A remotely steered millimetre wave launcher for electron cyclotron heating and current drive on ITER

Fusion Engineering and Design 85 (2010) 69–86

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A remotely steered millimetre wave launcher for electron cyclotron heating and current drive on ITER W.A. Bongers a,∗ , M.F. Graswinckel a , A.P.H. Goede a , W. Kasparek e , I. Danilov b , Á. Fernández Curto c , M.R. de Baar a , M.A. van den Berg a , A.J.H. Donné a , B.S.Q. Elzendoorn a , R. Heidinger b , P. Ivanov d , O.G. Kruijt a , B. Lamers a , A. Meier b , B. Piosczyk b , B. Plaum e , D.M.S. Ronden a , D.J. Thoen a , M. Schmid b , A.G.A. Verhoeven a a

FOM-Institute for Plasma Physics Rijnhuizen, Association EURATOM-FOM, PO Box 1207, 3430 BE Nieuwegein, The Netherlands1 Forschungszentrum, Karlsruhe, Association FZK-EURATOM, D-76021 Karlsruhe, Germany c Laboratorio Nacional de Fusión, Associación EURATOM-CIEMAT, 28040 Madrid, Spain d ELVA-1 mm-wave division, Riga, Latvia e Institut für Plasmaforschung, Universität Stuttgart, Pfaffenwaldring 31, 70569 Stuttgart, Germany

b

a r t i c l e

i n f o

Article history: Received 28 May 2009 Accepted 12 July 2009 Available online 25 August 2009 Keywords: Remote steering launcher Electron cyclotron heating and current drive ITER

a b s t r a c t High-power millimetre wave beams employed on ITER for heating and current drive at the 170 GHz electron cyclotron resonance frequency require agile steering and tight focusing of the beams to suppress neoclassical tearing modes. This paper presents experimental validation of the remote steering (RS) concept of the ITER upper port millimetre wave beam launcher. Remote steering at the entrance of the upper port launcher rather than at the plasma side offers advantages in reliability and maintenance of the mechanically vulnerable steering system. A one-to-one scale mock-up consisting of a transmission line, mitre bends, remote steering unit, vacuum window, square corrugated waveguide and front mirror simulates the ITER launcher design configuration. Validation is based on low-power heterodyne measurements of the complex amplitude and phase distribution of the steered Gaussian beam. High-power (400 kW) short pulse (10 ms) operation under vacuum, diagnosed by calorimetry and thermography of the near- and far-field beam patterns, confirms high-power operation, but shows increased power loss attributed to deteriorating input beam quality compared with low-power operation. Polarization measurements show little variation with steering, which is important for effective current drive requiring elliptical polarization for O-mode excitation. Results show that a RS range of up to −12◦ to +12◦ can be achieved with acceptable beam quality. These measurements confirm the back-up design of the ITER ECRH&CD launcher with future application for DEMO. © 2009 Elsevier B.V. All rights reserved.

1. Introduction 1.1. ITER electron cyclotron heating and current drive A 20 MW, 170 GHz millimetre wave system for electron cyclotron heating and current drive (ECH&CD) is currently under development for the nuclear fusion experiment ITER. The system comprises an array of 24 gyrotron sources rated 1 MW power each, coupled to individual transmission lines to feed millimetre wave launchers plugged into the ITER Ports. Diplexer switches in the transmission lines divide the power over one Equatorial Port and four upper port launchers [1]. To allow for future upgrades the beam-lines are

∗ Corresponding author. Tel.: +31 306096792; fax: +31 306031204. E-mail address: [email protected] (W.A. Bongers). 1 http://www.rijnhuizen.nl/. 0920-3796/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2009.07.001

designed for 2 MW CW operation. Development work reported here concerns the upper port launcher (UPL), or rather a specific design concept of one beam-line of the UPL that is subjected to low and high-power millimetre wave test.

1.2. What is the Issue High-pressure Tokamak plasmas are prone to neoclassical tearing modes (NTM) which develop on rational magnetic flux surfaces with q = 2/1, q = 3/2, which deteriorate plasma confinement and must be suppressed. Launching the millimetre wave beams from the ITER upper ports stabilises NTMs by driving a localised current on these q-surfaces [2–4]. Directing the millimetre wave beams to the pertinent q-surfaces requires agile steering and tight focussing of the beams. Steering is not conceived electronically (phased array), but mechanically by movable mirrors that are water-cooled.

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A straightforward approach would be to steer the beams by mirrors placed at the entrance of the Tokamak, the so-called front steering (FS) concept. Characteristically, the FS steering range is large and precise delivering a well-focussed beam over the steering range. Disadvantage is the close proximity of moving, water-cooled parts to the hostile plasma environment. The alternative, remote steering (RS) concept places the steering mechanism away from the plasma by making use of millimetre waveguide properties. A remotely placed steering system offers an obvious advantage in meeting the reliability and maintenance requirements of ITER [5]. 1.3. How is this issue addressed The remote steering concept was first proposed by [6] showing that, to some approximation, the normal modes of an oversized square corrugated waveguide (SCW) come back into phase after a fixed length L = (2a)2 /, where a is the waveguide width (44 mm in this case) and  is the vacuum wavelength of the millimetre waves (1.76 mm at 170 GHz). Making use of this property, a millimetre wave beam entering the SCW at a certain angle ϕ, exits this waveguide at the same, albeit mirrored angle. Thus, a millimetre wave beam can be steered remotely by steering the input beam at the entrance of the waveguide. The concept allows the mechanical steering unit to be placed several metres away from the plasma, such that neutron shielding can be accommodated and direct plasma exposure avoided. The concept is limited by the quality of a well-focussed beam over the steering range that can be practically achieved.

Fig. 1. Remote steering of a single beamlet through a fixed focusing mirror SM1 and a steerable flat mirror SM2. The steered beam exits the square corrugated waveguide (SCW) at the same but mirrored angle. Fixed front mirrors FM1 and FM2 focus the exit beam on the ITER target q-surface. One ITER upper port launcher (UPL) comprises six beamlets, grouped in Upper and Lower rows of three. Each UPL contains six individual SMUs, six SCWs and three pairs of FMs, each FM accommodating one upper and one lower beamlet [9]. Numbers 1–5 indicate the positions where beam profile measurements are taken. The ITER primary vacuum boundary is located at position 2.

dual mirror system whilst being controlled by one simple linear actuator. The SMU is contained in the secondary vacuum of ITER. The steered beamlet passes through a diamond vacuum window and a gate valve to enter into the SCW immersed in the ITER primary vacuum system. The steered beams at the SCW exit are shaped and folded by the FM system in periscope configuration such as to guide each beamlet around the blanket shielding. One ITER upper port launcher (UPL) contains six beamlets grouped in Upper and Lower rows of three, each beamlet steered by a dedicated SMU-SCW tandem.

1.4. What are the requirements

1.6. Validation of the remote steering design

The functional requirement of the ITER upper launcher includes a steering angle range of ±12◦ (accuracy ±0.1◦ ) achieved within a 1-s time frame, targeting the q = 2/1, q = 3/2 magnetic surfaces whilst staying in synchronism with magnetic island rotation. Polarization state of the millimetre waves launched needs to be elliptical for effective coupling to the O-mode electron cyclotron wave of the high density ITER plasma. Engineering requirements include a safely handled peak power load on the mirrors well below 10 MW/m2 , millimetre wave stray power below 1%, and operation under ITER environmental conditions (magnetic field 6 T, ultra-high vacuum, vibration from cooling and shock from plasma disruptions). Various options of the RS concept have been explored and optimised in terms of effectiveness in NTM suppression [7]. Optimisation of the quasi-optical design has been carried out by [8]. Recently, it was decided to abandon the RS concept in favour of FS because of the greater flexibility in focussing and steering offered by FS, needed in the explorative phase of ITER burn control where NTM stabilisation options need to be established. The RS concept now constitutes the back-up solution of the ITER upper port launchers, which still needs to be validated lest some unexpected problem with FS arise. For DEMO, RS offers advantages over FS as the higher and extended radiation generated by the nuclear fusion reactions will require a more robust design of launcher components facing the plasma. Steering range may be narrowed down applying knowledge acquired on appropriate plasma scenarios and their stabilization.

The design of the remote steering system requires experimental validation of steering range and beam quality as well as the mechanical movement of the SMU under vacuum. In this paper, initial validation is carried out on a mock-up of the ITER configuration and results are compared with design calculations. The mock-up consists of a gyrotron or, alternatively, a low-power source, followed by an ITER like circular corrugated waveguide with mitre bends, an SMU immersed in vacuum, a CVD diamond vacuum window and a square corrugated waveguide. At the waveguide exit, a shaped mirror represents the front mirror system.

1.5. Remote steering scheme The current remote steering launcher scheme consists of three elements, the steering mirror unit (SMU), the square corrugated waveguide (SCW), and the front mirror (FM) system. See Fig. 1. Evacuated circular corrugated waveguides (CCW) of 63.5 mm inner diam. transport the millimetre wave beam from the gyrotron source via mitre bends to a mode convertor that couples into the SMU. A novel sliding cam design of the SMU rotates and translates the

1.7. Background In the past validation of the RS concept at high millimetre wave power was limited to lower frequency (140 GHz) under ambient condition for Wendelstein-7X and ASDEX Upgrade [10]. Highpower experiments at 170 GHz were carried out by [11] using a simple flat copper steering mirror coupled to an evacuated SCW. Early verification of the RS principle by low-power measurements was carried out by [12]. 1.8. This paper This paper validates the design of an ITER like remote steering launcher through low- and high-power measurements at 170 GHz, the beam-line being evacuated in some cases. Measurements include near- and far-field beam power profiles, polarization state and complex amplitude measurements of the millimetre wave field. Diagnostics include calorimetric, thermo-graphic, polarization and heterodyne vector analyser measurement. Section 2 reviews the theory of optimising remote steering, in particular establishing the optimum length of the SCW and the characteristics of internal corrugations. Section 3 describes the experimental set-up for high-power measurements and Section 4 for low-power measurements. Section 5 presents the results of

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Fig. 2. Gaussian beam envelope propagating from the CCW waist (z = 0), through the SMU to the SCW entrance plane (z = 0.5). The steerable SM2 mirror changes axial position whilst steering, aiming the beam waist at the SCW entrance plane. The axial position of the beam waist at the SCW entrance shifts over the full ± 12◦ steering range as indicated.

low-power measurements and Section 6 the high-power measurements. Section 7 discusses and concludes the paper. 2. Theory The design of the millimetre wave launcher is based on quasioptical Gaussian beam propagation [13] characterized in free space by waist size and position. The theory of remote steering employing a square corrugated waveguide has been developed by [14,15]. 2.1. Gaussian beam propagation The Gaussian beam is imaged by the steering mirrors SM1 and SM2 from a beam waist at the CCW exit plane to a waist at the SCW input plane, see Fig. 2. Here, the CCW exit waist radius is w0 = 20.32 mm and the SCW entrance waist radius is w0 = 12.54 mm. For a steering angle ϕ = 0◦ this waist is centred at the SCW entrance plane. At increased steered angle, lateral and axial waist shift occurs, which at ϕ = 12◦ results in a maximum 98 mm axial displacement at the SCW entrance plane. The steering mechanism is designed to compensate for this axial and lateral beam waist shift. In order to optimise transmitted Gaussian beam power, a tradeoff is made between beam truncation requiring larger aperture SCW, and suppression of higher order mode loss requiring reduced aperture SCW. This leads to an optimum aperture size a2 of 44 mm × 44 mm as selected for the ITER design [16]. Beam losses at the entrance have been calculated for Gaussian beam truncation over the range of steering angles. Losses at the exit from misdirected beam power have been calculated in similar manner, considering the Gaussian main beam is reconstructed from the waist position at the SCW exit. The waveguide geometry is shown in Fig. 3. 2.2. Eigen mode spectrum Boundary conditions imposed by the oversized SCW determine the eigen mode spectrum of the beam modes. At increased steering angle ϕ, a phase shift arises between propagating modes that

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Fig. 4. Beam offset x [mm] at the exit of the remote steering waveguide SCW as a function of steering angle ϕ calculated from Eqs. (1) and (2). The input beam is centred at the SCW entrance plane. The optimal waveguide length Lopt is calculated for the angles ϕopt = 0◦ , 8.45◦ , 10.37◦ and 12◦ resulting in Lopt = 4391 mm, 4344 mm, 4321 mm and 4298 mm respectively.

produce, in addition to the main Gaussian lobe, side lobes and mirror beams at the SCW output. The phase shift between propagating modes may be reduced by reducing the length of the SCW [14]. Optimisation of the SCW length is included in the quasi-optical calculation by employing the approximation [17]: L=

1 4a2  1 + 1/2 sin2 (ϕ)

(1)

This formula expresses the dependence of the length of the remote steering waveguide on the steering angle ϕ. This length needs to be optimized for the entire steering range at some fixed value ϕopt . Another way of looking at this expression is that for a fixed waveguide length L the intersection point of output beam with waveguide axis shifts with increased steering angle in axial direction z, see Fig. 3. The lateral offset x ∼ z tan ϕ of the output beam over the steering range is expressed through z by: z ≈ L(a, , ϕ) − Lopt (a, , ϕopt )

(2)

The exit beam offset x is plotted as a function of steering angle ϕ in Fig. 4. Here Lopt serves as parameter, determined for a fixed value ϕopt . The above optimisation of waveguide length is taken one step further by allowing a beam offset at both the entrance and the exit of the SCW, thereby distributing the total beam power loss over the truncation loss at the entrance and the loss of misdirected beams at the exit of the SCW. A length correcting cam profile in the SMU producing an angle dependent offset can accomplish such beam shift. As a result, overall power loss is minimised. In Fig. 5 the calculation is shown of total beam power loss as a function of steering angle. The SCW length Lopt corresponding to ϕopt = 0◦ , 8.45◦ , 10.37◦ and 12◦ serves as a parameter. The input beam is assumed to be centred at the SCW entrance plane. Exception is the plot marked “10.37◦ symmetric”, which allows for a beam shift at both the entrance and the exit plane. The result of the optimisation process for minimum total power loss over the ±12◦ steering range is an SCW length Lopt = 4321 mm for ϕopt = 10.37◦ . A mirror steering mechanism that distributes the beam offset x equally over the SCW entrance and exit is required. 2.3. Corrugated waveguides

Fig. 3. Remote steering SCW producing a beam offset x ∼ z tan ϕ at the waveguide exit for large steering angle ϕ.

Corrugated waveguides are employed to reduce the skin current losses at the walls of the waveguide through modification of the boundary conditions for normal waveguide modes [19]. The bound-

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[10], given by multiplying this experimental value with the number N of geometrical reflections off the waveguide wall with length L for a certain steering angle ϕ: N=

Fig. 5. Total % power loss of input beam power (entrance and exit ports) calculated for a simple and a length-correcting steering mirror cam profile with SCW length as a parameter. The beam is focused at the centre of the SCW entrance plane, except for the case “10.37◦ symmetric” where beam offset is equally distributed over SCW exit and entrance plane The case “10.37◦ symmetric” yields the lowest average loss of 0.35% and lowest maximum loss of 0.65% over the ±12◦ steering range.

ary conditions imposed require the tangential magnetic fields at the corrugated waveguide walls to vanish and to preferentially propagate modes with low field strength at the sidewalls. In order to calculate power loss in the waveguide walls, consider the absorbed power a and reflected power r of an electromagnetic wave incident on a plane conducting surface [13]. In the E-plane (electric field parallel to the plane of incidence) and the H-plane (electric field perpendicular to the plane of incidence), the absorption (small absorption limit) becomes: aE = 1 − rE =

4 Rs cos ϑ Z0

(3)

and aH = 1 − rH = 4 cos ϑ

Rs Z0

(4)

Here  ϑ is the angle of incidence w.r.t. normal of the reflection plane,  0 /ε0 is the free space impedance and Rs =  0 / the Z0 = surface resistance of the material, where is the frequency of the wave and the DC conductivity of the material. For copper this conductivity is 5.8 × 107 −1 m−1 at room temperature. For normal incidence ϑ = 0 the equations reduce to aE = aH = 4(Rs /Z0 ), which for pure copper at 170 GHz attains the value of 0.114%. Normalizing the absorption to the reflection loss at normal incidence and transforming the angle of incidence ϑ to the steering angle ϕ = ␲/2 − ϑ, this yields the normalized absorption ˛nH of the H-plane electric field component of an electro-magnetic wave reflected from a plane conducting surface at a steering angle, ϕ: ˛nH = sin ϕ

(5)

Results for loss of corrugations, calculated at smooth and corrugated profiles as function of the steering angle, by means of a 2D FDTD algorithm [20], show the characteristic sin ϕ relationship, which was validated by experiment. The absorption of the corrugated wall of a waveguide can be experimentally determined by means of a three-element Fabry–Perot (FP) interferometer [21]. This technique compares the quality factor Q2 of a conventional two-element FP reference resonator with the quality factor Q3 of the resonator with the test sample inserted, effectively acting as a dielectric between the two reflectors. The absorption loss of the corrugated sample may thus be established as a function of steering angle and compared with the absorption of an ideal plane copper reflector represented by Eq. (5). The total absorption of the waveguide is, to good approximation

L tan ϕ a

(6)

where a the waveguide cross sectional dimension. This simple formula was checked by integrating over the wall currents in the SCW and was found to agree within a few percent with the exact calculation [10]. The phase shift between the components with parallel (E-plane) and perpendicular (H-plane) polarization of a plane wave reflected from this corrugated waveguide wall [20] must be kept zero. If the phase shift is not zero it leads to a change in polarization (depolarization) of the wave coupled to the SCW. This phase shift is also determined experimentally using the three-element FP resonator technique by measuring the shift in resonant frequency between Eplane and H-plane polarization. Tuning to the resonant frequency for zero frequency shift, the optimal SCW frequency can be established. The measured frequency shift relates to the phase shift by considering the free spectral range of the FP, which equals 2 phase difference. Again, by multiplying with the number of geometrical reflections N, given by Eq. (6), this yields the total phase shift for the waveguide. Since the phase shift for one reflection scales approximately linearly with the steering angle, ϕ, the total SCW phase shift scales as ϕ2 [25]. This scaling allows experimental results taken at one steering angle to be related to other steering angles. 3. High-power experimental set-up 3.1. Description beam-line The 170 GHz millimetre wave beam is generated by the European coaxial gyrotron [18]. An ellipsoidal focusing mirror, a beam truncation aperture and an ellipsoidal relay mirror shape the beam. This beam is injected into an evacuated circular corrugated waveguide (CCW) equipped with mitre bends. The mitre bend mirrors control the polarization state of the beam. The CCW output beam is directed at the steering mirror unit (SMU) immersed in a dedicated vacuum chamber representing the ITER secondary vacuum system. The mirrors are made out of Aluminium with no active water-cooling. Future development will include a water-cooled Copper Stainless Steel sandwich replacement. The steered beam passes through a gate valve and CVD diamond vacuum window, representing the ITER primary vacuum boundary, to enter the square corrugated waveguide (SCW). The SCW is built up from six straight sections of oxygen free high conductivity copper, most sections of approximately 80 cm length. Sections are procured from General Atomics (GA), a nuclear engineering firm of San Diego (USA), one additional section is procured from Dutch industry, Heeze Mechanics, and this is the only water-cooled section. Internally the SCW is furnished with corrugations of approximately 1/4 depth (depth 0.49 mm, period 0.66 mm), transverse to the waveguide axis, see Fig. 6. A shaped front mirror completes the experimental set-up. It represents the ITER front mirror system only in part as there is no double mirror periscope configuration in this mock-up. The entire beam-line is laser aligned to 0.03◦ precision (about 5 mm over 10 m distance) by mounting the laser on one of the mitre bends inside the CCW system. The laser was pre-adjusted in the mitre bend by rotating the un-mounted mitre bend in a lathe and minimising spot excursion at 5 m distance.

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Fig. 6. Design of the internal SCW corrugations by General Atomics. The profile is shaped such as to avoid bi-refringent behaviour or depolarization of the SCW and to maintain the polarization state of the propagating wave from input to output of SCW.

The experimental set-up is shown in Fig. 7. Total length of the facility is approximately 10 m. An expanded view of the vacuum immersed SMU is shown in Fig. 8. A layout of the SMU, explaining the translational–rotational mirror movement, is shown in Fig. 9. Key components include a fixed ellipsoidal mirror SM1 and a steerable flat mirror SM2 that is being moved over a cam profile by a linear actuator requiring an approximately 10 cm stroke to achieve the ±12◦ steering range. Measurements are taken at five locations along the beam-line including measurements of beam power, beam quality, polarization and beam steering performance. The five measurement locations are indicated in Fig. 1; the SMU entrance (1), the SCW entrance (2), the SCW exit near field (3), the SCW exit far field (4) and the focussing mirror far field (5). The target screen to the right is movable to the various measurement locations indicated in Fig. 1.

Fig. 8. Expanded view of the vacuum immersed steering mirror unit SMU. Key components shown are the circular waveguide CW system with polarizing mitre bends, the SMU fitted inside a vacuum vessel, the diamond window and vacuum valve and the square corrugated waveguide (SCW) onto which the steered beam is projected. The system may be vacuum sealed by two quartz windows at the CW entrance and at the SCW exit.

3.2. Diagnostics Diagnostics employed include calorimetry and thermography of the beam. Temperature rise of beam-line components is measured by thermocouples. Polarization is measured by a rotary probe moved in the beam-line. For each pulse the output power from the gyrotron is bolo-metrically established from the power intercepted by the gyrotron exit window.

Fig. 7. High-power mock-up of the ITER remote steering launcher (total length 10 m). Following the millimetre wave beam produced by the gyrotron to the left, the beam is shaped by mirrors and mode filter apertures before entering the circular corrugated waveguide (CCW) fitted with mitre bends. A beam waist positioned at the CCW exit plane defines the input beam of the vacuum immersed SMU. This waist is imaged on a second beam waist of the steered beam at the entrance of the square corrugated waveguide (SCW), see Fig. 2. The waveguide system is vacuum sealed by quartz windows at the CCW entrance and at the SCW exit in some cases. The ITER primary vacuum boundary is simulated by a CVD diamond window and vacuum valve at the SCW entrance. The beam profile and beam power is measured at five locations along the beam-line; the entrance of the SMU, the entrance of the SCW, the SCW exit near and far field and at the FM, see Fig. 1. The entire system is laser aligned to 0.03◦ precision (5 mm over 10 m distance). The beam-line and the diagnostics are surrounded by Faraday cages in order to reduce interference from stray millimetre wave radiation.

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tive to one electric field component only. The antenna is fitted with a Macor stray radiation absorber cone, two attenuators and a Schottky diode detector. The assembly is suspended inside an acrylic tube and can be rotated and translated through the beam to measure the polarization angle ˛ and the ellipticity tan ˇ of the polarised wave to ±1◦ accuracy at ±0.3◦ resolution. 4. Low-power experimental set-up 4.1. Heterodyne scheme

Fig. 9. Layout of the SMU. The mm-wave beam emanating from the circular waveguide CW system is focused by a fixed ellipsoidal mirror SM1 and steered by a flat mirror SM2 that is being moved over a cam profile tilting and translating the mirror by a linear actuator requiring an approximately 10 cm stroke to achieve the ±12◦ steering range specified. The beam waist is kept centred at the SCW entrance over the steering range. The SMU dimensions indicated are the ones used in the quasi-optical calculation of beam propagation presented in Fig. 2.

Beam profiles are measured by taking infrared images of a target PVC foil (0.4 mm thickness, 400 mm × 800 mm area) observed by an infrared camera (3.4–5 ␮m). The pixel resolution of the PVC screen is approximately 1.5 mm × 3 mm. An electrically heated 100 mm × 100 mm wire raster serves as a dimensional reference frame. Peak temperatures are measured at a frame rate of 50 Hz, faster than the PVC thermal diffusion time (s). IR camera temperatures are black body calibrated to an accuracy of ±2 ◦ C. Each high-power shot is initialised by a reference IR measurement with no power that is subtracted from the high-power temperature distribution to avoid bias yielding a precision of ±0.1 ◦ C. The PVC foil is highly absorbing and not sensitive to the angle of incidence of the steered millimetre wave beam (εr = 3.2 and tan ı = 150 × 10−4 yielding 96% absorption). Two images are taken under an angle with the screen to avoid direct beam impact on the camera and are then synthesized by image reconstruction to form a normal incidence image, enhancing pixel resolution at the same time. The calorimeter is of water-cooled ballistic type, calibrated to ±5% accuracy. The thermo-graphic beam profile measurement is accurate to ±5% and precise to ±1%. A calibration curve establishes the relationship between infrared temperatures at pixel resolution and beam energy, as explained in Appendix A. The polarization measurement device shown in Fig. 10 consists of a rotary probe equipped with a D-band waveguide antenna sensi-

Low-power heterodyne measurements are a powerful means to characterise beam optical parameters, including amplitude, phase and polarization of the millimetre wave radiation. A local oscillator signal is non-linearly mixed with the measured signal, down converted and measured. The heart of the system is a state of the art 10 MHz–20 GHz Agilent Technologies Vector Network Analyser (PNA series E8352B) containing one transmitter and four receivers. A custom built up/down convertor system, designed by ELVA4 and FOM1 , using active high output power multipliers and a nonharmonic low-loss balanced mixer, allow for complex amplitude measurements of the electric wave field over the 167–173 GHz frequency range at a 120 dB dynamic range. This large dynamic range is needed because low gain open-ended receiver waveguides are employed for achieving high spatial resolution and, the diverging beam requires high dynamic range to cover axial and lateral beam distribution. Two receivers allow for simultaneous measurement of the complex electric field in orthogonal direction from which polarization information is retrieved. A schematic of the heterodyne system is shown in Fig. 11. The Agilent Technologies Vector Network Analyser in the centre serves both as the millimetre wave source for the transmitter signal as well as detector for the receiver signal. The frequency is swept from 8.016 GHz to 8.516 GHz. A phase locked dielectric resonator oscillator adds 5.9 GHz to the source signal resulting in a pump signal of 13.916 GHZ to 14.416 GHz. The receiver channel is slightly shifted in frequency to 5.91 GHz to reduce cross talk between the coaxial cables. The source signal is amplified by approximately 33 dB to compensate for coaxial cable loss and then passed through an amplifying power divider to feed both transmitter and harmonic reference signal mixer. This reference signal serves to phase lock the receiver signal. The transmitter frequency is up-converted by a factor 12 to reach the 167–173 GHz transmitter frequency at 5–10 mW power output. To transmit this signal from a D-band waveguide into the 63.5 mm circular waveguide of the remote steering (RS) system, a 842 mm long mode convertor or taper is employed, designed by IAP Nizhny Novgorod, achieving HE11 mode purity better than 99.5% at output. 4.2. Diagnostics

Fig. 10. Rotary probe for polarization measurement consists of a standard D-band waveguide antenna (lhs) fitted with a Macor stray radiation absorber cone (white element on first disc), two adjustable circular symmetric 30 dB attenuators and a Schottky diode detector (rhs). The system is suspended in an acrylic tube by discs 1, 4 and 5 from left and mechanically supported by a wooden structure equipped with bearings that allow rotation driven by a DC servo motor. The assembly is fixed on a XY position slide that moves the probe over the SCW output at 1.5 m distance.

A double probe fabricated from standard D-band waveguides detects the electric field amplitude and phase simultaneously in two orthogonal directions from which polarization information is derived, see Fig. 12. The probe is mounted on an X–Y–Z stage, powered by three 100/mm stepper motors, to provide a spatially resolved antenna pattern of the millimetre wave beam. In practice, the positional accuracy obtained is 0.1 mm limited by hysteresis and sagging. Vibrations are allowed to die out in a 100 ms measurements time delay. Thus, axial variations producing phase shift noise are avoided. The two waveguide probes are connected directly to individual balanced mixers that down convert the signal frequency and, after 35 dB low-noise amplification, connect to the Vector Network Analyser. The local oscillator signal of the balanced mixer is generated by adding 5.91 GHz to the source signal resulting in a

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Fig. 11. The 167–173 GHz Vector Network Analyser (VNA) based on a commercial VNA equipped with a custom built up-convertor as source and a down-convertor as receiver. Two receivers measure the complex wave amplitude simultaneously to derive polarization information. Employing active multipliers and normal balanced mixers in the frequency conversion, low conversion loss is achieved, unlike harmonic mixers that produce high conversion loss. Cross talk in the system is reduced by an additional frequency shift resulting in different output and input frequencies of the VNA. The system provides for a high dynamic range of 120 dB at 1 Hz resolution bandwidth needed to diagnose the diverging beam at high spatial resolution.

13.926–14.426 GHz frequency range. This signal is then amplified by 42 dB and multiplied by a factor 11 to produce a local oscillator range of 153.193–158.693 GHz of the receiver. The resulting Intermediate Frequency signal range is 13.8067–14.3067 GHz. Phase measurements need a reference signal of frequency equal to the measured signal. This signal is generated by the 10 MHz reference derived from the VNA and mixed upward with the transmitter

pump frequency to the 11th harmonic. Feeding this signal to the receiver, which is at exactly the same frequency range, a phase reference signal is created. A future development includes swept frequency measurements that allow identification of reflected and trapped power by the use of fast Fourier transform into the time domain. The gyrotron source, which is a free running device, cannot be phase locked to the waveguide polarization measurement device to serve as a reference signal for the network analyser limiting this device to low-power measurement [22]. This limitation does not exist for the rotary probe (Fig. 10). On the other hand, the rotary probe cannot distinguish between left and right handed elliptical polarization. Both methods have been compared for accuracy at low power, by measuring the signal of a well-characterised polarizing mitre bend. A circular scan of the rotary probe measures the field intensity as function of angle. The field maximum yields the angle ˛ of the major axis of the elliptic polarization. The field minimum yields the minor axis angle and subsequently tan ˇ, the ellipticity of the millimetre wave field [23]. For the low-power measurements ˛ and ˇ are derived from the complex amplitudes of the measured vertical and horizontal E-field amplitudes with their phase difference, the equations are given in [24]. Results of both polarization diagnostics are compared with the same universal polarising mitre bend and are shown to be in good agreement, see Fig. 13.

Fig. 12. Polarization measurement device consisting of two orthogonal D-band waveguides are connected to the mixers of the receiver system to measure simultaneously H-plane and E-plane polarised wave components. The assembly is mounted on an XYZ stage for beam antenna pattern measurement. The cross polarization error caused by waveguide misalignment is less than −35 dB.

5. Low-power measurements The SCW length is optimised for steering range, according to theory (Section 2) at the slightly reduced length of 4321 mm cor-

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Fig. 13. Polarization angles ˛ and ˇ of a universal polarizing mitre bend are measured by the dual channel rotary probe (Fig. 10) and the D-band probe (Fig. 12) and compared with specified angle. Here, ˛ is the orientation of the elliptic wave vector and tan ˇ, the ratio between major and minor axis yielding ellipticity. The two independent diagnostics are in good agreement.

responding with an optimal steering angle of 10.37◦ at 170 GHz. This length has been established by swept frequency measurements over the range 167–173 GHz, employing the waveguide at its original length of 4389 mm. Fig. 14 shows the Gaussian content of far-field (2 × Rayleigh distance) planar scans at 500 mm distance from the SCW exit perpendicular to the waveguide axis employing the dual waveguide probe scanning an area of 800 mm × 200 mm. The result yields an optimum SCW frequency of 171.54 GHz averaged over the range of steering angles for the 4389 mm waveguide length. Scaling by Eqs. (1) and (2) to 170 GHz operation yields the optimal waveguide length of 4321 mm. Fig. 15 shows beam profile measurements, recorded by the dual waveguide probe over the ±12◦ steering range at 170 GHz and optimal SCW length. The input beam waist is centred at the SCW entrance plane. Shifting the waist axially ±5 mm does not seriously affect this result. These measurements refer to the case where the electric field is in the plane of steering (plane of incidence or Eplane), the worst case in terms of performance. Due to differences in path length, the positive angle beam profiles are slightly different from the negative angle results. Far-field measurements shown in Fig. 16 performed in the equatorial plane of the beam steered at −6◦ angle show a beam power

Fig. 14. Gaussian beam content as a function of frequency for a range of steering angles derived from far-field beam pattern measurements, employing the dual waveguide probes of Fig. 12. Optimal performance (averaged over steering angle) occurs at 171.54 GHz for a 4389 mm long SCW. Tuning to optimal performance at 170 GHz, the waveguide was shortened to 4321 mm. A 98% Gaussian mode coupling of the steered output beam at outer steering angle is achieved, the remaining power being lost as stray radiation.

Fig. 15. Far-field low-power distribution (170 GHz) measured by the XY scanning dual D-band waveguide probe over the steering range +12◦ , +6◦ , 0◦ , −6◦ and −12◦ (shown top to bottom). The power appearing in the side lobes at ±12◦ is about −20 dB. The Gaussian content of the main lobe is better than 98%. Grid area 800 mm × 200 mm.

contour in dB vividly portraying the main Gaussian beam squirting out of the SCW with the stray power radiated in side lobes. Total Gaussian content of the SCW output beam over the steering range is very good at 98%. Side lobes of approximately −20 dB appear at extreme steering angles. Polarization measurements performed at the SCW exit using the rotary probe show a small change in polarization state of the

Fig. 16. Beam power contour measurements at 6◦ steering angle in the equatorial beam steering plane, showing 98% of the power confined to the main Gaussian beam and side lobe power at the −20 dB level.

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Table 1 Low-power polarization measurements of the SCW at 12◦ steering angle, employing the rotary probe for horizontal E-plane (upper row) and 45◦ input polarization (lower row), confirm that the input polarization state is approximately maintained over SCW steering range to within a few degrees. Input E field

˛

ˇ

tan ˇ

˛ = 0◦ ˛ = 45◦

1◦ 47◦

4◦ 0◦

0.07 0

SCW propagating beam at 12◦ steering angle over the 167–173 GHz frequency range, see Table 1. In order to characterize the SCW corrugations, a small sample of the SCW wall was subjected to test at IPF University Stuttgart5 employing the 3-mirror FP resonator technique [21]. The absorption (ohmic loss) and the phase shift between the parallel (E-plane) and perpendicular (H-plane) field components are determined relative to that of an ideal copper plate for varying angle of incidence. Fig. 17 shows the results of the ohmic loss calculation which method has been proven by measurements on a similar corrugated plate [10]. The calculation is done for both the E-plane and the H-plane polarizations as a function of steering angle normalized at the theoretical values of an ideal copper plate at 170 GHz, which at normal incidence is 0.114%, see Eq. (5). The results show an absorption enhancement of a factor 1.5 for H-plane polarization and a factor 4.4 for E-plane polarization as compared with an ideal plane copper conductor plate. Fig. 18 plots the resonance frequency shift for E- and H-plane polarization, as a function of the frequency. A linear relationship is found. In this measurement the steering angle is set at ϕ = 20◦ (angle of incidence ␪ = 70◦ ). The phase shift between Eand H-plane polarised waves relates to this resonant frequency shift through the free spectral range of the FP (367 MHz) corresponding to 360◦ phase shift for a two reflection round trip over the waveguide test sample. For one reflection the phase shift is derived from the measured frequency shift by multiplying with 180◦ /367 MHz = 0.49. The total phase shift (depolarization) of the SCW at ϕ = 20◦ is approximately given by multiplying this measurement with the number of reflections N, see Eq. (6), in this case N = 35.7. The result shows that zero phase shift occurs around a frequency of 171.64–172 GHz. For the nominal operation frequency of 170 GHz a 1.2◦ phase shift occurs, leading to a total SCW phase shift of 42◦ for 20◦ steering angle. As shown in Section 2, this result scales with ϕ2 [25], hence at ϕ = 12◦ steering angle the calcu-

Fig. 18. Determination of the optimum frequency of the corrugation profile. Around 171.64–172 GHz the phase shift between the E-plane and H-plane polarization is zero, thus avoiding bi-refringent reflection, which is required for maintaining the polarization state of the mm-wave beam transported through the SCW.

lated maximum phase shift between H- and E-plane polarization is 0.36 × 42◦ = 15◦ 6. High-power measurements Demonstration of high power, long pulse length (500 s ITER requirement) operation is a non-trivial task as small percentage power losses along the beam-line due to absorption, beam truncation and stray radiation show up as (too) high heat loads. High-power tests have been carried out at the FZK-IHM facilities2 supported by FZK staff [26]. For this purpose the ITER prototype high-power 170 GHz coaxial gyrotron was employed, see Fig. 19. Diagnostics for frequency, pulse shape, energy and power were supplied by FZK. The measurements reported here are limited to 1 to 2 kJ energy content, limited by the performance of the gyrotron at the time (2005) [18]. Pulse length could be varied from 1 ms to 10 ms, maximum power is limited to 400 kW. At the time of the tests, the mirror system inside the coaxial gyrotron was not optimized resulting in a lowest order Gaussian output beam content of 58%. Special matching optics to improve the Gaussian beam content are designed by IPF University Stuttgart, which include a vignetting absorber on the first beam shaping mirror and a beam scraping aperture between the first and second mirror, as shown in Fig. 20. 6.1. Input beam characteristics

Fig. 17. Absorption ˛ (ohmic loss) of the corrugated waveguide wall for polarization in H-plane (blue, ˛ = 1.5 sin ϕ) and E-plane (magenta, ˛ = 4.4 sin ϕ), as a function of steering angle ϕ at 170 GHz. The dotted curve represents the theoretical absorption for a plane copper plate, normalized at ˛ = 4Rs /Z0 for normal incidence, i.e. ϕ = 90◦ , see Eq. (5). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Prior to experiments, the input beam of the steering mirror unit (SMU) was characterised thermo-graphically at position 1, Fig. 1, at 50 cm distance from the output plane of the CCW, which is about twice the Rayleigh distance, thus establishing the far-field beam power distribution. The lowest order Gaussian beam content is 70% determined from fits to this measurement employing the coupling coefficient method described in Appendix A. The beam power profile measured is shown in Fig. 21. The spot radius of the fitted Gaussian beam is 38.4 mm, significantly larger than the 24.6 mm calculated. This discrepancy is attributed to the relatively low Gaussian beam content of the input beam yielding a Gaussian fit less well defined. The input beam characteristics for the SCW are determined at the output of the SMU at position 2, Fig. 1, positioned at 34 cm distance from the nominal SCW entrance plane measured in the direction of beam propagation. The result of Fig. 22 again yields a much broader beam (about 30 mm radius) as compared with the theoretical value of 21 mm radius.

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Fig. 21. High-power input beam profile of the SMU thermo-graphically recorded at position 1, Fig. 1, at 50 cm distance from the CCW output plane. Employing a vignetting absorber on the first mirror and a vignetting aperture between the first and the second mirror improved the first order Gaussian content from 58% to 70%. Spot radius (−12 dB contour) is wx = 38.8 mm, wy = 38.4 mm, centre x0 = −0.2 mm, y0 = 0.7 mm. Total power is approximately 300 kW.

Fig. 19. ITER prototype European 170 GHz coaxial gyrotron used as input millimetre wave source for the high-power test of a mock-up remotely steered millimetre wave launcher.

Fig. 22. High-power exit beam profile of the SMU recorded at 34 cm distance from the SCW entrance plane, in direction of beam at position 2, Fig. 1, with the beam steered at 0◦ . Gaussian content is 80% with spot radius wx = 32.0 mm, wy = 27.7 mm beam, beam centre x0 = 5.28 mm, y0 = 9.68 mm.

Design calculations (Fig. 2) are based on a beam waist radius of 20.32 mm positioned at the CCW output plane with a flat phase front from where the Gaussian beam is propagated through the steering system. In order to reconcile measured beam parameters with theory a best Gaussian beam fit to the far-field measurements at the SMU entrance and exit yield an initial beam condition at the CCW output plane with beam radius of 17.3 mm and phase front curvature of 0.49 m. These parameters are used in the subsequent sections as a basis for comparison of measured and calculated beam properties. 6.2. Beam steering performance near field In order to compare measured steered beam performance with theory, the near-field beam pattern was recorded at the SCW output plane as a function of beam steering angle, placing the thermographic diagnostics directly at the SCW output flange. Results are shown in Fig. 23. A Gaussian fit to these near-field measurements yield a steered beam shift as predicted by theory (Eqs. (1) and (2) an Fig. 4), provided a beam offset of −2.5 mm from the SCW central axis is taken into account, see Fig. 24. 6.3. Beam steering performance far field Fig. 20. The millimetre wave system feeding the steering unit under vacuum, showing ellipsoidal beam shaping mirrors, vignetting aperture and the circular corrugated waveguide fitted with polarising mitre bends. The beam path is indicated by the yellow line. The position marked by the orange arrow indicates where the input beam reference measurements took place employing the calorimeter and the infrared camera, recording beam patterns on the PVC screen placed at 50 cm distance from the CCW output.

Fig. 25 demonstrates the steering of the high-power beam by far-field measurements on location 4, Fig. 1, thermo-graphically recorded at 50 cm distance from the SCW exit plane. Picture inserts list the Gaussian content, energy and the beam central position for every steering angle. The measurement series refers to horizontal polarization. Similar results are obtained for vertical polariza-

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Fig. 24. Measured beam offset x (mm) at the exit of the remote steering waveguide compared with theory. Good agreement is found when including a 2.5 mm beam offset (least square difference minimized to find the offset). The SCW length is 4321 mm, based on a 10.37◦ steering angle optimisation.

tion. At large steering angles a ghost beam at mirrored angle appears. This is predicted by theory where phase slippage between waveguide modes leads to output beams in the opposite direction [15]. Comparison of the measured beam radius of the far-field beam power distribution with calculated TEM00 Gaussian content propagated through the SMU-SCW tandem, shows good agreement over the range of steering angles for all polarization directions, as is shown in Fig. 26. The output beam characteristics are derived from the known starting point of the TEM00 part of the Gaussian beam, located at the output of the SCW, and the far-field pattern measured. In Fig. 27, the lowest order Gaussian coupled TEM00 mode contents for two polarization states are shown. Maxima are identified at 0◦ steering and at the optimum SCW design angle 10.4◦ , consistent with theory [17,27], see also Fig. 5. The beam steering angle measured at the SCW far field is compared with input steering angle over the ±12◦ steering range. At 12◦ steering, the steered beam lags behind by about 1.7◦ with respect to the input steering angle, see Fig. 28. This difference is the so-called “squint angle”. Comparison of the measured squint angle with theory [27] is plotted in Fig. 29. Measured values show a larger squint than predicted. The deviation is attributed to the relatively large higher order TEM mode content (20%) of the input beam at the SCW entrance plane. 6.4. Beam power efficiency

Fig. 23. High-power near-field beam patterns measured at the SCW output plane for horizontal input polarization (plane of beam steering). The centre coordinate of a first order Gaussian fit to the beam is shown in brackets together with the steering angle. The beam steering is as predicted, see Fig. 24.

The overall power efficiency of the RS system is determined from the input power to the SMU and output power from the SCW. The SCW output power is derived from thermo-graphic far-field beam profile measurements calibrated against calorimetric measurement. The input power to the SMU derives indirectly from the bolometric power intercepted at the gyrotron output window serving as a proxy for the gyrotron output power. This bolometric power is calibrated against beam profile measurements of the far-field CCW power, which in turn are calibrated calorimetrically. Thus, the absolute power loss of the RS system at 0◦ steering angle is found to be of the order of 8% (±5%). This figure relates to a vertically polarised beam and is compensated for TEM00 mode content change. The error bar is large because the input power cannot be measured directly. The relative power loss of the steered beam is determined both calorimetrically and thermo-graphically at the SCW far field. The calorimeter shows a 5% relative beam energy loss at the extreme angles compared with 0◦ steering. Thermo-graphic measurements are consistent with this 5% loss for 45◦ beam polarization. For horizontal and vertical polarization the thermo-graphic measurements yield a 15% loss integrated over the side lobe pixel power con-

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Fig. 26. Measured SCW far-field beam radius as a function of steering angle fitted to the lowest order Gaussian mode for three polarization directions compared with theory. There is good agreement provided the adjusted beam initial condition is used.

Fig. 27. Coupled TEM00 mode content measured for two polarization directions (H and 45◦ ) as a function of steering angle. Optima in coupling are found at 0◦ and at 10.4◦ as predicted by theory by [17,27].

or H-plane and 30◦ ). By rotating the probe, the polarization profile was sampled at 19 points. The detector signals are linearised using the diode detector sensitivity curve and converted into E-field values. Through these E-field values a cosine profile is fitted to obtain

Fig. 25. SCW output far-field beam profile measurements at 50 cm distance from the SCW exit plane (250 kW, 8.76 ms pulse, horizontal input polarization). Each picture displays the beam angle, its centre coordinates, the lowest order Gaussian content and the total energy. The beam offset, compared with the near-field measurements, is in the same direction, albeit larger in magnitude. Ghost beams at opposite angle are observed as predicted [15].

tent, see Fig. 30. The difference with calorimetric measurement is attributed to power contained in strongly diverging higher order modes that are not captured by the thermo-graphic diagnostics. 6.5. Polarization The polarization state of the beam was measured by moving the rotary probe into the beam at location 4, Fig. 1. Angular accuracy of the probe was established at 0.1◦ . The beam polarizations are measured at the edge of the beam and at the centre of the beam for three input polarization directions (horizontal or E-plane, vertical

Fig. 28. Output steering angle as a function of input steering angle measured at the SCW far field by establishing the first order Gaussian mode content. Both insteering plane and out-of-steering plane beam positions are shown. Polarization is horizontal. At large steering angle the steered beam lags behind the input steering angle, a so-called squint view of the beam develops.

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0◦ and 12◦ is about 10◦ . This value is consistent with calculations presented in Section 2. The accuracy of the measurement is limited by the fact that the input polarization is only approximately known. Also, the rotary probe is moved from the centre to the edge of the beam during the measurements, sampling a higher order mode content at the edge of the beam. 6.6. Front mirror far-field and vacuum operation

Fig. 29. Comparison of the squint angle measured with theoretical calculations [27] as a function of steering angle. Experimentally, a stronger squint is observed than predicted by theory.

Fig. 30. The relative power loss of the far-field steered millimetre wave beam as a function of steering angle for three polarization directions. For 45◦ polarization the relative loss with steering angle is approximately 5% within the measurement accuracy. For horizontal (E-plane) and vertical (H-plane) polarization direction the power loss at extreme steering angle is about 15%.

the polarization parameters ˛ and ˇ. An excellent fit is obtained for all steering angles. In Table 2 the polarization parameters ˛ and ˇ are tabulated as a function of steering angle, input polarization and x–y position in the beam. Using [24] the horizontal- and vertical E-field amplitudes with their phase difference could be determined from ˛ and ˇ. Looking to the measurement at 30◦ the difference in phase between

The far-field beam pattern, after reflection by the front mirror (FM), is measured in similar manner as before employing the thermo-graphic diagnostics system at 600 mm distance from the FM. The steering angle has been varied over the ±12◦ steering range. The results shown in Fig. 31 yield a slightly higher Gaussian content of the beam, explained by longer propagation distance diverting higher order gyrotron modes out of the beam. Side lobes at −16 dB at extreme steering angle are observed, as predicted [15]. Note the slanted projection of the beam patterns compared with previous measurements. The FM causes this, directing the beam out of the horizontal plane at the q-surfaces targeted in ITER geometry. Unlike previous measurements, some of these far-field measurements are carried out under high vacuum (5.9 × 10−7 mbar) with the steering mirror system sealed at the entrance of the CCW and the exit of the SCW (position 3) by quartz windows. Plotting the relative beam power loss over the range of steering angles, Fig. 32, we note an additional 15% vignetting loss attributed to the presence of the quartz windows. These quartz windows, however, do not represent the ITER configuration. In addition to power loss, the Gaussian beam content (lowest order Gaussian coupled TEM00 mode) is considered when changing the window situation: no windows, with diamond window at entrance of SCW and full vacuum conditions the system end-capped by quartz windows. A relatively large loss in Gaussian content is due to the quartz windows, not representing the ITER configuration. No influence in mode content due to the diamond window is observed. The variation over steering angle agrees with theoretical prediction [17,27], see also Fig. 5 (Fig. 33). 7. Discussion and conclusion The remote steering design for the ITER upper launcher for electron cyclotron heating and current drive at 170 GHz operation has been validated by low and high-power measurements, partly carried out under vacuum. The low-power heterodyne measurements show that remotely steered beams over the ±12◦ steering range can be achieved with better than 98% Gaussian beam content. Gaussian beam propagation is in good agreement with theory. Measure-

Table 2 The results of the high-power polarization measurements as a function of the steering angle. Parameter is approximate input polarization to a few degrees and radial position of the probe. The polarization parameters ˛ and the ellipticity tan ˇ show a small variation over steering angle and radial position consistent with theory. Steering angle (◦ ) 0 0 +12 +12 +12 +12 0 0 0 0 +12 +12

SCW input polarization ◦

∼30 ∼30◦ ∼30◦ ∼30◦ ∼Hor. ∼Hor. ∼Hor. ∼Vert. ∼Vert. ∼Vert. ∼Vert. ∼Vert.

OX (mm)

OY (mm)

0 0 −345 −345 −337 −337 0 0 0 0 −340 −340

57 57 57 57 47 52 62 62 17 0 7 7

˛ (◦ ) −29.3 −28.3 −28.0 −28.0 −8.1 −7.9 −15.4 −73.8 −78.1 −82.2 −79.8 −79.9

ˇ (◦ )

tan ˇ E-ratio

+4.2 +4.4 +8.3 +8.2 +16.3 +16.0 +8.0 +8.5 +5.3 +6.8 +4.3 +6.3

0.07 0.08 0.15 0.14 0.29 0.29 0.14 0.15 0.09 0.12 0.08 0.11

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Fig. 32. Relative loss over the steering angle at the far field of the front mirror. Several situations are examined: with and without diamond window, and with and without quartz vacuum sealing windows fitted to establish vacuum operation. A slight 2% power loss is observed from the diamond window representing the secondary vacuum boundary of the ITER Launcher [28–30]. An additional 15% vignetting loss is observed from the quartz windows, which do not represent the ITER situation.

ment and theory on beam offset as a function of steering angle enabled the optimal length of the square corrugated waveguide (SCW) to be established. Polarization state is found to be approximately maintained over the steering range. The corrugation profile was characterised for absorption and phase shift in accordance with theory. High-power experiments reported are limited in beam power and beam quality delivered by the European co-axial gyrotron at the time of measurement (2005). Nevertheless, good agreement is found for Gaussian beam propagation at all polarization directions, provided input beam characteristics are adapted to represent the low Gaussian content. Near-field measurements at the SCW exit show good agreement with beam-offset as predicted by theory. Far-field measurements show approximately 80% coupled Gaussian mode content, varying with steering angle as predicted. The squint angle observed at high steering angles is larger than predicted by theory. The discrepancy is attributed to the spurious mode content of the gyrotron. In practice, to achieve the ±12◦ required, the system needs to be designed for ±13◦ or possibly ±14◦ to allow

Fig. 31. Front mirror far-field measurement of the high-power beam. A higher Gaussian beam content is observed. At 12◦ steering a −16 dB side lobe is visible as predicted [15]. In these measurements the remote steering system was brought under vacuum by sealing the entrance CCW and the exit SCW by quartz windows. The slanted beam pattern represents the out of plane steering by the FM directing the beam to the ITER poloidal magnetic flux surface targeted. In the ITER configuration only a diamond window, placed at the SCW entrance, acts as the primary vacuum boundary of the ITER Upper Launcher. Additional tests, with and without diamond window (position 2), show a ∼2% loss in beam power over the steering range, which must be attributed to vignetting loss since calculated tan ı losses are significantly smaller [28–30]. A 2% vignetting loss, albeit small in relative terms, will present a serious power loading problem when ramping up to long pulse length.

Fig. 33. Coupled TEM00 contents with and without diamond window and with quartz windows fitted at both ends of the remote steering system to create vacuum conditions. There is good shape agreement at 0◦ and 10.4◦ steering angle predicted by [17,27], also illustrated by Fig. 5. Gaussian mode content has increased because of higher order modes diverted out of the beam.

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for this squint angle margin. Steered beam efficiency goes slightly down at extreme steering (45◦ polarization). The polarization state is approximately maintained over the beam cross section. The overall transmission of power in the Gaussian beam from input steering system to output SCW could not be established directly, but is estimated to be approximately 92% going down to approximately 87% at ±12◦ steering angle for horizontal (E-plane) and vertical (Hplane) polarization. This high loss figure is probably due to the high order mode content of the input beam. In future, this figure should come down significantly to less than 1% specified in order to be suitable for operation at the 1 MW level and long pulse operation. The quartz windows, vacuum sealing the system in mock-up geometry, influenced both mode content and steered beam power. In ITER geometry, however, these quartz windows are absent. Instead, a diamond window representing the ITER primary vacuum boundary is shown not to influence the steered beam mode content. Beam power loss accounts for up to 2% mainly caused by vignette loss and needs to come down significantly to allow long pulse high-power operation. Operation of the steering mirror unit under vacuum experienced no problems. The high voltage breakdown limit was not reached and no arcing was observed for beam powers up to 400 kW. Sophisticated diagnostics and data analysis schemes, including heterodyne and thermo-graphic techniques have proved essential in validating the design of the ITER millimetre wave launcher. It allowed beam steering efficiency, beam profile, Gaussian mode content and polarization state to be determined and compared with theory. Future diagnostics development includes swept frequency measurements that will allow identification of reflected and trapped power by the use of fast Fourier transform into the time domain. Our measurements show that going from low to high-power operation, power loss increases by an order of magnitude at extreme steering angle (2–15%) owing to the reduced Gaussian beam content and the presence of higher order modes. This raises the question of how sensitive the remote steering concept is on gyrotron source characteristics and, how this sensitivity reflects on the technical specification of the gyrotron source. A similar question relates to the front steering concept and it would be interesting to compare both concepts on this aspect. Future challenges in developing the ITER remote steering concept include high-power long pulse duration test. Building on Wendelstein 7-X experience of 30 min high-power pulse duration test [31], stray radiation and absorbed or trapped power could lead to overheating of beam-line components. Thus, validation of the remote steering design concept needs to be followed up by highpower long pulse duration tests. A detailed development plan needs to be worked out, including engineering model, prototype and operational model of the assembled ITER upper port launcher before hardware is deemed ready for installation on the ITER machine. High-power tests need to be specified at each stage of model development. An appropriate Test Bed needs to be defined in order to perform the pre-installation test at high power and long pulse duration.

views and opinions expressed in this publication do not necessarily reflect those of the European Commission.

Appendix A. The temperature rise of the PVC target screen produced by millimetre wave beam impact is measured by an Infra-Red camera (Jenoptik Variotherm type 014104) fitted with 1.4/25 objective placed behind the screen. Pixel resolution is 3.1 mm × 1.5 mm, doubled by forming a composite image from two viewing angles. Temperature background is measured prior to each beam pulse and subtracted to achieve better than ±0.1 ◦ C precision determined by the A/D convertor bit resolution. The target screen, shown in Fig. 34, is positioned at 50 cm distance from the circular waveguide exit (location 1, Fig. 1). In order to establish the relationship between temperature profile and beam energy, the peak temperatures are calibrated against a ballistic calorimeter. By varying the millimetre wave pulse length the input beam energy is varied. Subsequently, for the same sequence of pulse lengths the beam energy is measured by a ballistic calorimeter. Results are shown in Table 3 and plotted in Fig. 35.

Fig. 34. The beam target screen made out of a highly absorbing PVC sheet of 0.4 mm thickness stretched over a rigid 400 mm × 800 mm frame. Heating wires at 100 mm period mark the reference coordinates. A movable IR camera measures the temperature profile at two orientations with respect to the beam.

Acknowledgements This work is supported by NWO, ITER-NL and the European Communities under the contract of Association between EURATOM and FOM and was carried out within the framework of the European Fusion Program, EFDA contract TW5-TPHE-ECHULB3 (2005). We are grateful to the Forschungszentrum Karlsruhe, for providing their high-power Gyrotron source and millimetre wave test facilities and their experimental, analytical and technical support. The

83

Fig. 35. The temperature to energy sensitivity curve of the PVC target screen.

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Table 3 Calibration of beam energy against temperature rise of the PVC target screen. Gyrotron pulse length is varied. Shot pair no.

Pulse time (ms)

Calorimeter (J)

Power (kW)

Tpeak (◦ )

Tstart (◦ )

T (◦ )

53 and 60 54 and 61 55 and 62 56 and 63 57 and 64 58 and 65 59 and 67

0.83 1.766 2.718 3.694 4.654 5.654 6.174

241 521 772 1049 1280 1549 1648

290 295 284 284 275 274 267

38.04 52.41 72.03 91.87 119.85 139.48 149.22

27.60 27.63 27.75 27.59 27.79 27.87 27.91

10.44 24.78 44.28 64.28 92.06 111.61 121.31

From these measurements, one can derive the temperature T to energy E sensitivity function:



E(T ) = −1053 + 2.513

5

1.756 · 10 + 7960T

(7)

The total energy Etotal contained in the beam profile is calculated by summing over all pixels: Etotal = Cpvc

0.4

Area mm Xpixels Ypixels



Xpixels Ypixels

i=1

E(Tmeasured(i,j) )

(8)

j=1

Here the screen area Area is normalized by the number of pixels Xpixels and Ypixels . The constant Cpvc 0.4 mm derives from the calorimetrically measured energy of the beam pulse: Cpvc

0.4 mm

= 6.302 × 10−4 mm−2

(9)

The energy profile is built up from individual pixel values Epixel : Epixel = Cpvc

0.4 mm

Area E(Tmeasured(i,j) ) Xpixels Ypixels

(10)

Summing over the measured surface area s, this energy profile Epixel is equated to the power profile times the pulse length P(s) p . In order to determine the lowest order Gaussian mode content of the millimetre wave beam, the coupling coefficient C0,0 is calculated,

defined as [13]:

 C0,0 =



s

∗ ds fE0,0

ff ∗ ds s



(11)

E E ∗ ds s 0,0 0,0

This equation couples the measured field distribution f to the lowest order Gauss–Hermite electric field value (E0,0 ) [13]. In the coupling procedure the beam size and the centre of the Gaussian distribution are varied such that the coupling coefficient is maximized by applying the “fminsearch” algorithm. In order to account for thermal noise in the measurement, relating to power noise in the calculation, negative values are permitted by substituting ff* = P(S). In determining the Gaussian beam content, this is an important factor in the integration of ff* over the area s, which would otherwise lower the Gaussian content significantly. Fig. 37 illustrates the Gaussian beam fit method by applying Eq. (11) to the beam profile measurements taken at several positions along the beam-line for 0◦ steering angle. Fig. 36 shows the same at ±12◦ steering. In these measurements noise is observed at the profile edges and is correctly accounted for by allowing negative values for P(s). A large pedestal in the beam power profile suggests stray power present in the beam emanating from the CCW. A mirror beam is observed at 12◦ steering angle.

Fig. 36. Far-field SCW output beam profile for 12◦ steering measured at beam position 4, Fig. 1 (horizontal input polarization) in the two beam planes: (a) and (b) steering angle −12◦ and (c) and (d) steering angle 12◦ . The dotted line shows the coupled lowest order Gaussian mode. A mirror beam is observed in measurement (a).

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Fig. 37. Far-field beam profile measurements in the two orthogonal beam planes taken at three locations along the beam-line (vertical input polarization): (a) and (b) SMU entrance position 1, Fig. 1; (c) and (d) SCW entrance position 2, Fig. 1; (e) and (f) SCW exit at position 4, Fig. 1. The dotted line shows the lowest order coupled Gaussian beam mode. Noise at the edge of the measured profile needs to be treated correctly in determining the Gaussian beam content by Eq. (11) allowing negative power values. A large pedestal is observed around the main beam entering the SMU corresponding to stray power.

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