Chou, W

Mori, Y

Muto, M

Shirakabe, Y

Takagi, A

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DESIGN AND TEST OF A BEAM TRANSFORMER AS A CHOPPERW. Chou, FNAL, Batavia, Illinois, USAY. Mori, M. Muto, Y. Shirakabe, A. Takagi, KEK, Tanashi, JapanAbstractThis paper introduces a new type of chopper, which is sim-ilar to a beam transformer first discussed by R. Wideroe[1].It is based on the fact that the RFQ has a rather small energywindow. A pulsed beam transformer that provides 10% en-ergy modulation to the beam in front of an RFQ can ef-fectively chop the beam. It has fast rise- and fall-time anda short physical length. A prototype, which consists of acavity and a high voltage pulsed power supply, has beenconstructed and tested. Two types of magnetic materialswere tried for making the core of the cavity – the Finemetand the ferrite Philips 4M2. While the former gave goodperformance, the latter failed the test. Results from thebench measurements and a plan for beam tests are brieflydescribed. For details the readers are referred to Ref. [2].1 INTRODUCTIONIn a high intensity proton accelerator, a beam chopper is of-ten necessary in order to reduce particle losses during injec-tion from the linac to the first circular machine. A choppercan also serve other purposes such as to create a gap in abunch train so that the extraction loss due to kicker rise-timecan be eliminated.There are various ways to chop the beam, e.g., ion sourcechopping, transverse deflecting, etc. This paper introducesa new type of chopper, which is similar to a beam trans-former first discussed by R. Wideroe.[1] It is based on thefact that the RFQ has a rather small energy window. Bothsimulations and measurements show that, a 10% energyerror in a beam before it enters the RFQ can effectively cutthe transmission efficiency down to zero. For an RFQ withan injection energy of 50 keV, a 10% error is 5 keV. There-fore, a pulsed beam transformer that provides a 5 keV en-ergy modulation to the beam in front of an RFQ can servethe purpose of a chopper. There are several advantages ofthis type of chopper.1. Fast rise- and fall-time.As will be seen below in our bench test, a 40 ns rise-time has been achieved with a thyratron switch. Itwill be shortened to 10-20 ns when a solid state switch(e.g., the HTS transistor) is used.2. Short physical length and chopping at low energy.The beam transformer has a length of just about 10cm. The chopped beam is at the RFQ injection energy(which is 50 keV at the JHF).Email: chou@fnal.gov2 DESIGN2.1 Components of a chopperThe chopper consists of a pulsed power supply and a cav-ity. The latter contains several magnetic cores and worksas a transformer. When a voltage waveform is provided inthe primary circuit, an acceleration voltage with the samewaveform will be generated across the gap as the secondary.An ideal voltage waveform is a series of squares, each withsteep rise and fall edges and a flat top. The height of thesquare is 5 kV (which could be higher or lower, depend-ing on the design of the RFQ). The repetition rate (rep rate)of the squares are determined by the injection rf frequency.The spacing between two neighboring squares is equal tothe chopped beam length.It is necessary to use a bipolar waveform. A monopo-lar waveform will generate significant flyback voltage on aninductive load when the power supply is switched off, andalso has a large dc component.Take the JHF as an example. The bucket length of theJHF Booster at injection is 500 ns (2 MHz). Assume one-half of the beam will be chopped. The required voltagewaveform is: +2.5 kV for 250 ns, –2.5kV for the next 250ns in a period of 500 ns. When this voltage is transmittedto the beam through an accelerating gap, the beam energywill alternate between EH− + 2.5 keV and EH− – 2.5 keV,where EH− is the H− ion source energy, which can be setto 47.5 keV. Thus, at the exit of the RFQ, a 250 ns gap willbe created in the beam every 500 ns.2.2 Magnetic coreThe magnetic core is a critical part of the beam transformer.It must be able to stand high magnetic field Brf and havehigh permeability . Two types of materials have beentested for making the core — the Finemet and the Philips4M2 (which is a ferrite). Their magnetic parameters can befound in Ref. [3].When a voltage waveform v(t) is applied, the averagefield is Bave =Rv(t)dt=A, in which A is the flux area ofthe core. Because the Finemet can be used at much higherBave ( 5 kG or higher) than that for the 4M2 ( 100 G),it makes the Finemet a preferred candidate.The permeability of the Finemet and 4M2 has a strongdependence on the frequency. Assume the pulse has a pe-riod !0T = 2 and a length of 2 as shown in Figure 1.The Fourier spectrum lines are n!0, n = 0, 1, 2, ... For theJHF, T = 500 ns, !0 = 2 2 MHz. Thus, the permeabil-ity value at 2 MHz gives a reasonable estimate. It is 2800for the Finemet and 170 for the 4M2. This is another reasonwhy the Finemet is a better material for a chopper.0-7803-5573-3/99/$10.00@1999 IEEE. 565Proceedings of the 1999 Particle Accelerator Conference, New York, 1999w t(t)2aV( w t)2p(T)V0Figure 1: A series of voltage pulses with square waveform.2.3 Pulsed power supplyThe power supply is a bipolar voltage source. It provides12V0 square pulses at a given rep rate (2 MHz for the JHF).The number of pulses in a burst is determined by the in-jection time. It is about 0.3 ms for the JHF, which cor-responds to 600 pulses. The relative length of +12V0 and−12V0 should be a variable so that the chopping length canbe adjusted to meet different needs at the injection.The switch is the most important part of the power sup-ply. It must be able to deliver high voltage (several kV),high current (several tens of amperes) at high rep rate (sev-eral MHz). It must also have short turn-on and turn-off time(a few tens of nanoseconds). There are some triodes andtransistors that are commercially available and can servethe purpose, for example, the fast high voltage transistorswitch HTS 81-09 from a German company Behlke. Thepeak power of the power supply is high (hundreds of kW).But the duty factor is low. For the JHF, the duty factor isbelow 1%.3 MEASUREMENTSA simple cavity was constructed as shown in Figure 2. Itconsists of a magnetic core, a copper shield, a one-turn coil(the primary circuit of the beam transformer) and a stainlesssteel beam pipe. The pipe has a 22-mm long ceramic gap(the secondary of the transformer).A high voltage, low rep rate (3 Hz) kicker power supplywas used in the experiment. This is a monopolar voltagesource. It uses a thyratron as the fast switch and a pulseforming line (PFL) with a characteristic impedance 25 Ω.A dummy load of 25 Ω is added in parallel to the primarycircuit for matching the PFL impedance.Both the Finemet and 4M2 were tested. The dimensionsof the Finemet core are: OD = 58 cm, ID = 32 cm, d = 2.5cm. That of the 4M2 are 50 cm, 20 cm and 5 cm, respec-tively. The measured primary and secondary (i.e., the gap)voltages are shown in Figures 3-4. Finemet:It is seen that the gap voltage has a nearly square shape.The rise-time is about 40 ns, which is determined bythe thyratron. The fall-time is a bit longer. The am-plitude difference between the primary and secondaryvoltage is small, which indicates good coupling. Themaximum gap voltage reaches about 7.5 kV. The cor-Ceramic Gap Beam PipeV0Metal ShieldMagnetic Core10(cm) 0 5Figure 2: A test cavity used in the measurements. It con-sists of a magnetic core, a metal shield, a one-turn coil anda stainless steel beam pipe with a ceramic gap.responding primary voltage is about 8 kV. In this test,one achieved Bave = 4.9 kG and Bmax = 6.7 kG. 4M2:In Fig. 4, it is seen that there is a fast fall off in thevoltage waveform after the switch is turned on. Thereare two reasons for this behavior: (1) The inductanceof the 4M2 is smaller than the Finemet, the decay isfaster. (2) The maximum Brf of the 4M2 is rather low.At high voltage, the performance of the 4M2 is deteri-orated. Moreover, when the switch is turned off, thereis a voltage jump to the oppositeside (flyback voltage).Because the thyratron rep rate is low, a separate test usinglow voltage high rep rate power supplies (waveform gener-ators) was performed. The measurements were done at 2MHz and 7 MHz. The results are similar to that shown inFigs. 3-4 and can be found in Ref. [2].4 PLAN FOR BEAM TESTSA new power supply and a prototype chopper have been de-signed and are under construction. The power supply willuse two HTS 81-09 transistors for a bipolar operation, asshown in Fig. 5. Each transistor can operate at 8 kV, 90 Aat a burst frequency of 2.5 MHz. The pulse length can bevaried from 200 ns to infinity.566Proceedings of the 1999 Particle Accelerator Conference, New York, 1999Figure 3: The primary and secondary voltage waveformwhen a Finemet core is used. The rise-time is about 40 ns.Figure 4: The primary and secondary voltage waveformwhen a 4M2 core is used.The chopper will use three Finemet cores. The dimen-sions of each core are: OD = 50 cm, ID = 16 cm, d = 2.5cm. The total inductance at 2 MHz is about 50 H.This chopper will be installed on the HIMAC linac for a+HVCBCB-HVTriggerInputNOTRsRs RbCb RL LHV PulseOutputHTSHTSFigure 5: A circuit diagram of a bipolar (push–pull) highvoltage source using two HTS transistors as the switches.beam chopping experiment in the fall of 1999.5 SUMMARYIt is feasible to construct a beam transformer to provide 5kV (or higher) pulses with short rise- and fall-time and a flattop. When it is placed in front of an RFQ, it can be used asa beam chopper.The Finemet has high Brf and high . It is a good mate-rial for the magnetic core. The 4M2 gave poor performancein the tests because its Brf and  are too low.A bipolar high voltage source with high current and fastswitches is the preferred power supply.There are several issues that have not yet been addressed,including: (1) The tolerable voltage variation on top of thewaveform; (2) The effect of low energy (50 keV) H− parti-cles bombarding the RFQ.6 ACKNOWLEDGEMENTSThere were useful discussions with D. Swenson, D. Ra-paria, D. Anderson, A. Ueno, M. Yoshii, N. Tokuda and J.Griffin. One of the authors (W. Chou) would like to thankthe KEK for its kind hospitality during this work.7 REFERENCES[1] R. Wideroe, Archiv fur Electrotechnik, Vol. 21, p. 387(1928).[2] W. Chou et al., KEK Report 98-10 (1998), and the referencestherein.[3] T. Uesugi et al., JHP-31, KEK (January 1997).567Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

Design and test of a beam transformer as a chopper

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