RFQ sparking experiments

220

Nuclear Instruments and Methods in Physics Research A278 (1989) 220-223 North-Holland, Amsterdam

RFQ SPARKING EXPERIMENTS A. GERHARD'), J. BRUTSCHER 2), H. KLEIN 2), M. KURZ

2)

and A. SCHEMPP

2)

1 ) GSI, Postfach 110 552, D-6100 Darmstadt FRG zi Institut für Angewandte Physik der Johann Wolfgang Goethe Universität, Robert Mayer Str. 2-4, D-6000 Frankfurt am Main 1, FRG

Beam currents in heavy-ion RFQs are strongly dependent on the maximum accelerating and focusing field strength . The maximum beam current intensity in an RFQ injector is directly proportional to the maximum applicable voltage. On the other hand this voltage is limited by sparking . Therefore sparking experiments are essential to determine practical voltage limits, especially in cw operation. The rf breakdown limit, for instance, depends on electrode geometry, gap width, electrode material and duty cycle and is not accurately predictable. The influence of the pulse length and duty cycle has been investigated in RFQ structures. The experimentals results are presented. 1. Introduction In RFQs the rf fields are used both for focusing and acceleration ; therefore high fields well above the Kilpatrick criterion [11 are required in many cases. To ensure reliable operation with these high fields, sparking experiments have been done . For a more precise measurement of the gap voltage a computer-based X-ray detector system has been used, so the maximum gap voltage can be measured independently of the cavity parametes, e.g . Q-value. The influence of the electron current [2] between the electrodes on the breakdown limit has been investigated in addition . As resonators quarter-wave coaxial structures and four-rod RFQs are used at resonance frequencies of

26 .7, 108 .4 and 216 MHz. The 108.5 MHz resonator is a normal quarter-wave coaxial structure [3], whereas for 216 MHz [4] a double quarter-wave coaxial resonator was built. The four-rod RFQ operating at 108.5 MHz [5] is shown in fig. 1 . The RFQ for 26 .7 MHz is a four-rod spiral RFQ [6]. ?. Experimental setup In all experiments the electrodes were made of standard industrial OFHC copper. The electrodes of the quarter-wave coaxial structures have been polished . In all investigations the gap voltage was measured via the bremsstrahlung spectrum . The number of breakdowns of the rf amplitude during the rf pulses was counted automatically . The value of the breakdown voltage is defined arbitrarily by a spark rate of one per rf-minute. The rf time can easily be calculated from the product of operating time and duty cycle. The vacuum pressure varied during operation from 1 x 10 -7 to 2 x 10 -6 hPa. 3. Experimental results

Fig. 1 . Scheme of the 108.5 MHz RFQ; all dimensions in cm, gap 6.5 mm, ro = 6.5 mm, rod diameter 12 .7 mm . 0168-9002/89/$03 .50 O Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)

After construction, tuning and testing at low field level at Chalk River [5], the 108.5 MHz four-rod RFQ was conditioned at Frankfurt, starting at low field levels and increased carefully up to the breakdown limit. The R Pvalue of 408 kQ was determined by the measurements of the gap voltage via bremstrahlung spectra, and rf power by a power meter. In fig. 2 the breakdown voltage is plotted versus duty cycle . The measurements have been done with a constant pulse length of 1 ms and an increasing repetition frequency, starting at 20 Hz . In a certain range between

22 1

A. Gerhard et al. / RFQ sparking experiments UB

.

[kVl 180

160

r IMMENNEEMEN

2.2 Kp 00

140 x 120

x

1 .7 Kp

x

x x

x

x

100

P

80

x

60 40

100 dc

2 45 10 20 50 [%1 3DIV

IWENEEMEEMN NEEMMMEMEM MMMMMMMMMM REGO

A:

B:

1 V

2 V

+ +

OmV T: 200ns SNG DC OmV D :\ B

Fig. 2. Breakdown voltage versus duty cycle for the 108.5 MHz RFQ; pulse length 1 ms, before (" ) and after (X) mechanical changes, 1 Kilpatrick = 77 kV .

Fig. 3. Rf signal of the pickup loop (A) and trigger signal (B) of the oscilloscope during sparking ; f«, = 26 .7 MHz, gap width 3.5 mm, pulse length 1 ms, pf = 30 Hz .

4% and 50% duty cycle the breakdown voltage shows an exponential decrease . This decrease could be measured with electrodes formed as a disk [7], too, using the quarterwave coaxial resonator at 108.5 MHz. Using the original RFQ, breakdown measurements could be done up to 8.5% duty cycle. For greater duty cycle the RFQ showed a thermal instability. Therefore the rods were fixed with four rings, two on every side near the end of the rods . But surprisingly the breakdown voltage is about 25% lower for the stabilized RFQ. A possible explanation for this behaviour may be that the surface was rougher than the original one, because after brazing of the rings the RFQ was treated by a sand blasting . Therefore the tests will be repeated with a polished surface. First sparking tests have been done with a 27 MHz spiral four-rod RFQ [6] also with a pulse length of 1 ms and repetition frequencies of 30 and 150 Hz . Table 1 shows the results. Fig. 3 shows the breakdown of the rf field in the cavity measured by the pickup loop . The average duration of the breakdown is similar to the measurements in other cavities [4,81. The gap voltage collapses within 200 ns, i.e . 5 rf cycles . Fig. 4 shows a photograph of the electrode surface after sparking . The

photograph was taken by a light microscope . On the surface are grooves caused by twisting off on a lathe and craters caused by sparking . The grooves have a dimension about 5 p.m . The craters have diameters from 10 up to 300 [Lm and estimated depths up to several ltm. A part of the electrode surface shows an accumulation of small craters (< 2 p m) . These craters could be generated by sparks with lower energy . After surface finishing before sparking, experiments have been done : grooves on this electrode surface were in the order of 30 wm . These electrodes were carefully conditioned in pulsed (pulse length 10 ms, repetition rate 100 Hz) and cw power operation for 5 hours. The surface of these electrodes represents the influence of conditioning very

Table 1 Breakdown and Kilpatrick voltage for a 27 MHz RFQ Pulse length [ms]

Duty cycle [%]

UKApa«,ck

[kV]

UB [kV]

1 1

3 15

44 44

138 114

Fig. 4. Photograph of an OFHC copper surface after sparking; diameter of the grooves - 5 Wm, diameter of the crater - 300 [Lm. IV . ACCELERATOR RESEARCH

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A. Gerhard et at. / RFQ sparking experiments I tE)

elf

eU B ~-- AE---{

0' E

Fig. 6. Bremsstrahlung spectrum by Kuhlenkampf [9]. (1 X 10 -9 V-1 ), Z the atomic number, i the electron current, e the electron charge, UB the gap voltage and E the energy of the photons. The intensity IF in the

Fig. 5. Microphotograph of a crater, with an inner and outer region ; diameter - 300 ~tm.

well and demonstrates the distinction between a conditioning effect and sparking . During the conditioning the electrode surface was demolished like an "electropolishing" effect, while sparking damaged the surface. A microphotograph of a crater (fig . 5) shows an inner and an outer region [4] of the crater . The inner region is marked by an irregular honeycombed structure which could be caused by an impact of a microparticle. These impacts lead to the melting and emission of the electrode material . In other tests the influence of the electron current between the electrodes on the breakdown limit has been investigated. The bremsstrahlung spectrum is measured for the time t and at a distance d between the gap and detector . With this spectrum a real spectrum is calculated, using the detector efficiency and the transmission of the used absorbers. For the calculations of this spectrum only the high-energy part of the measured spectrum is used, because the low-energy part is falsified by Compton scattering. The total energy EF of all photons within the energy interval [eUA, eUB ], see fig. 6, is calculated from the real spectrum . An interval length AE = e(UB - UA) of 10-20 keV is chosen arbitrarily. The corresponding intensity of the bremsstrahlung spectrum is described by Kuhlenkampf [9]: 2 I(E) = 2CiZ/e (eUB - E), where I(,, is the spectral intensity, C a constant

10 nA

100

150

200

U9 [ kV 1

Fig. 7. Electron current vs gap voltage for different electrode geometries ; pulse length 10 ms, repetition frequency 100 Hz ; X: 6.5 mm gap width, structure : four-rod RFQ, fR =108 .5 MHz; o: 4 mm gap width, structure: Jß/4 coaxial resonator, JR =108 .5 MHz, geometry: disks with 40 mm diameter ; ": 4 mm gap width, structure: a/4 coaxial resonator, JR =108 .5 MHz, geometry: disks with 28 .2 mm diameter ; El : 5 mm gap width, structure: a/4 double coaxial resonator, J R = 216 MHz, geometry : squares with an edge length of 35 mm ; o: 8 mm gap width, structure: a/4 coaxial resonator, fR =108.5 MHz, geometry : disks with 40 mm diameter ; dashed line : breakdown limit.

A. Gerhard et al. / RFQ sparking experiments

photon energy interval [eUA , eUB ] of the spectrum, see fig. 6, is I,=

feUBI(E) eUA

dE=CiZ(i1E/e) z .

(2)

The relation between IF and EF is given by the lifetime t of the detector and the area of the detector crystal, af : EF

= IFta f /4,trd z ,

where d is the distance between the detector and the gap. With eq. (2) one gets the electron current EF41Tdz (i1 E/e )

CZta f

Eq. (4) was used to determine the electron current between the OFHC copper electrodes at four different gap widths for two radio frequencies (108 and 216 MHz) and two different rf structures . Fig. 7 shows the results. The electron current varied between 40 and 10 mA and showed an exponential dependence on the gap voltage as expected . Sparking occurred above 1 mA electron current independently of gap width and frequency for the chosen geometry . This result may indicate that for sparking a minimum electron current is necessary.

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the sparking process is independent of the used structure and rf frequency. From the measurements of the electron one can perhaps derive a new sparking criterion. They will be continued and a special interest will be given to different electrode surface treatments to enhance the breakdown limit and to investigate the dependence of the breakdown limit on the electrode material . References [1] W.D . Kilpatrick, Rev . Sci. Instr. 28 (1957) 824. [21 J. Brutscher, Diploma Thesis, Frankfurt, 1988. [3] A. Gerhard et al ., IEEE Trans. Electron Devices EL-20 (1985) 709. [4] M. Kurz, Diploma Thesis, Frankfurt, 1988 . [5] R. Hutcheon et al ., PAC 87 (1987), IEEE no. 87, ch . 2387-9 . [6] A. Schempp, EPAC, Rome, 1988, to be published in Nuovo Cimento. [71 A. Gerhard et al ., ibid . [81 A. Gerhard et al ., Proc . 13th ISDEIV, Paris, 1988, eds. J.M . Buzzi and A. Septier (Les Editions de Physique, Paris, 1988) p. 492. [9] H. Kuhlenkampf and L. Schmidt, Ann. Phys. (Leipzig) 43 (1943) 494.

4. Conclusions Rf sparking tests have been performed with different rf structures and radio frequencies . All results show that

IV . ACCELERATOR RESEARCH