to be dealt with are two distinct lower magnitude contributions, the first at around 204 nTp-p in the "a.c." frequency range of 1.6 Hz - 1.0 KHz (mainly pervasive fields from neighborhood electrical power distribution equipment), and another at about 2900 nTp-p in the lower frequency range of .016 to 1.6 Hz (pervasive geomagnetic disturbances from elevators and vehicular traffic). To compensate these remaining fields, a 30 dB (attenuation factor of 32) active-feedback system covering a frequency range of .001Hz to 1.0 KHz is specified with an installed configuration similar to that of Figure 2. Once the equipment is in place and activated, we note that maximum residual fields across the EM site are reduced to slightly less than 100 nTp-p, or about 80% of the FEG instrument's susceptibility threshold.
Since the instrument manufacturer's published interference figure of 125 nTp-p is fairly conservative, a comfortable margin now exists between the compensated fields and the EM susceptibility threshold.
In sum, without too much effort and at a small percentage of the overall site cost, we have solved an atypical, worst-case interfering magnetic fields problem! In general, by applying a basic understanding of source characteristics, useful survey data and a few key technical resources, it is possible to remedy most EM site magnetic field interference problems quickly and at reasonable cost.
Assistance of The MuShield Company, Goffstown, NH (USA) 603-666-4433, in preparation of this article is gratefully acknowledged.
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