Three photoinjector modifications were undertaken at CEBAF to help ensure successful completion of the PREx and Qweak parity violation experiments: the development of a pockels cell high voltage switch that provides stable voltages at 960 Hz helicity flip rate with 60 μs rise/fall time, the installation of a two-Wien-filter spin flipper for slow spin reversal, and the installation of a new photogun with inverted insulator geometry that operates at higher bias voltage

Adderley, P. A.

Clark, J.

Covert, S.

Grames, J.

Hansknecht, J.

Machie, D.

Poelker, M.

Stutzman, M. L.

Suleiman, R.

Surles-Law, K.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2012-11288-3Colloquia: PAVI11IL NUOVO CIMENTO Vol. 35 C, N. 4 Luglio-Agosto 2012Photoinjector improvements at CEBAF in support of parityviolation experimentsP. A. Adderley, J. Clark, S. Covert, J. Grames, J. Hansknecht,K. Surles-Law, D. Machie, M. Poelker(∗), M. L. Stutzman and R. SuleimanThomas Jefferson National Accelerator Facility - Newport News, VA 23606, USAricevuto il 4 Ottobre 2011; approvato il 5 Maggio 2012pubblicato online il 4 Luglio 2012Summary. — Three photoinjector modifications were undertaken at CEBAF tohelp ensure successful completion of the PREx and Qweak parity violation experi-ments: the development of a pockels cell high voltage switch that provides stablevoltages at 960Hz helicity flip rate with 60μs rise/fall time, the installation of a two-Wien-filter spin flipper for slow spin reversal, and the installation of a new photogunwith inverted insulator geometry that operates at higher bias voltage.PACS 29.27.Hj – Polarized beams.PACS 29.25.Bx – Electron sources.PACS 21.10.Hw – Spin, parity, and isobaric spin.1. – IntroductionThe Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab hasproved to be an ideal facility to conduct parity violation experiments, providing reliablehighly polarized electron beams with sufficiently small helicity correlated beam asym-metries that have enabled measurement of increasingly smaller physics asymmetries, asillustrated in table I (experiments listed in chronological order). Recent parity violationexperiments PREx and Qweak represent significant challenges for the accelerator divisionbecause the physics asymmetries are less than 1 ppm, which imposes tight toleranceson helicity correlated beam properties. A conscious effort was made to improve pho-toinjector accelerator systems to meet these demands, including the development of apockels cell high voltage switch that can provide spin flipping at 960Hz, the installationof a two-Wien-filter spin flipper for slow helicity reversal [1], and construction of a newphotogun that operates at higher bias voltage [2]. These systems are described below.(∗) E-mail: poelker@jlab.orgc© Societa` Italiana di Fisica 152PHOTOINJECTOR IMPROVEMENTS AT CEBAF ETC. 153Table I. – Parameters and beam specifications for parity violation experiments at CEBAF.The upper portion of table are experiments which have been completed with beam specificationsachieved.Experiment Energy I Apv Charge Position Angle Size(GeV) (μA) (ppb) (ppm) (nm) (nrad) (dA/A)HAPPEx-I 3.3 40 15050 200 12 12 –G0-Forward 3.0 40 3000–40000 300± 300 7± 4 3± 1 –HAPPEx-II 3.0 55 1580 400 1 0.2 –HAPPEx-III 3.484 100 22100 200± 100 3± 3 0.5± 0.1 10−3PREx-I 1.056 100 657± 60 100± 130 2± 3 1 10−4Qweak 1.162 180 288± 5 100± 10 2± 1 30± 3 10−4PREx-II 1.0 70 500± 15 100± 10 1± 1 0.3± 0.1 10−4Moeller 11.0 85 36± 0.6 10± 10 0.5± 0.5 0.05± 0.05 10−52. – Fast pockels cell high voltage switchPolarized beam experiments measure the scattering asymmetry at a target by flippingthe spin direction of the electron beam at a relatively fast rate. This is accomplishedvia an electro-optical device called a pockels cell at the photoinjector drive laser table.By applying positve or negative high voltage to the pockels cell, the helicity of the laserlight and hence the electron beam, can be flipped. Prior to PREx and Qweak, all parity-violation experiments flipped polarization at 30Hz, which served to cancel 60Hz beammotion.The first pockels cell high voltage switch used at CEBAF was homemade and utilizedtwo MOSFET switches capable of driving the cell to +/−2700V in less than 1μs. Unfor-tunately, at each helicity transition, the voltage would oscillate—or ring— for approxi-mately 500μs before stabilizing. In addition, MOSFET drive circuitry requires relativelyhigh current and this caused a voltage droop of the regulated high voltage power sup-plies powering the circuitry. This voltage droop introduced a long settling time-constantthat was evident when using a pseudo-random helicity flip pattern. Specifically, distinctasymmetry values were observed depending on the state of the previous helicity pair (orquartet, or octet, etc.). This phenomenon was termed pockel cell memory, however inhindsight, this behavior was not related to the pockels cell as much as it was related tothe non-constant voltage applied to the pockels cell. Thinking that problems were relatedto the homemade nature of the switch, it was replaced with a commercial system thatwas expected to provide much faster rise/fall times and constant voltage throughout theduration of each helicity state (according to the vendor). Surprisingly, the new switch(that also used MOSFET technology) produced nearly identical results. Furthermore,and perhaps more problematic, the commercial switch radiated helicity information thatcould be picked up by nearby electrical systems. These observations suggested ringingwas not a drive circuit problem, but were associated with peizo-electric properties of theKD*P cell. Indeed we found that in order to get a shorter transition to a new helicitystate, we had to slow the application of voltage to a critically damped rate to avoidinducing piezo-electric ringing. Whereas a 50 ns switch speed would induce ringing thatmade the electron beam unusable for 500μs, a slower switch rate that carefully rampedthe voltage over 50μs provided quality electron beam within 60μs.154 P. A. ADDERLEY, J. CLARK, S. COVERT, ETC.Fig. 1. – Pockels cell high voltage switch that relies on optical diodes.Knowing that high-speed MOSFET switches were no longer necessary or desirable,a new pockels cell high voltage switch was constructed using high-voltage optical diodesthat “reverse conduct” when light is applied via LEDs shining on the diode (fig. 1).A custom TTL drive circuit was designed to sequentially apply light to two opticaldiodes in series, and thereby flip the polarity of the voltage applied to the pockels cell.The optical diodes are suitably fast, providing 60μs rise/fall times (1.4% deadtime forPRex at 240Hz flip rate and 5.8% deadtime for Qweak at 960Hz) and the capacitance ofthe switch circuit is very small. Consequently, voltage stability throughout each helic-ity state is markedly better compared to previous switches, with “pockels cell memory”small enough as to be ignored. Finally, the simplicity of the optical diode switching givesus a vastly improved level of confidence in our ability to control parasitic currents thatwere previously leaking or radiating into the helicity data acquisition system. Future ex-periments require even faster flipping and the optical diode approach might be improvedto reduce rise/fall times further.3. – Two-Wien spin flipperHelicity-correlated variations in the electron beam properties can generate false asym-metries that affect the parity-violating physics asymmetry the experiment is trying tomeasure. To address this problem, it is customary to reverse the helicity of the beampolarization (relative to a fixed helicity clock) on a daily basis, which serves to cancel thefalse-asymmetry contribution to the parity-violating asymmetry. This slow reversal alsohelps identify small helicity-correlated electronic pickup between accelerator electronicsystems and the experiment data acquisition system which can introduce comparativelylarge offsets in the experimental asymmetry measurement. Prior to 2009, the only slowspin reversal available at CEBAF was provided by changing the polarity of the drivelaser light by inserting an optical halfwave plate immediately upstream of the drive laserpockels cell, however, this method cancels only some of the helicity-correlated beam vari-ations, namely those related to pockels cell residual birefringence [3]. For the PRExand Qweak experiments, a new slow reversal method was deemed necessary to cancel-outother effects, in particular, helicity-correlated beam size variations that can be introducedvia pockels cell focusing. The new slow reversal mechanism changes the polarity of theelectron beam directly and is composed of two orthogonal Wien filters with interveningsolenoids.PHOTOINJECTOR IMPROVEMENTS AT CEBAF ETC. 155Fig. 2. – Top: The Wien filter spin manipulator used a CEBAF (magnet not shown in the cuta-way view). Bottom: schematic of the two-Wien-spin flipper composed of vertical Wien filter,spin rotating solenoids, and horizontal Wien filter. Beam travels left to right, both diagrams.A Wien filter is a device with static electric and magnetic fields perpendicular toeach other and to the velocity of charged particles passing through it (fig. 2, top). Unitcharged particles with a velocity of βc = EB remain undeflected in passing through theWien filter, while the spin is rotated in the plane of the electric field. A Wien filterhas been used at CEBAF [4] for years to orient the spin direction of the beam at theinjector by an amount (modulo 2π) equal but opposite to the spin precession introducedas the beam passes through the arcs and transport lines to the halls. This sets thespin direction of the beam longitudinal to the direction of beam motion at the target.The CEBAF Wien filter can provide ±π2 rotation at beam energies up to 140 keV. Atlarge spin angles, a Wien filter introduces astigmatic focusing of the beam that can becorrected using quadrupole magnets.A solenoid magnet provides effective focusing of a low momentum (0.1–1MeV/c)electron beam, where the focal length is inversely proportional to the integral of thesquare of the magnetic field, given in the paraxial approximation by 1/f ∼ ∫ B2zdz [5].A solenoid is similarly well suited to rotate the spin of a low energy electron beam byan amount proportional to∫Bzdz, with spin rotated about the longitudinal magneticfield. It is common to use two solenoids in series to simultaneously set both the opticalfocusing and spin rotation.156 P. A. ADDERLEY, J. CLARK, S. COVERT, ETC.Fig. 3. – Diagrams of the two recent gun designs: (a) conventional large-bore cylindrical insulatorand (b) inverted gun with compact tapered insulator that extends into the gun vacuum vessel.Both drawings have same scale with top flanges 10 diameter. The cathode/anode gap is 6.3 cmfor both designs.We designed and built a 4π spin manipulator using two orthogonal Wien filters andtwo intervening solenoid magnets, as shown in fig. 2, bottom. The first Wien filterprovides a π2 spin rotation into the vertical plane. The intervening solenoids provide theproper focusing and rotate the polarization into the horizontal plane, in either direction,by simply changing the direction of the current through each solenoid. The second Wienfilter orients the polarization in the horizontal plane to cancel the spin precession in theaccelerator, as described above. The main advantage of this scheme is that the Wienfilter settings are not changed to enact a spin flip, only the direction of the currentthrough the solenoid magnets is changed. This is appealing because, in principle, theelectron-beam optics are identical in both configurations. In practice, it takes about onehour to change the direction of the spin, with time devoted to steering adjustments toregain the injector beam orbit.4. – Inverted gun operating at 130 kV bias voltageThe Qweak experiment demands very high current from the photogun (∼ 180μA),roughly a factor of two greater than typical high current experiments at CEBAF. Itis critical that the electron beam passes through photoinjector apertures with mini-mal interception (apertures that set the spatial and longitudinal acceptance of the ac-celerator) because aperture loss introduces fluctuations in beam current and position.These fluctuations increase the width of the charge and position asymmetry distribu-tions, thereby reducing the accuracy of these measurements. At nominal CEBAF gunvoltage 100 kV, the beamloss at injector apertures is significant (> 30%) when deliv-ering 180μA to a single hall. This beamloss stems from space charge induced emit-tance growth and can only be reduced by operating the photogun at higher bias volt-age. Increasing the bias voltage on the first CEBAF load locked gun (fig. 3a) was notan option, because it already suffered low level field emission that degraded the vac-uum near the photocathode which resulted in very poor photocathode lifetime. Toaddress this problem, a new photogun was built that employed a compact, tapered ce-ramic insulator that extends into the vacuum chamber (fig. 3b). This gun geometryPHOTOINJECTOR IMPROVEMENTS AT CEBAF ETC. 157 0 10 20 30 40 50 60 70 80 90 100 0 50 100 150 200 Transmission (%) Photogun Current (uA) Transmission vs Gun Voltage 130kV 115kV 100kV 85kV 70kV Fig. 4. – Transmission through photoinjector apertures versus photogun beam current andbias voltage. Significantly improved transmission is obtained at 130 kV compared to nominal100 kV of previous photogun designs. (Note that the RF buncher was de-energized during thesemeasurements.)is commonly referred to as an inverted gun design, a reference to the first such im-plementation by M. Breidenbach et al., at SLAC [6]. The inverted insulator designhelped to eliminate field emission because it provided a means to increase the distancebetween biased and grounded parts of the photogun not related to beam delivery andmore importantly, the design significantly reduced the amount of metal biased at highvoltage, so there was less metal to generate field emission. The new inverted photogunwas installed during the summer shutdown of 2009 and biased to 150 kV without inci-dent. It operates reliably at 130 kV without measureable field emission for the Qweakexperiment. Transmission measurements were made versus beam current for differentbias voltages. Figure 4 illustrates significantly improved transmission through injec-tor apertures compared to operation at lower voltages. These measurements were ob-tained with an RF buncher cavity de-energized. With this buncher cavity energized,and the gun bias voltage 130 kV, transmission at Qweak beam current 180μA approaches90%.∗ ∗ ∗Notice: Authored by Jefferson Science Associates under U.S. DOE Contract No. DE-AC05-84ER40150 and with funding from the DOE Office of High Energy Physics andthe Americas Region ILC R&D program. The U.S. Government retains a non-exclusive,paid-up, irrevocable, world-wide license to publish or reproduce this manuscript for U.S.Government purposes.158 P. A. ADDERLEY, J. CLARK, S. COVERT, ETC.REFERENCES[1] Grames J. M., Adderley P. A., Benesch J. F., Clark J., Hansknecht J., Kazimi R.,Machie D., Poelker M., Stutzman M. L., Suleiman R. and Zhang Y., in Proceedingsof 2011 Particle Accelerator Conference (PAC’11), New York NY (2011).[2] Adderley P. A., Clark J., Grames J., Hansknecht J., Surles-Law K., Machie D.,Poelker M., Stutzman M. L. and Suleiman R., Phys. Rev. ST Accel. Beams, 13 (2010)010101.[3] Paschke K. D., AIP Conf. Proc., 1149 (2008) 853.[4] Sinclair C. K., Adderley P. A., Dunham B. M., Hansknecht J. C., Hartmann P.,Poelker M., Price J. S., Rutt P. M., Schneider W. J. and Steigerwald M., Phys.Rev. ST Accel. Beams, 10 (2007) 023501.[5] Lawson J. D., The Physics of Charged Particle Beams (Oxford University Press) 1977.[6] Breidenbach M., Foss M., Hodgson J., Kulikov A., Odian A., Putallaz G., RogersH., Schindler R., Skarpaas K. and Zolotorev M., Nucl. Instum. Methods A, 350(1994) 1.

Photoinjector improvements at CEBAF in support of parity

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Photoinjector improvements at CEBAF in support of parity violation experiments

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