Wakefield Acceleration in Structures Manoel E. Conde Argonne National Laboratory High Energy Physics Division 9700 S, CassAve. Bldg. 362 Argonne, IL 60439 Abstract. Wakefield acceleration in dielectric loaded structures is discussed in this paper. We present a description of the dielectric wake field accelerator concept, comparing some features of the collinear and the two beam accelerator configurations. The Argonne Wakefield Accelerator Facility (AWA) is discussed in detail, including major upgrades that are presently taking place. The basic features and capabilities of the facility are presented, and the dielectric wakefield acceleration results are briefly summarized. Possible variants of the two beam accelerator configuration are discussed, and work on planar dielectric structures in various institutions is presented. We conclude this report mentioning prospective achievements of dielectric wakefield accelerating structures. INTRODUCTION The High Energy Physics accelerator community has no clear path to design linear colliders with energies higher than a few TeV. Therefore, new technology needs to be developed in order to make these higher energies accessible. The new technology needs to address the following three issues: (a) the energy source from which energy will be transferred to the beam; (b) the structure or medium that will support high accelerating gradients; (c) a means to transport and distribute the energy to the beam. We believe that electron beam driven accelerating schemes can make important contributions to these three issues. Electron beam driven accelerators can be based on structures or plasmas. The latter is discussed in another paper in these proceedings [1]. This paper will report on the status of the development of dielectric based wakefield accelerators. Wakefield accelerators based on metallic structures (e.g. CLIC project at CERN [2]), and the use of active media for energy storage [3] will not be discussed here. DIELECTRIC WAKEFIELD ACCELERATOR CONCEPT Dielectric wakefield accelerators are dielectric loaded waveguides that are excited by the passage of electron beams. The wakefields generated by the "drive" bunches correspond to the Cherenkov radiation emitted by the electrons as they traverse the structures with a speed higher than the radiation group velocity supported by the structures. These devices are typically constructed as hollow cylindrical dielectric CP647, Advanced Accelerator Concepts: Tenth Workshop, edited by C. E. Clayton and P. Muggli © 2002 American Institute of Physics 0-7354-0102-0/02/$19.00 63 tubes tubes inserted inserted into into cylindrical cylindrical metallic metallic waveguides, waveguides, or or as as dielectric dielectric slabs slabs attached attached to to the is the walls walls of of rectangular rectangular metallic metallic waveguides. waveguides. The The simplicity simplicity of of these these devices devices is expected expected to to make make them them relatively relatively inexpensive. inexpensive. The The absence absence of of irises irises should should allow allow operation the operation atat high high accelerating accelerating gradients gradients without without electrical electrical breakdown breakdown problems problems in in the structures. structures.These These dielectric dielectric structures structures compare compare favorably favorably with with conventional conventional iris iris loaded loaded structures structuresininterms termsof ofdamping damping of of higher higher order order modes modes [4]. [4]. Figure Figure 11 shows shows the the longitudinal longitudinal cross cross section section of of aa cylindrical cylindrical dielectric dielectric loaded loaded passage of an accelerator. accelerator. The The intensity intensity of of the the accelerating accelerating field field W Wzz generated generated by by the the passage of an electron bunch of charge Q and bunch length σ is given approximately by electron bunch of charge Q and bunch length <JzZ is given approximately by é æ π σ ö2 ù z WZ ≈ 3 exp expê − 2 ç ç λ ÷÷ ú 2 /2 ê n è ø úû a ë where the n-th n-th where aa isisthe the inner inner radius radius of of the the dielectric dielectric tube, tube, and and λkn is is the the wavelength wavelength of of the mode high modesupported supportedby by the the structure. structure. The The expression expression shows shows that that itit is is desirable desirable to to have have high charge charge bunches bunches traversing traversing small small diameter diameter structures. structures. An An example example of of aa dielectric dielectric structure, structure,including includingthe the beam beam parameters parameters and and resulting resulting wakefield wakefield is is given given in in Table Table 1. 1. Q 2b 2b 2a ε Q Cu FIGURE FIGURE1.1. Longitudinal Longitudinalcross crosssection section of of aa cylindrical cylindrical dielectric dielectric loaded accelerator. TABLE1.1. Example Exampleof ofcylindrical cylindricaldielectric dielectric loaded loaded structure. structure. TABLE Innerdiameter diameterof ofdielectric dielectric(2a) (2a) 6 mm Inner Outerdiameter diameterof ofdielectric dielectric(2b) (2b) 10 mm Outer 10 Dielectricconstant constant((s) 4.6 Dielectric ε) Chargeofofelectron electronbunch bunch(Q) (0 100 nC nC Charge 100 Bunchlength length((<r 1 mm mm Bunch σz)z) RFpower powergenerated generated(P) (P) 400 MW MW RF 400 RFfrequency frequency (f(f)) 19 GHz RF 19 Acceleratinggradient gradient(W (W MV/m___________ Accelerating z) z)_________________________9292 MV/m bunchtrain train of of four four 100 100nC nCbunches bunches or or eight eight 50 50 nC nC bunches bunches traversing traversing the device AAbunch describedininTable Table11would would generate generate an an accelerating accelerating gradient gradient of over 300 MV/m. described Dielectric wakefield wakefield accelerators accelerators can can be be operated operated in in the collinear collinear configuration, Dielectric where the the same same structure structure isis used used to to decelerate decelerate the the drive drive bunches bunches and and to accelerate the where probe bunches. That is, an electron bunch traverses the dielectric loaded structure, probe bunches. That is, an electron bunch traverses emitting radiation and establishing the wakefield in the structure; this drive bunch is emitting radiation and establishing the wakefield in followed by another electron bunch (probe or witness bunch) which is accelerated by followed by another electron bunch (probe or witness the wakefield present in the structure. the wakefield present in the structure. 64 Alternatively, accelerators can can be be operated operated inin the the two twobeam beam Alternatively, dielectric dielectric wakefield wakefield accelerators accelerator where two two dielectric dielectric loaded loadedstructures structuresare areused used(Fig. (Fig. accelerator configuration configuration (TBA), (TEA), where 2). the drive drive beam beam isis coupled coupled out out of of the the decelerating decelerating 2). The The RF RF power power generated generated by by the structure loaded structure, structure, where where the the witness witnessbeam beamisis structure and and into into aa second second dielectric dielectric loaded accelerated. accelerated. Schematic Diagram of Argonne Wakefield Step-Up Transformer High Amplitude Wave (Compressed) \ Low Amplitude Wave (uncompressed) Low Diel. Const. Material High Diel. Const. Material I—> Intense Drive Beam Accelerated Beam FIGURE 2. 2. Schematic Schematic of of aa dielectric dielectric wakefield FIGURE wakefield two two beam beam accelerator, accelerator, also also known known asas step-up step-up transformer. transformer. The two two beam beam accelerator accelerator scheme scheme offers The offers several several advantages advantages over over the the collinear collinear mode of of operation. operation. In In the the collinear collinear configuration, mode configuration, the the so-called so-called transformer transformerratio ratio cannot have have aa value value greater greater than cannot than two two ifif the the longitudinal longitudinal distribution distribution ofof charge chargeisis symmetric with with respect respect to to the the center symmetric center of of the the bunch; bunch; that that is, is, for forsymmetric symmetricbunches bunchesthe the rate at which the witness beam gains energy cannot exceed twice the rate at rate at which the witness beam gains energy cannot exceed twice the rate atwhich whichthe the drive beam beam loses loses energy. energy. This This limitation limitation is drive is easily easily avoided avoided in inthe thetwo twobeam beamaccelerator accelerator scheme, where the RF pulse can be compressed in the second structure, scheme, where the RF pulse can be compressed in the second structure,both bothininthe the transverse plane (essentially by having the second structure with smaller dimensions) transverse plane (essentially by having the second structure with smaller dimensions) and in the longitudinal dimension (by using a higher dielectric constant material in the and in the longitudinal dimension (by using a higher dielectric constant material in the second structure, with the corresponding lower group velocity for the radiation). The second structure, with the corresponding lower group velocity for the radiation). The possibility of RF pulse compression, with the consequent higher accelerating gradient possibility of RF pulse compression, with the consequent higher accelerating gradient in the second structure, is the reason why this configuration is also known as "step-up in the second structure, is the reason why this configuration is also known as “step-up transformer". transformer”. Both configurations can make use of multiple drive bunches, in the form of a bunch Bothallowing configurations make usethe of multiple drivethe bunches, in the formway, of a bunch train, for the can build up of fields inside structure. In this both train, allowing for the build up of the fields inside the structure. In this way, both schemes can achieve higher accelerating gradients; the collinear case, however, is still schemes can achieve higher accelerating gradients; the collinear case, however, is still subject to the transformer ratio limit of two. subject to the transformer ratio limit of two. Both schemes can make use of multiple stages of acceleration (and deceleration) to Bothhigher schemes make use multiple stages of acceleration (and(ordeceleration) reach finalcan energies. Thisofwould require several drive bunches several driveto reach higher final energies. This would require several drive bunches (or several drive bunch trains) to traverse several structures. While this is possible in the collinear bunch trains) to traverse several structures. While this is possible in the collinear 65 configuration, it it is is certainly certainly simpler simpler in in the the two two beam beam accelerator accelerator case, case, because because of of the the configuration, greater flexibility flexibility in in terms terms of of the the beam beam optics. optics. greater ARGONNE WAKEFIELD WAKEFIELD ACCELERATOR ACCELERATOR FACILITY FACILITY (AWA) (AWA) ARGONNE The Argonne Argonne Wakefield Wakefield Accelerator Accelerator Facility Facility (AWA) (AWA) was was designed designed to to study study The wakefield acceleration. acceleration. The The facility facility uses uses aa high high charge charge drive drive beam beam to to excite excite wakefield wakefields, and and aa low low charge charge witness witness beam beam to to probe probe the the wakefields. wakefields. These These two two beams beams wakefields, can be be made made to to propagate propagate through through the the same same structure, structure, to to study study collinear collinear acceleration, acceleration, can or can can travel travel to to two two separate separate structures structures (Fig. (Fig. 3). 3). or FIGURE 3. 3. Schematic of the the Argonne Argonne Wakefield Wakefield Accelerator Accelerator beamlines. beamlines. FIGURE Schematic of The drive drive beam beam is is generated generated by by aa half-cell half-cell photocathode photocathode RF RF gun gun running running at at 1.3 The 1.3 GHz. The bunch charge can be varied from 10 to 100 nC, with a bunch length of to GHz. The bunch charge can be varied from 10 to 100 nC, with a bunch length of 15 15 to 35 ps ps FWHM. FWHM. The The drive drive gun gun is is followed followed by by two two linac linac tanks tanks (1.3 (1.3 GHz) GHz) that that bring bring the the 35 beam energy energy up up from from 22 MeV MeV to to 15 MeV. beam 15 MeV. The 44 MeV MeV witness witness beam beam is is generated generated by by aa 66 1½ cell photocathode photocathode RF RF gun. gun. The The The A cell bunch charge charge is is typically typically 0.1 0.1 nC nC with with aa bunch bunch length length of of 88 ps ps FWHM. FWHM. Both Both guns, guns, as as bunch well as as the the linac linac tanks, tanks, are are powered powered by by aa single single 30 30 MW MW L-band L-band klystron. klystron. well The laser laser system system consists consists of of aa dye dye oscillator oscillator (496 (496 nm) nm) followed followed by by aa dye dye amplifier amplifier The and an excimer amplifier; a doubling crystal is followed by a second excimer amplifier and an excimer amplifier; a doubling crystal is followed by a second excimer amplifier (248 nm), nm), bringing bringing the the final final pulse pulse energy energy to to 88 mJ, mJ, with with aa pulse pulse duration duration of of 66 to to 88 ps ps (248 FWHM. FWHM. Over the the past past few few years, years, several several experiments experiments have have been been successfully successfully carried carried out out at at Over the AWA facility in both the collinear and the two beam accelerator configurations the AWA facility in both the collinear and the two beam accelerator configurations [5]. All these these experiments experiments used used dielectric dielectric loaded loaded cylindrical cylindrical waveguides, waveguides, with with [5]. All operating frequencies ranging from 7 to 20 GHz. The dielectric materials are typically operating frequencies ranging from 7 to 20 GHz. The dielectric materials are typically Magnesium-Calcium-Titanate based based ceramics ceramics (MCT), (MCT), with with dielectric dielectric constants constants Magnesium-Calcium-Titanate spanning from from 44 to to 40. 40. Accelerating Accelerating fields fields of of about about 15 MV/m have have been been measured measured in in spanning 15 MV/m collinear acceleration acceleration experiments. experiments. A A two two beam beam accelerator accelerator experiment experiment operating operating at at collinear 7.8 GHz GHz demonstrated demonstrated that that more more than than 90 90 % % of of the the RF RF power power generated generated by by the the drive drive 7.8 66 beam yielding an an accelerating acceleratingfield fieldofof beam(4(4MW) MW)was wascoupled coupled into into the the second second structure, structure, yielding 77MV/m. Figure 4 shows the energy modulation of the witness beam as it enters the MV/m. Figure 4 shows the energy modulation of the witness beam as it enters the dielectric loaded structure at different values of the RF phase. dielectric loaded structure at different values of the RF phase. FIGURE4.4. Plot Plotshowing showingthe thewitness witness beam beam energy energy as as aa function FIGURE function the the delay delay beam beam the the witness witnessand andthe the drivebeams. beams. drive Plasmawakefield wakefieldacceleration acceleration experiments experiments have have also Plasma also been been carried carried out outatatthe theAWA AWA facility. Electron beam focusing and acceleration in the underdense (blowout) facility. Electron beam focusing and acceleration in the underdense (blowout)regime regime werefirst firstobserved observedwith withthe theAWA AWA beam beam [6]. [6]. were Much higher accelerating gradients can be achieved achieved if Much higher accelerating gradients can be if the the quality quality of ofthe thedrive drivebeam beam increases. Smaller emittances allow the operation of smaller diameter increases. Smaller emittances allow the operation of smaller diameter structures, structures, whichyield yield much much higher higher accelerating accelerating fields. fields. Shorter which Shorter electron electron bunches bunches would would also also permit operation at higher RF frequencies. The field superposition resulting from the permit operation at higher RF frequencies. The field superposition resulting from the operation with longer bunch trains would allow the RF fields to build up to higher operation with longer bunch trains would allow the RF fields to build up to higher levels as well. For these reasons the AWA facility is currently undergoing several levels as well. For these reasons the AWA facility is currently undergoing several major upgrades. majorAupgrades. new photocathode RF gun has been built to replace the old drive beam gun. The newhad photocathode RF gun built to 2replace theRF oldpower drive was beamavailable. gun. The oldAgun been designed and has builtbeen when only MW of old gun had been designed and built when only 2 MW of RF power was available. This limited amount of RF power had led to the construction of a half-cell gun, This limited amount soft of RF power had led to at thetheconstruction of athehalf-cell gun, yielding a relatively 2 MeV electron beam gun exit, with consequent yielding a relatively soft 2 MeV electron beam at the gun exit, with the consequent high emittance and long bunch length. The new 1 ½ cell gun is expected to produce a high emittance long The new 1 VT.bunch cell gun is expected to produce 7.5 MeV beamand with 12 bunch MW oflength. RF power. Similar charges (10 – 100 nC) willa 7.5 MeV beam with 12 MW of RF power. Similar bunch charges (10 100 will be generated by the new gun, but with shorter bunch lengths (2 – 5 ps rms) andnC) much belower generated by the new gun, but with shorter bunch lengths (2 5 ps rms) and much emittances (30 – 200 π mm mrad). The new gun (Fig. 5) has been conditioned lower (30 - 200 mm mrad). new gun (Fig.beam 5) has been the conditioned up to emittances 12 MW of power, andnproduced the The first photoelectron during last few uphours to 12ofMW of power, and produced the first photoelectron beam during the lastonce few operation of the old laser system. The beam will be fully diagnosed hours of operation of the oldsystem laser starts. system. The beam will be fully diagnosed once operation with the new laser operation with the newnew laserlaser system starts. Installation of the system was completed a few days before the start of Installation of The the new system was of completed daysTsunami before the start of this workshop. new laser system consists a Spectraa few Physics oscillator this workshop. The newregenerative system consists of aandSpectra Physics Tsunami oscillator followed by a Spitfire amplifier two Ti:Sapphire amplifiers (TSA followed by a Spitfire twolength Ti: Sapphire 50). It produces 1.5 mJregenerative pulses at 248amplifier nm, with aand pulse of 6 to 8amplifiers ps FWHM(TSA and 50). It produces mJ to pulses at 248 pulse lengthweofhave 6 to had 8 psrunning FWHMthis and a repetition rate1.5 of up 10 pps. Thenm, verywith briefa experience a repetition rate of up to 10 pps. The very brief experience we have had running this 67 new that its its power power stability stability and and beam beam profile profile quality qualityare areindeed indeedmuch much new system system confirms confirms that better the old old laser laser system system could could provide. provide. better than than what what the FIGURE 5. Picture of of the the new new AWA AWAdrive drivegun gununder underinstallation. installation. The third item being upgraded upgraded is is the the photocathode photocathode material. material. The The old old drive drive gun gun operated routinely with magnesium photocathodes, being able to produce up to magnesium photocathodes, being able to produce up to100 100nC nC of charge charge with with 55 mJ 10~−44).). The The new new drive drive of mJ of of UV UV laser laser beam beam (quantum (quantum efficiency efficiencyof of 11 x× 10 beam has has been beam been commissioned commissioned with with aa copper copper photocathode, photocathode, but but that that will will soon soon be be replaced by replaced by aa cesium cesium telluride telluride photocathode photocathode [7], [7], which which isis expected expectedtotohave haveaaquantum quantum efficiency close close to 10%. We efficiency to 10%. We need need aa quantum quantum efficiency efficiency of of about about 1% 1% inin order order toto generate 64 bunches of 50 nC with 1.5 mJ of laser energy at 248 nm. generate 64 bunches of 50 nC with 1.5 mJ of laser energy at 248 nm. VARIANTS OF VARIANTS OF THE THE TWO TWO BEAM BEAM ACCELERATOR ACCELERATOR SCHEME SCHEME There are are two There two possible possible variants variants of of the the dielectric dielectric two twobeam beam accelerator acceleratorscheme: scheme:(a) (a) the structure where the drive beam loses energy can be used as a power the structure where the drive beam loses energy can be used as a power source source for for metallic accelerating accelerating structures; metallic structures; (b) (b) aa dielectric dielectric loaded loaded structure structure can canbe bepowered poweredby byaa conventional RF source. conventional RF source. Both of of these Both these options options are are currently currently being being pursued pursued by by the the AWA AWA group group inin collaboration with other research institutions. Duly Research, in collaboration collaboration with other research institutions. Duly Research, in collaboration with with CERN and and ANL, CERN ANL, is is building building 21 21 GHz GHz dielectric dielectric loaded loaded structures structurestotobe betested testedasaspower power sources using sources using the the CTF CTF II II electron electron beam beam at at CERN CERN as as the the drive drive beam beam [8]. [8]. These These structures are expected to generate more than 150 MW of RF power at 21 GHz. structures are expected to generate more than 150 MW of RF power at 21 GHz. The AWA The AWA group group has has built built 11.4 11.4 GHz GHz dielectric dielectric loaded loaded prototype prototype structures structurestotobebe tested at NRL using their X-band magnicon as the RF power source [9]. tested at NRL using their X-band magnicon as the RF power source [9]. 68 PLANAR DIELECTRIC WAKEFIELD STRUCTURES W-band dielectric loaded structures have been built and tested using the NLCTA electron beam at SLAC [10]. These planar dielectric structures were constructed by inserting two alumina slabs into a rectangular waveguide section. The transverse dimensions of the structures are of the order of hundreds of micrometers, with a 720 um gap for the passage of the 300 MeV electron beam. One of these devices was configured in a ring resonator circuit, where the measurements indicated that 200 kW of circulating power generated an accelerating field of 20 MV/m. A collaboration between Yale, Columbia and Omega-P is developing planar dielectric loaded structures with transverse dimensions of the order of tens of micrometers. They plan to chop the electron beam from the accelerator LACARA [11], in order to make 1 um long bunches of 1 pC charge. A bunch train composed of ten of these small bunches, each spaced by 20 jim, is expected to generate an accelerating field of about 600 MV/m. CONCLUSION Substantial progress is being made in the development of dielectric loaded structures as an alternative to the use of metallic structures. The AWA upgrades will make it a powerful tool for the study of electron beam driven accelerators and the exploration of higher accelerating gradients. The new AWA beam will be capable of demonstrating gradients of the order of 200 to 300 MV/m in dielectrics, in both collinear and step-up transformer structures. This intense beam will be able to generate hundreds of megawatts of RF power with frequencies in the range of 30 to 100 GHz, with pulse lengths of tens of nanoseconds, assuring a prominent place for beam driven schemes in the quest for multi TeV colliders. ACKNOWLEDGMENTS This work was supported by DOE, High Energy Physics Division, Advanced Technology Branch, under Contract No. W-31-109-ENG-38. REFERENCES 1. Barov, N., "Plasma-Wakefield Acceleration Experiments", in these proceedings. 2. 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