Experimental results on MEIRA characteristics and some typical operational problems

NUCLEAR

INSTRUMENTS

EXPERIMENTAL

AND

METHODS

RESULTS

TYPICAL OPERATIONAL

139

(1976) 57-63;

©

NORTH-HOLLAND

ON MEIRA CHARACTERISTICS

PUBLISHING

CO.

AND SOME

PROBLEMS

G. LEMPERT, I. C H A V E T and M. K A N T E R

Soreq Nuclear Research Centre, Yavne, Israel Various types of factors affect the deflection radius of a separator and consequently its mass calibration versus the magnetic field setting. In particular, the effects of the source magnetic field and of the shift of the effective boundaries with increasing field values are described in detail. Results of accurate measurements of the deflection radius for M E I R A are reported and interpreted. Measurements of the third order aperture aberrations arc in good agreement with the expected curve. The image curvatures for trajectories with the same ~ were found to be practically independent of the value of c~, thus achieving the main objective of tile M E I R A optical design. Three typical operational problems encountered during the trial runs of M E I R A and their solutions are described: lack o f space-charge neutralization of the ion beam, frequent high-voltage breakdowns in the source region, permanent shortcircuits between the source and the electrode under certain conditions.

1. Introduction

The design of the M E I R A isotope separator has been described in detail at the previous conference1). After its assembly was completed, various measurements were undertaken concerning the ion source characteristics, beam properties and optical features of the machine in order to be able to choose the correct operating conditions to obtain the best performance in each separation. Some of the results concerning the beam and the ion source are described in other contributions 2, a) to this conference. In what follows, the results of measurements of the effective deflection radius and the image aberrations are reported. Also, some typical operational problems encountered during the trial runs of ME1RA are described, as well as the solutions found for each of them. 2. Effective deflection radius

The effective deflection radius R is an important optical characteristic which is linked to the correct magnetic field setting for any given mass or what is commonly called the mass calibration of the separator. Usually, the reproducibility and accuracy of this calibration is not satisfactory. Some measurements were made on M E I R A to elucidate the nature of the various factors affecting the value of R. Three types of such factors may be distinguished: - Factors determining a constant modification of the radius value. These consist in construction tolerances and errors relative to the design values, and their effect on R can be easily calculated. They will not be further discussed. - Operational factors affecting the radius for a given

mass. It has been commonly found that the correct field setting for a given mass varies from day to day due to changed operational conditions. Factors affecting the radius according to the mass value and which are obviously linked to the field intensity and to saturation phenomena. 2.1. OPERATIONAL FACTORS

2.1. l. Magnet history A m o n g the operational factors, the most unpredictable and disturbing one is the magnet history. a) Short-term history. When the magnet is set for a new mass value, it takes some time for it to stabilize, after which the final setting can be established. For M E I R A , which has a field-regulated magnet, one needs to wait at least 10-15 rain until the drift at the required field setting is reduced to a negligible level, that is: to less than 10-5 T. b) Long-term history. However, even after the magnet has stabilized, the setting may differ from that of another day, if the magnet history has been different, obviously because of hysteresis phenomena. Significant hysteresis effects were expected for M E I R A because of the squat-e-cornered boundaries of the magnet pole-pieces where the local field is much higher than in the main gap. Table 1 shows some setting differences noted between field adjustments starting from rest position and from maximum value. In the worst case a difference of 0.28 m T was obtained indicating a relative radius variation of 1.2x 10 .3 or 6.5% in the dispersion per unit mass difference. The effect is not very serious, except for very accurate mass calibration. It was found empirically that the effect can be minimiIII. I S O T O P E S E P A R A T O R S

58

G. L E M P E R T et al.

TABLE 1 Effects of the long-term history of the magnet on the field setting%

M

12 28 129

B

AB

(naT)

(mT)

157 240 518

0.14 0,28 0.10

AB

__ = B

AR

-__ R

0.9 1.2 0.2

x 10 - 3

% of D ( A M = I )

Fig. 1. Effect of incorrect orientation of the beam in the xy or horizontal plane on the position of the virtual object. Steering of the beam is achieved by translation of the electrode along y or its rotation around z (R=).

2.1 6.5 4.9

" Examples of differences (AB) in the field setting (B) for a given mass (M) between adjustments o f the field starting from its rest position and f r o m its m a x i m u m value. The corresponding variations of the relative deflection radius AR/R and image shifts expressed as fractions of the unit dispersion (D) are shown in the two last columns.

zed when the magnet history includes an overnight rest after which the magnetic field is raised to the desired value only by increasing steps. 2.1.2. Position of source and collector slits Lack of precision in the positioning of the source and collector slits is an important operational factor affecting the deflection radius. The relative variation of R is obviously given by relation (1) for an error Ay" in the collector position and by relation (2) for an error Ay' in the source position, where D is the dispersion and G,, the radial magnification.

AR

1 Ay"

R

2 D

. . . . .

AR

1 Ay'

- -

R=2D

,

x

(1)

G~.

Iy

(2)

2.1.3. Electrode movements Variations of electrode position and orientation as required by the operating conditions may also affect the effective deflection radius. The beam is usually centered at the magnet entrance by rotation of the electrode around the z-axis or by its translation along y (fig. 1). Regardless of which steering method is used, lack of reproducibility in the centering of the beam at the magnet entrance leads to variations in the virtual object position in the y direction and consequently in the effective deflection radius, even if the aperture aberrations are perfectly corrected. Knowing the focal power of the electrode lens, the magnitude of the effect due to electrode translation can be easily calculated. For a typical case, it was found that a lateral electrode shift of 1 ram, yielding a beam

deviation of 15 mrad, leads to a relative radius variation of 2.2 x 10 . 4 with fair agreement with the calculated value. Concerning the electrode distance, it was found that, provided the beam is correctly centered, the longitudinal position of the electrode (along x) has no significant effect on the radius. In short, the variable effects of this factor can be practically eliminated by centering the beam correctly.

2.1.4. Source magnetic fieM Unlike the previous operational factors which can be minimized by appropriate procedures as described, the source magnetic field cannot be eliminated except for some special sources. Fortunately, its effect on the main field setting can be taken into account with sufficient accuracy, as will be shown. The source magnetic field extends a long way outside the source and, if it has the same direction as the main field (which is the case for MEIRA), the erects will be added and the required main field setting will be lower. For the reverse case, the effect will be reversed. Results of measurements show that this decrement of the field setting B is linear with the source field intensity B~ (fig. 2). The field setting can thus be easily extrapolated to zero source field and the decrement AB related

44.9

\\

44,8

--•44.7 E

44.6

44.5 4 4.4 0

'

10

'

2o

B s (m~esla)

Fig. 2. Variation o f the main field setting B with the source field intensity Bs. The measurements were made with a hydrogen beam centered in the magnet entrance after each variation of Bs.

MEIRA CH ARACT E RIS T ICS

0o3 ]

59

I0

0.8

I

2

5

I0

2 g

15

M

m

Fig. 3. Ratio of field setting decrement AB to source field intensity B~ as a function of the mass M o f the ions analyzed.

to this value. Even more interesting is the fact that the slope of this curve for a given value of the source, or the ratio AB/B~, is practically independent of the mass, tha{ is, of the main field value, as shown by measurements taken for masses between 1 and 14 (fig. 3). Indeed, it can be shown that this ratio AB/B, is independent of the field value (and therefore mass) or of the high voltage, and is given by the simple relation AB = I__~_~GZ~ B~ R2 D '

(3)

where Is is a characteristic integral of the source field normalized to the main radius R. The last ratio of eq. (3) is characteristic for each separator 4) and, in our case, is given by the expression: Gr

2 / ( E sin ~b + 1 - c o s qb)

(4)

D where L' and q~ are defined as in ref. 1. The characteristic integral I~ is defined as follows: I~ =

~XxsI-IB~(=~ x dx,

(5)

EL Bs(0) where Bs(x) is the source field intensity measured in the median plane, in the z direction, at a distance x from the source field axis; XEL and Xsn are the distances from this axis of the electrode and the magnetic shield, respectively, at the magnet entrance boundary. The relative value of the source magnetic field for MEIRA was measured as a function of the distance x and is shown in fig. 4. Because of the factor x, I s is very sensitive to the distant tail of the source field curve. We have also calculated the values of/s and the corresponding ratios AB/B s for three values of the integral upper limit (fig. 4). The experimental value of 0.0219 for AB/B~ may be considered to be in good agreement with the calculations, taking into account the partial shielding of the curve tail by structural iron components.

0.6

0.4

I s = 0.025

0.028

0.050 m2

0.0245

0.0260

1

0.2

0.2

O~

0.6

08

I-[.0

~/

1.4

x[m)

Fig. 4. Relative value of the source magnetic field B~ as a function of the distance x from the field axis for the M E I R A source magnet (pole pieces diam. 0.10 m; gap 0.34 m). Calculated values of the characteristic integral Is and of the ratio AB/B~ are shown for the distances x = 0.6, 1.0 and 1.4 m = Xsa.

2.2. FACTORSDEPENDINGON B (OR M) The factors depending on the main field intensity (or on the mass) involve essentially the fringe-field characteristic integrals, r/*, I~ and Sin, as defined by Wollnik and EwaldS). From the beginning, during the magnet design the values of these integrals must be estimated; this estimation may more or less approach the actual values of the completed magnet. What is more important, these values depend strongly on the field intensity because of saturation phenomena, especially with our square-cornered poles. Any increment in q*, which defines the effective field boundary, will require an adequate increase in the deflection radius. It can be shown that this increment dR is given by the following relation assuming that Arl* is the same at the magnet entrance and exit,

AR = Arl* - -E'

2 D cos s"

+ -Gr]~ cos e' '

(6)

where L', L", e', e" are defined as in ref. 1. On the other hand, the integrals I~ and S~ affect the exact positions of object and image as defined by the "corrected ideal trajectory" according to Wollnik and EwaldS). Any variations in I~ and Sm will modify R in the same way as errors in positioning the object and image discussed previously. 2.3. EXPERIMENTALMEASUREMENTOF R Taking into account all the operational factors described, the deflection radius was measured as a III. I S O T O P E S E P A R A T O R S

60

G. LEMPERT et al.

function of mass with the results shown in fig. 5. The curve is remarkably smooth, in agreement with an estimated precision of 2 x 10-4 in the measurement of R. The only exception is a group of three points displaced upwards slightly. This discrepancy is probably due to a brief power failure which occurred shortly before these points were measured, thus disturbing the magnet history. The estimated values for ~/*, I, and S~ in our design were 0.33, 0.i6 and 0.08 in units of gap width• The actual values measured directly on the magnet before assembly were at 0.8 T: 0.35, 0.18 and 0.23 respectively. On the basis of these actual values, the corrections for the design radius of 625 m m were calculated and yielded an effective deflection radius of 631.6 mm, compared to the value 630.3 taken from the experimental graph (fig. 5), the discrepancy is 1.3 m m or 2/1000 of the radius. This relatively small discrepancy indicates that the most important factors affecting the value of R have indeed been considered. 2.4. CONCLUSION Summarizing, it may be said that, provided the correct procedures are adopted, sufficiently accurate measurements of R can be made to establish a reliable mass calibration of the machine under varying operating conditions. A m o n g the factors considered, the most disturbing is the magnet history which, in fact, determines the practical limit of the calibration accuracy.

3. Optical aberrations

3.1. APERTURE ABERRATIONS The aperture aberrations of M E I R A , that is aberrations due to terms homogeneous in c~ (~ being the angle between the trajectory and the plane xz), were measured by isolating narrow beams (12 mrad wide) with an appropriate diaphragm at the magnet entrance for c~ values f r o m - 8 0 to + 1 0 0 m r a d . The results obtained with argon under different conditions show, in general, good agreement with the expected third order aberration curve shown in fig. 6, calculated according to Ludwig6). This indicates that for a wellshielded magnet, the effective boundary follows closely the profile of the mechanical boundary since, in our case the second order was corrected and the third order was not corrected at all1). At the time of the design, because of bad previous experience with other magnets we did not expect the effective boundary to follow so closely the shape of the mechanical boundary. We, therefore, thought that profiling this boundary to the third order would be a useless refinement. I f the mechanical boundary had been realized according to the calculated third order profile, we would have been spared most of the shimming which has to be done now. Looldng at the details of fig. 6, we see that the aberrations measured on different days do not differ significantly, although the focus distance was found to vary somewhat. Ay (mm) 4

642 • .~_-._--" experimental data ~

\

2

theo~ehcal

cur've

658

-0.08 'CC," 634

-2

k -<.~

'k\ x

't "

t

630

2

I

5

I

I0

f

20

I

50

I

I

lO0

-4

P

\i 1

200

M i

0.0 5

' O.I

O.'2 B (tesla)

d .5

' 0.8

Fig. 5. Effective deflection radius R as a function of the mass M or of the main field intensity B.

Fig. 6. Aperture aberrations Ay of the MEIRA magnet for argon. • O Measurements taken on two different days with the magnet field value set from zero. + Measurements taken with the magnet field set from its maximum value. The theoretical curve represents the calculated third-order aperture aberration.

61

MEIRA C H A R A C T E R I S T I C S

However, with a purposely disturbed magnet history, we obtained small variations in the aberration results, amounting to about 1/4 of a ram. For higher field values, the empirical aberration curve was found to vary slightly, as shown by results obtained for argon, xenon and bismuth (fig. 7). We will probably need different sets of shims for different mass ranges as was done at OrsayT). All these results were obtained with a very weak beam current (a few pA) in order to isolate the magnet aberrations from space charge and extraction effects. Recent measurements made with a 20 mA argon beam did not show significant variation from the data presented in fig. 6. 3.2. IMAGE CURVATURE ABERRATIONS

Obviously, correcting the aperture or median plane aberration does not guarantee a perfect image over its whole height. Consider a curtain of rays with an inclination a, that is a group of rays having the same c~value (fig. 8). Assume, for the time being, that all the trajectories intersect at a single point at the magnet entrance boundary (fig. 8a), that is A]~ = 0, where +_Aft is the angular dispersion in the vertical plane of the trajectories emitted from a single point on the source slit (fig. 8b). The trajectories of this curtain will then form a sharp curved image characterized by its distance d from the axis z" and by its curvature indicated in the figure by the arrow a for a given image length. Correcting the aperture aberrations will make the images of all such curtains coincide only at their midheight by eliminating all variations in d. However, the individual images for various values of c~will not fully

coincide unless their curvatures also are independent of ~. At the previous Marburg Conference, we explained that this was indeed the main objective of the optical design of M E I R A as expressed by eq. (3) of ref. 8. We have measured the curvature arrows for a span 70 mm high, for curtains with e values form - 8 0 to + 80 mrad. The results (fig. 9) show a probable maximum variation of less than 0.1 mm which is just detectable considering that the estimated measurement precision is 0.05 ram. Obviously, by correcting the median-plane aberrations by appropriate shimming, the individual images will coincide over their whole length. It must be noted that the average experimental value obtained for the image arrow for a 70 mm span, namely - 0 . 6 6 mm (fig. 9) is significantly higher than the calculated value: - 0 . 2 3 ram. The reason for this discrepancy is not yet known. 3.3. ABERRATIONSDUE TO IMPERFECTCROSSING Returning to the curtain of rays (fig. 8), there is actually a small angular dispersion +Aft in the vertical plane for each trajectory which may lead to some widening of the image, unless the magnet parameters •

Z"

[magnet :

,Z

I-A

.

ct

Ay (mrn) 4

Fig. 8. (a) Image obtained by a curtain of rays with inclination crossing perfectly in the vertical direction at the magnet entrance boundary. (b) A small angular dispersion Aft in the vertical plane may lead to a slight widening of the image line.

__argon

\

+ ....... b i s m u t h

~

~'%~I,

,

a(rnm) 2

. . . . .

~\_~>.~

-0.08

a(rad)

0.08

I

-0.08

-2

i

I

i

,

T

,

i 0.08

e(rad)

-4

Fig. 7. Experimental aperture aberrations values.

Ay for different field

Fig. 9. Curvature arrow (a) measured for a 70 mm span of the image trace of individual curtains of rays (fig. 8) as a function of the inclination ~ of the curtain. l l I . ISOTOPE SEPARATORS

62

G. L E M P E R T

have been so chosen as to avoid this type of aberration, as expressed by eq. (4) of ref. 8. Preliminary measurements were made with a sub-milliampere beam and a vertical diaphragm aperture of 1 2 m m (Aft= __3.6 mrad), comparing the image width at a z coordinate of _+35 m m with the image width in the median plane. N o difference could be detected between the peakwidths at 1/2 height, but there was a probable increase of 0.05 m m in the peak width at 1/10 height. The width itself was 0.40 m m at 1/2 height and 0.67 at 1/10 height for an emission slit width of 1.0 m m and collector slit width of 0.2 mm. The magnitude of this aberration is therefore negligible as expected. 3.4. CONCLUSION Values obtained for the aberrations, due to variations of image curvature and to imperfect crossing, are of the order of 0.1 m m or less (for a span of 70 mm). Also, the experimental third order aperture aberration follows the theoretical curve closely. When this aberration is reduced by proper shimming to the same order of magnitude as the preceeding ones, it will be confirmed that a single optical component, namely a magnetic prism can yield images practically free of aberration terms to the third order, including heterogeneous, as well as homogeneous terms.

et al.

when the barrier electrode was at a relatively high negative potential which was more than sufficient according to rubber membrane analog measurements. After trying many unsuccessful modifications of the electrode structure it was possible to obtain a neutralized beam only by enclosing it in a metallic box between the electrode and the partition separating the source chamber from the main separator enclosure (fig. 10b). Meunier, from Orsay, pointed out that this difficulty may be due to the small electrode width allowing some of the acceleration field to overflow and extract electrons from the beam. On the basis of this assumption, we replaced the box by large wings on both sides of the electrode (fig. 10c) which was indeed very successful. Thus we could eliminate the box which prevented the free pumping of the region behind the electrode as provided by the original design. 4.2. SPARKING IN THE SOURCE REGION Another problem was the frequent sparking in the source region at expected or very unexpected locations. It was found that the frequency of these discharges increases with beam current intensity, source magnetic field and electrode distance.

)orts

4. Typical operational problems

chamber

4.1. BEAM NEUTRALIZATION The first important problem encountered was the impossibility of obtaining a neutralized beam, even

electrode electrode Gm

:I

II

l rr__~ S/

_

,

:I

-~F_~r rC x,,!~.. ', II

Ial section

\

C~'

b

C

Yig. 10. Schematic horizontal view o f the electrode region: S, ion source; E~, barrier electrode; Eg, g r o u n d electrode; P, partition between source c h a m b e r a n d m a i n separator enclosure; V, v a c u u m p u m p s ports. (a) Original design: the b e a m is n o t neutralized despite the large negative potential on Eb (-- 1 kV) c o m p a r e d to the source potential (40 kV). (b) First empirical solution: the b e a m is enclosed in a box B, split to permit the variation o f the electrode distance. T h e b e a m is neutralized even w h e n the Eb potential is less t h a n --500 V. (c) Final solution: the electrode width is increased by lateral wings W shielding the b e a m f r o m the acceleration field. T h e free p u m p i n g o f the region behind the electrode is restored.

section

Yig. 11. Schematic d i a g r a m o f the source c h a m b e r and its internal c o m p o n e n t s showing the m o s t i m p o r t a n t modifications achieved to reduce sparking. These modifications, identified by figures, are explained in the text.

MEIRA C H A R A C T E R I S T I C S

Numerous modifications were made in the source chamber to reduce or eliminate the geometrical factors enhancing these discharges. The most important of these modifications are illustrated in fig. 11 showing the source vacuum chamber in a horizontal and vertical section. We followed three obvious guidelines: 1) Avoid concentration of magnetic fields. Therefore, the magnetic poles (4) inside the source chamber were eliminated. 2) Avoid concentration of electrostatic fields. Thus, as much as possible, corners were rounded on the electrode (2) and the vacuum chamber (8) and the source was enclosed in a smooth envelope (3). 3) Reduce the length of electrostatic lines of force as much as possible. Thus, wings (l) and grids (7) were added. This reduction in the length of electrostatic lines of force is especially important along the magnetic lines of force. For that reason plates (6) and screens (5) were installed. No screens were placed at the lateral sides of the beam to avoid interfering with the pumping of the beam region. A fourth important principle is the need to thoroughly outgas the source and electrode before operation. At M E I R A we do it by reducing the electrode distance to a few m m and heating the arc chamber to about 1000°C for 1-2 h. With the improvements described, the occurrence of momentary discharges is maintained at a satisfactorily low level. 4.3. SHORT-CIRCUITS Discharges of another type, quite different from those described above, occur under certain conditions and consist of virtual short-circuits of the hv supply. These can be stopped only by shutting down completely the arc current in the source. These discharges may be triggered by random sparking of the previous type or when the sides of the beam strike the electrode, provided that the ion beam current is above some difinite threshold. This threshold decreases, that is the phenomenon is aggravated, with decreasing source field, decreasing electrode distance and increasing emission slit aperture angle. Typically, the threshold may be 60 m A for a source field of 10 m T and an aperture angle of 70 ° and can be made to rise above 90 m A for a source field of 15 m T and an aperture angle of 60 °. These phenomena may be explained as follows. In

63

series with our hv power supply is a thermionic diode 9) which limits the current. The diode itself shares an important fraction of the voltage, reducing the potential of the source. This voltage drop increases the beam divergence momentarily. I f conditions are such that the new momentary beam width strikes the electrode, secondary electrons may be produced which multiply the current value. I f this new value of current exceeds the diode limit, the short-circuit may be locked. A simple proof of this mechanism is the fact that by modifying the cathode heating current, that is its current limit, the threshold beam current for the phenomenon is also modified. The multiplication factor between source and electrode will depend of course on the electrode distance, initial total divergence and source magnetic field if the multiplication mechanism involves also secondary ions. This multiplication ,factor was measured at different conditions and was found to reach values as high as 10 for extreme conditions. The adverse effect of a wide slit angle may be due to the large value of the parasitic divergence produced2). This is the reason we prefer to use a 60 ° aperture angle for the emission slit instead of a 70 ° aperture angle or the true Pierce-shaped profile. After solving these and other less important operational problems and after achieving a better understanding of the separator operation, as described in this and other contributions 2' 3), we are planning to start true production runs. There is no doubt however that much remains to be done to measure and improve the machine performance. References ~) I. Chavet, M. Kanter, I. Levy and H.Z. Sar-El, Proc. 8th Int. EMIS Conf., Sk6vde, Sweden (1973) (eds. G. Andersson and G. Holm6n; Chalmers University of Technology, G6teborg, Sweden) p. 191. 2) I. Chavet, M. Kanter and M. Menat, these proceedings, p. 47. 3) G. Lempert and I. Chavet, these proceedings, p. 7. 4) I. Chavet, Nucl. Instr. and Meth. 45 (1966) 340. s) H. Wollnik and H. Ewald, Nucl. Instr. and Meth. 36 (1965) 93. 6) R. Ludwig, Z. Naturforsch. 22a (1967) 553. 7) j. Camplan and R. Meunier, Nucl. Instr. and Meth. 57 (1967) 252. 8) I. Chavet, Proc. Int. Conf. on Electromagnetic isotope separators and the techniques o f their applications, Marburg, Germany, (1970) (eds. H. Wagner and W. Walcher, Physikalisches Institut tier Universitfit Marburg) p. 366. 9) M .Kanter and I. Chavet, these proceedings, p. 69.

III. ISOTOPE SEPARATORS